XXXI. MINERALOGY AND CRYSTALLOGRAPHY…

XXXI. MINERALOGY AND CRYSTALLOGRAPHY

THE MINERALS—CHARACTERISTICS—CRYSTALS AND THEIR FORMS—DESCRIPTIONS OF MINERALS.

MINERALS are constituent parts of the earth. All parts of minerals are alike. There are simple minerals and mixed. The former are the true minerals, and are generally considered under the heading MINERALOGY. The others constitute a branch of GEOLOGY, as they form aggregate masses, and as such compose a large portion of the earth. We must learn to distinguish minerals and crystals as inorganic forms of nature. In the animal and vegetable kingdoms we have forms which are possessed of organs of sight, smell, taste, and certain structures indispensable to their existence and development. But in minerals we have no such attributes. They are INORGANIC, and have a similar structure; a fragment will tell us the story as well as a block of the same mineral. These inorganic substances are possessed of certain attributes or characteristics. We find they have FORM. They have chemical properties, and they behave differently when exposed to light and electricity. They are generally solid. All the elements are found in the mineral kingdom, and a mineral may be an element itself, or a chemical combination of elements. These compounds are classed according as the combination is more or less simple. An alliance of two elements is termed a binary compound, of three a ternary compound, forming a base and an acid.

We have learnt from our chemistry paper that there are between sixty and seventy elementary bodies in nature. When we speak of “elements,” we do not mean to apply the popular and erroneous definition of the word. Earth, air, fire, and water are not elements; they are compounds, as we have seen. The list of elements has been given; we will now give the names of the more important minerals. We have no space for a detailed description, but in the British Museum the cases contain some hundreds, and the student will find them classified and described with the greatest care, and according to the arrangement of Berzelius.

PRINCIPAL MINERALS AS ARRANGED BY PROFESSOR ANSTED.

I.

Diamond. Graphite. Anthracite. Coal. Lignite. Bitumen. Amber. Sulphur. Quartz. Amethyst. Agate. Chalcedony. Flint. Jasper. Opal.

II.

Sal-ammoniac. Nitre. Rock-salt. Borax.

III.

Witherite. Spar. Strontianite. Celestine. Calc-spar. Marble. Dolomite. Fluor-spar. Gypsum. Apatite. Magnesite. Corundum. Sapphire. Emery. Turquoise. Alum-stone.

IV.

Cyanite. Christolite. Clay. Fullers-earth. Garnet. Iolite. Jade. Emerald. Beryl. Felspar. Obsidian. Pumice. Talc. Serpentine. Zircon. Hornblende. Asbestos. Augite. Diallage. Topaz. Tourmaline. Lapis-lazuli. Chrysoberyl.

V.

Wolfram. Molybdenite. Chromite. Pitch-blende. Uranite. Pyrolusite. Wad. Manganese-spar. Arsenic. Realgar. Orpiment. Antimony (grey). Bismuth. Blende. Calamine. Spartalite. Tinstone. Galena. Pyromorphite. Iron-pyrites. Mispickel. Magnetic iron ore. Micaceous iron. Hematite. Spathic iron. Cobalt. Copper. Oxides of copper. Copper pyrites. Azurite. Malachite. Mercury. Cinnabar. Silver. Gold. Platinum. Palladium.

The above is the arrangement best suited for beginners.

Professor Nichol prefers the following arrangement:—

ORDER I.—THE OXIDISED STONES.

Quartz. Felspar. Scapolite. Haloid stones. Leucite. Zeolite. Mica. Serpentine. Hornblende. Clays. Garnet. Cyanite. Gems. Metallic stones.

ORDER II.—SALINE STONES.

Calc-spar. Fluor-spar. Heavy-spar. Gypsum. Rock-salt.

ORDER III.—SALINE ORES.

Sparry iron ores. Iron salts. Copper salts. Lead salts.

ORDER IV.—OXIDIZED ORES.

Iron ores. Tinstone. Manganese ores. Red copper ores. White antimony ores.

ORDER V.—THE NATIVE METALS.

ORDER VI.—SULPHURETTED METALS.

Iron pyrites. Galena. Grey antimony ore. Grey copper ore. Blende. Ruby-blende.

INFLAMMABLES.

Sulphur. Diamond. Coal. Mineral resins. Combustible salts.

These are only a portion of the minerals, but it would be scarcely interesting to give the list at greater length. In the foregoing we recognize the metals and various combustible and non-combustible substances familiar to us, existing, as people say sometimes, in “lumps.” But if any one will take the trouble to examine a “lump,” he will find the shape is definite and even. These regular forms of the minerals are called CRYSTALS, from the Greek word krustallos, ice. The term was originally applied to quartz, for in olden times it was thought that quartz was really congealed water. We can define a crystal as “an inorganic solid bounded by plane surfaces arranged round imaginary lines known as axes.” It must not be imagined that crystals are small bodies; they may be of any size. There are crystals of many hundredweight; and although the usual crystal is comparatively small, it may be any size.

Crystallization has occurred by cooling, or by other natural means; and we can form crystals by evaporation from certain salts deposited in water. So we may conclude also that the evaporation of water in the early periods deposited many forms of crystals. We have crystals in the air, such as snowflakes, which are vapours crystallized. Carbon, when crystallized, is the diamond. Boron is very like it. Oxygen cannot be crystallized. Alumina makes sapphires and ruby with silica. Alumina and earth give us spars, tourmaline, and garnets. Limestone also has beautiful forms, as in Iceland spar. Crystals, therefore, are certain forms of nature, corresponding in the inorganic kingdom to the animals and plants of the organic.

Let us look a little more at these. Here we have a group of crystals of different forms. Earths are metals combined with oxygen, and the principal earths are alumina, lime, and silica. To these three we are chiefly indebted for the ground we live on, and from which we dig so many useful metals and other minerals. Earths are coloured by the substances mixed with them. We can thus find copper, silver, gold, lead, etc., by noting the appearance of the soil. True earths are white. Strontia and baryta are also earths, and the latter is used in firework manufactories. Our chief assistants are ALUMINA, which furnishes us with bricks and slate; LIME, which gives us marble or stones for building in a carbonate form. Quicklime, by which is meant lime freed from the carbonic acid, is well known; and plaster of Paris is only lime and sulphuric acid in combination. The SILICATES, such as sand and flint, are in daily demand. Agate, cornelian, Scotch pebbles, rock-crystal, etc., belong to the same family. Even our gems are crystallized earths, and, as already stated, diamonds are merely carbon.

Stone, as we know, is quarried; that is, it is dug out of the earth. But perhaps many readers do not know why a stone-mine is called a “quarry.” Most kinds of stone (granite and marble are the exceptions) are found in layers, or strata, rendering them easy of removal. The blocks of stone are cut with reference to these layers in a more or less square manner, and “squared up” before they are carried away. Thus the term “quarry,” from an old French word, quarré, or carré, as now written, signifying a square. In granite quarries the stone being very hard is bored, and loosened by means of gunpowder or dynamite blasting. Slate, on the contrary, is easily divided into slabs. We will now resume the subject of Crystals.

We have said that crystals vary in size, and this variety may be traced, in the cases of crystallization from fluids, to the slowness or the rapidity of the cooling process. If the work be done slowly, then the crystals obtain a size commensurate with the time of cooling, as they are deposited one upon the other. The form of minerals is the first important point, and to ascertain their forms and structure we must study CRYSTALLOGRAPHY. We shall find faces, or planes,—the lines of contact of any two planes,—called edges, and the angles formed where these planes meet. We may add that crystals have, at least, four planes, making six edges and four angles. Nearly all crystals have more than this, for the forms are, if not infinite, very numerous, and are divided into six (by some writers into seven) different systems or fundamental forms from which the varieties are derived. The axis of a crystal is an imaginary line drawn from an angle to the opposite one.

The first form, the monometric, or cubic system, with three equal axes at right angles, is represented by fig. 436. This crystal is limited by eight equilateral triangles. It has twelve edges and six angles. If we describe a line from any one angle to an opposite one, that line is called an axis, and in the case before us there are three such axes, which intersect each other at right angles.[23] Such crystals are regular octohedra. There are irregular forms also, whose axes do not come at right angles, or they may be of unequal length. The substances which we find crystallized in this form or system are the diamond, nearly all metals, chloride of sodium (salt), fluor-spar, alum, etc.

When we say in this form we do not mean that all the minerals are shaped like the illustration (fig. 436). We shall at once see that the system admits of other shapes. For instance, a regular crystal may have been cut or rubbed (and the experiment can be made with a raw turnip). Suppose we cut off the angles in fig. 436; we then shall have a totally different appearance, and yet the crystal is the same, and by cutting that down we can obtain a cube (fig. 437). Take off its angles again we obtain a regular octohedron once more, as shown in the diagram opposite.

We will exhibit the gradations. Suppose we cut fig. 437; we will obtain (fig. 438) the cube. The next is merely the cube with angles and edges cut off; and if we proceed regularly we shall arrive at fig. 442, the rhombic dodecahedron, or twelve-sided figure, whose equal planes are rhombs.

We can, by taking away alternate angles or edges situated opposite, arrive at other secondary crystals. From the original octohedron we can thus obtain figs. 443 and 444. These are known as tetrahedron. The pentagonal dodecahedron is another secondary form (fig. 445).

The cube, or hexahedron, the octohedron, and the rhombohedron are all simple forms, being each bounded by equal and similar faces, or surfaces. We can thus understand how certain primary or original natural forms of crystals can be changed in appearance by connection. Of the various substances crystallizing in this system we find salt, iron pyrites, gold, silver, copper, and platinum, and the sulphide of lead called galena, in the cube or hexahedron form. The diamond and fluor-spar, alum, etc., appear in the first form (I), fig. 436 (octohedron). The cube, we see, has six equal faces, eight equal angles, and twelve equal edges. Galena, as will be observed from the illustration herewith, shows this peculiarity in a very marked manner (fig. 446).

The second crystalline form is the HEXAGONAL, and in this system three of the four axes are equal and in the same plane, inclined at an angle of 60°, with a principal axis at right angles to the others. In crystals of this system are found quartz and calc-spar.

The third system is termed the QUADRATIC or the diametric. This form has three axes, all at right angles, two being equal and the other longer or shorter than the former two. In this system crystallize sulphate of nickel, zircon, oxide of tin, etc.

The fourth, or RHOMBIC system, or the trimetric. Here we have three rectangular axes, all unequal and intersecting at right angles. The sulphate and nitrate of potassium crystallize in this system.

The fifth is the oblique, or MONOCLINIC system, which displays three unequal axes, two of which are at right angles; the third, or principal axis, is at right angles to one and oblique to the other of the preceding. Ferrous sulphate, tartaric acid, and gypsum crystallize in this system.

The sixth, or TRICLINIC system, or the doubly oblique. In this system we have three axes differing in length, and all forms which can be arranged about these unequal and oblique axes. Sulphate of copper will be found in this group. The system has been called anorthic, or triclinic, because the axes are unequal and inclined, as in the oblique prism based upon an obliqued angled parallelogram. Axinite crystal, as annexed, will show one form in this system.

As may be gathered from the foregoing, it is not easy to determine a crystalline form with certainty,—a great part of the crystal may be invisible. A crystalline mass is a mineral, which consists of an arrangement of crystals heaped together. If it does not possess these the mineral is amorphous, or shapeless. We will now endeavour to describe some of the physical characteristics of minerals.

The GONIOMETER (see fig. 455) is the instrument used for measuring the angles of crystals. Wollaston’s reflecting instrument is most generally used. It consists of a divided circle, graduated to degrees, and subdivided with the vernier. The manner of working is easy, though apparently complicated. The vernier is brought to zero, when an object is reflected in one face of the crystal. The crystal is turned till the same object is viewed from another face. The angle of reflection is then measured, and can be read off on the circle.

We have already referred to the physical characteristics of the minerals, and one of these attributes is cohesion. When we find a substance is difficult to break, we say it is “hard.” This means that the cohesion of the different particles is very great. Minerals vary in hardness; some are extremely difficult to act upon by force, and a file appears useless. At the other side we find some which can be pricked or scratched with a pin; and these degrees of hardness being put as extremes, we can in a manner relatively estimate the hardness of all other minerals. We can test this by scratching one against another; whichever scratches the other is the harder of the two, and thus by taking up and discarding alternately, we can at length arrive at a comparative estimate of the hardness of all. Such a scale was arrived at by Mohs, and arranged in the following order. The softest mineral comes first:—

1. Talc. 2. Gypsum (rock-salt). 3. Calcareous spar. 4. Fluor-spar. 5. Apatite-spar. 6. Felspar. 7. Quartz. 8. Topaz. 9. Corundum. 10. Diamond.

Talc, we see, is the softest, and diamond the hardest. Thus “diamond cut diamond” has passed into a proverb expressive of the difficulties one “sharp” person has to circumvent or “cut out” another. Diamond is used by glass-cutters. When geologists wish to express the degree of hardness of any substance, they mention it with reference to the foregoing list; and if the substance be harder than fluor-spar, but not so hard as felspar, they determine its hardness five, or perhaps between five and six, or between four and five, according as it is harder or less hard than apatite. Thus hardness, or power of cohesion, resistance to exterior force and pressure, is a prime characteristic of the mineral kingdom. The file is the best test.

We now come to another phase of the physical character of our minerals—cleavage. This is the term employed to express the facility of cutting in a certain direction which in the mineral is its direction of cleavage. Take mica, for instance. There is no difficulty in separating mica into thin layers; we can do so with our fingers. The layers, or flakes, or laminæ are so arranged that they exhibit less cohesion in one direction than when tried in other ways. We cut with the grain, as it were in the direction of the fibre when wood is concerned. Here we have another popular saying expressive of this,—“against the grain,”—which signifies an act performed unwillingly and unpleasantly. Cleavability, therefore, means cutting with the grain, as it were, and various minerals are possessed of different degrees of cleavage. It sometimes happens that electric excitement is observed when cleavage takes place. One place will become positive, and the other negative. Mica, arragonite, and calcareous spar will exhibit this action after cleavage or pressure. When a crystal of tourmaline is heated, it will develop positive electricity at one end of its principal axis, and negative at the other. Even if it be broken, the extremities of the fragments will exhibit similar phenomena, and so far like a magnet, which, as we have seen, possesses this attribute of “polarity.” But a curious fact in connection with this is that, if the heating cease the polarity ceases for a second or two, and yet as cooling goes on the polarity is restored, with the difference that the positive end has become negative, and the end previously negative has come over to the opposite pole. Electricity, therefore, must be hidden away in every portion of our globe, and will some day be proved to be the mainspring of all life.

Fracture in minerals is also to be noticed. Those substances which we cannot laminate we are obliged to break, and we may require to break a mineral in a direction different from or opposed to its direction of cleavage. Under such circumstances we must break it, disintegrate it, and observe the fracture. Sometimes we shall find the surfaces very even, or uneven, or what is termed conchoidal. This is observable in the breaking of flint. There are various ways in which minerals display fracture, and the particular manner and appearance denotes the class to which the mineral belongs.

We may pass over the question of the specific gravity of minerals, as we have in a former part explained this. It is important, however, to ascertain the specific gravity. As a general rule, minerals containing heavy metals are of high specific gravity.

But the relation of minerals (crystals) with regard to light is of great interest and importance. When we were writing of polarization, we mentioned the faculty a crystal has for double refraction, by which it divides a ray of light into two prolonged rays taking different directions, the plane of vibration of one being at right angles to that of the other. This property is not possessed by all crystals. Some act as ordinary transparent media. Some crystals transmit only one polarized ray, and tourmaline is called a polarizer; and if light be passed through it to another polarizer, it will be transmitted if the latter be similarly held; but if the second be held at right angles to it the ray will be stopped. We can easily understand this if we suppose a grating through which a strip of tin is passed; but the strip will be stopped by bars at right angles to it. The coloured rings in crystals can be observed when a slice of a double refracting crystal is examined. The rings are seen surrounding a black cross in some instances, and a white cross in another. The effect when examined in the polariscope is very beautiful. Selenite is probably the best crystal for exhibiting colours.

Minerals sometimes reflect, sometimes refract light; they are said to possess lustre and phosphorescence. All these properties may be considered as belonging to the crystals which are transparent, semi-transparent, translucent, or opaque, according to the degrees in which they permit light to pass through them. All minerals are electric or non-electric, and the variety can be ascertained by rubbing and placing the mineral near the electrometer. But all do not exhibit magnetic properties. Taste and smell are strongly marked in some minerals—salts, for instance, and sulphur; some are soapy to the touch, some appear cold to the fingers. Chemistry is very useful to us in determining the nature of the mineral, and the amount of it enclosed in the substance under examination. These delicate operations are termed qualitative and quantitative analysis. The application of heat is increased by means of the blowpipe, which is in effect a small bellows. We can thus, and particularly by means of the oxy-hydrogen blowpipe, obtain a very intense heat with little trouble. When the fragments of a mineral are held in the flame by platinum “tweezers,” or tongs, then the fusibility of the substance, and the colour of the blow-pipe flame will be of great assistance in determining the nature of the mineral. It is also curious to observe the different forms into which the various substances expand or contract under the influence of the blowpipe. We may have a rugged slag, an enamel, or a glass, or a bead, or “drop” of metal. The varied substances produce various colours—yellow, green, orange, or red, according to circumstances. Strontia is a vivid red, copper is green, lime orange, and so on.

It is very little use to attempt a study of mineralogy without some acquaintance with chemistry. In dealing with minerals, and in studying geology, we must try to keep our knowledge of chemical science in our minds, and thus fortified we can more easily understand the steps leading to the classification of minerals. It is impossible to teach mineralogy or geology from books. Nature must be studied, the specimens must be seen, the earth must be examined. The advance in mineralogy may be—probably will be—slow, but crystals will teach something; and when we can pass a viva voce examination in chemistry and crystallography, expressing, by the symbols, the various substances under discussion, we shall have made a considerable advance in the science. We shall have an idea of the component parts of various substances, and be able to class the various minerals according to their chemical constitution. Beginning with the metalloids, we shall pass to the metals and various compounds, salts, resinous substances, etc., such as amber.

It is impossible in the space at our command to describe all the minerals, and yet it is necessary to enumerate the most important. We may, therefore, take them in the following order. It should be added that most of the simple minerals occur in comparatively small quantities, but sometimes we find them in aggregate masses (rocks). We append a table.

SYNOPTICAL TABLE OF THE MINERALS.

First Class.—Metalloids. Sulphur. Boron. Carbon. Silicium (Silicon).

Second Class.—Light Metals. Potassium. Sodium. Ammonium. Calcium. Barium. Strontium. Magnesium. Aluminum.

Heavy Metals. Iron. Manganese. Cobalt. Copper. Bismuth. Lead. Tin. Zinc. Chromium. Antimony. Arsenic. Mercury. Silver. Gold. Platinum.

Third Class. Salts. Earthy resins.

SULPHUR is found in Sicily and Italy and other parts of Europe, in a native state, but as such has to be purified. The crystals take the form as shown in the margin. Cleavage imperfect; it is brittle. Sulphuric acid is a very important combination, and a very dangerous one in inexperienced hands. Sulphur combines with a number of elements, which combinations are “Sulphides.” (See Chemistry section.)

SELENIUM is a metalloid resembling sulphur, but less common. It is inodorous.

BORON is usually found near volcanic springs, and in combination with oxygen. It is soluble. Taste, acid bitter, and white in colour; friable. It is known as SASSOLINE, or boracic acid. (See Biborate of Soda for one of the borates.)

CARBON is one of the most important of our minerals. In the form of coal we have it in daily use, and in the form of diamond it is our most valuable gem. In the latter form it is the hardest of all minerals, a powerful refractor of light, lustrous, and transparent. It is found in the East Indies and Brazil; more lately Cape diamonds have been brought to Europe, but they do not equal the Eastern gem. Almost fabulous prices have been given for diamonds, which, after all, are only carbon in a pure state. Another form of carbon is graphite (plumbago, or blacklead). It is much used for pencils and in households. It is found in Cumberland, and in many other localities in Europe and Canada.

Carbon appears in one or other of the above forms in regular octahedrons or their allied shapes. Anthracite, another form of carbon, is used as fuel for strong furnaces. It leaves little “ash,” and is smokeless when burned. Coal, in all its forms, is evidently derived from wood. Thousands of years ago vegetable matter must have been embedded in the ground and subjected to carbonization. There are different kinds of coal, all of which come under one or other of the following heads: cubical coal, slate coal, cannel coal, glance-lignite,—the last being, as its name implies, an imperfect development of wood; it is a brown coal. We are not here concerned with coal as a fuel. Charcoal is also a form of carbon prepared from wood and finds a counterpart in coke, which is prepared from coal. Carbon, as we have already seen, plays an important part in electric lighting and in the Voltaic Battery. Peat, or as it is called in Ireland, “turf,” is one of the most recent of the carboniferous formations. It is much used as fuel. It is cut from moors (“bogs,” as they are sometimes called), and the various deposits can be traced. Bog-oak is no doubt the first step towards peat, as peat is the step towards coal. The brown turf is newer than the black, and both kinds may be seen stacked in small square “bricks” along the Irish canals and in the yards of retailers of fuel.

SILICON. Silica occurs generally in combination with alumina, and never in a free state. In combination with oxygen it is called silicic acid. Silica, when crystallized, is usually called quartz.

QUARTZ has several varieties. We need only enumerate them, they will all be immediately recognized. We give illustrations of the crystals of quartz (fig. 459):—

1. Rock crystal appears in beautiful six-sided prisms.

2. Amethyst is coloured by protoxide of manganese, supposed by the ancients to be a charm against drunkenness.

3. Common quartz, or quartz rock, forms granite in combination, and is also known as “cat’s-eye,” “rose” quartz, etc.

4. Chalcedony, sometimes termed cornelian: used for seals, etc.

5. Flint: much used in potteries. “Flint and steel” have been superseded by phosphorus.

6. Hornstone: something like flint, resembling horn.

7. Jasper: of various colours; opaque and dull in appearance.

8. Silicious slate: a combination; used as a whetstone.

9. Agate: a mixture of quartz, amethyst, jasper, and cornelian; very ornamental.

10. Opal: a peculiar variety, containing water. It is not found in the form of crystal, but in vitreous masses. Its changeableness of hue is proverbial. The “noble” opal is much prized.

11. Smoky quartz, or cairngorm.

12. Onyx and Sardonyx.

We now arrive at some minerals which contain metals.

POTASSIUM. This metal is so frequently combined in minerals with alumina that we may refer to it with the latter in sequel. There are two natural potassa salts—nitre, and sulphate of potassa. Nitre is known as saltpetre, and is of great use in medicine. It is the chief ingredient in the composition of gunpowder.

SODIUM. We have a number of minerals in this group—viz., nitrate of soda (nitratine), which occurs in large quantities in Peru; rock salt, chloride of sodium, known as salt. It crystallizes in the cubic system. Colour usually white, but it occurs in secondary rocks in company with gypsum, etc. It is sometimes of a reddish colour, or even green and yellow. Biborate of soda is borax, and is found in and on the borders of a Thibetian lake. There are several other combinations with soda: the sulphates of soda—viz., thenardite and glauberite, anhydrous and hydrated respectively, carbonate of soda, and so on.

AMMONIA combinations occur in lava fissures, and are not often met with in consequence of their volatile nature.

CALCIUM. This forms an important group of the minerals, which are very white in colour, and not very hard in substance. Calcium is the metallic basis of lime. Fluoride of calcium, known as fluor-spar, most frequently crystallizes in cubes in the first system. Anhydrite is the anhydrous sulphate of calcium. The hydrated sulphate is called gypsum. One variety of the hydrated sulphate is selenite, another is known as alabaster. Apatite, or asparagus stone, and pharmacolite are in this group.

Carbonate of lime, not content with one system of crystals, makes its appearance in two. It is therefore divided into two minerals—namely, calcareous spar and arragonite. In the former it possesses various forms, as will be observed in the accompanying diagrams. It is a very important mineral, as will be readily acknowledged; it enters largely into the composition of all shells and bones. The minute shells, deposited by millions at the bottom of the sea, have combined to raise our chalk cliffs. Carbonate of lime is a constituent of water, as the deposits at the bottoms of kettles, upon the sides and bottoms of water-bottles, and the stalactites all testify. A little good vinegar will quickly dissolve this deposit. Calc-spar is crystallized, and the Iceland spar is celebrated. Marble, which is another form of carbonate of lime, is white, hard, and granular. It is sometimes varied, but the pure white is the most valuable. Chalk, we know well, is soft, and is useful for writing. We have also aphrite, schiefer spar,—compact limestone in various forms,—and finally, arragonite, called from the place of its nativity, Arragon,—a colourless and somewhat transparent vitreous crystal.

BARYTES. The sulphate of baryta is known as heavy spar; the crystals are of tabular forms, but numerous modifications exist. One of the forms is represented in the margin.

STRONTIUM is the metallic basis of strontia. Sulphate of strontium is celestine, the mineral which colours the blow-pipe flame a fine crimson. There are certain varieties. Strontia salts are chemical preparations. A beautiful pyrotechnic “red fire” is produced by mixing nitrate of strontia with sulphur, antimony, charcoal, and chlorate of potassia.[24] There is a carbonate of strontia in the same crystalline system.

MAGNESIUM. With this metal we have a large group of minerals. Magnesite is carbonate of magnesia, and occurs as talc-spar. The magnesium limestone crystallizes as bitter spar. This dolomite is like marble or common limestone, according to colour. Talc is a combination of magnesia with silicic acid. The hydrated carbonate is termed “white magnesia.” The sulphate of magnesia is found in Siberia, and we have boracite, and native magnesia called periclase. The sulphate is generally present in mineral waters, such as the Seidlitz and Epsom Springs. Large masses have been found in the extensive caverns of Kentucky and Tennessee, etc.

Meerschaum is a hydrated silicate of magnesia. It is found in Anatolia and Negropont, also in France and Australasia. Serpentine is another similar composition. It is found in Cornwall, where it is carved into various ornaments. It is sometimes called snakestone. There are many other hydrated silicates of magnesia—viz., gymnite, picrosmine, pycrophyll, etc.

There is another family allied to magnesia, called AUGITES. These minerals are black or dark-green, and are contained in lava and basalt: AUGITE and HORNBLENDE are the chief representatives of this family. The former crystallizes in the fourth system (see fig. 463), and there are several varieties—diallage, bronzite, diopside, etc. HORNBLENDE belongs to the same system, and is a large factor in the composition of gneiss, syenite, and porphyry. Tremolite is a hornblende, and asbestos (amianthos), and mountain-cork are also varieties. The attribute of asbestos for sustaining heat is well known, and may be usefully employed for fire-proof purposes. The well-known jade-stone of China and calamite are other varieties.

ALUMINUM, or ALUMINIUM, gives us a large class of minerals. It is the metallic basis of alumina, which, combined with silica, is the chief component of our clay. Silicic acid and this base combine to form many minerals, and contains nearly all the precious stones. Corundums consist of pure alumina, and crystallize in the hexagonal system. The following stones are varieties of this mineral:—Sapphire, a beautiful blue; ruby, a red oriental; topaz, yellow oriental; amethyst, violet; all being sapphires more or less. The finest crystals are found in the East Indies in the sands of rivers and diluvial soils. The common corundum is very hard, and is used for polishing. Emery is well known, and is found in mica-slate. It is of a bluish-grey colour, and is also a polisher.

ALUM forms another family, of which we may first mention aluminite, a “basic sulphate” of alumina and found in small quantities. Alum-stone is found in Italy. Alum occurs in large crystallized masses. (See illustration, fig. 464.) There are different minerals with a composition very similar to alum, in which the potassa base of alum is supplied by others. Thus we have the potassa alum, soda alum, manganese alum, ammonia alum—all being very nearly of the same constituents, and having similar crystals in the regular system, and are thus termed isomorphous, or similarly-formed. The potassa, or potash alum, is the commonest form, and is found abundantly in England, on the Continent of Europe, and the United States. Soda alum is called salfatarite, and magnesia alum pickeringite; manganese alum is apjohnite; phosphate of alumina is wavellite.

There are compounds of alumina and magnesia called SPINELS. They are hard minerals, and the same isomorphous changes take place with them as are observable with the bases of alum. There are therefore varieties such as the spinel ruby found in the East Indies, very red in colour; the balas ruby not so red, and the orange-red, termed rubicelle. Ceylon is remarkable for some fine specimens of spinels. Chromite is like the spinel, but is known as chrome iron.

ZEOLITES are principally compositions of silica and alumina. They contain water, and are white, vitreous, and transparent. There are several varieties of them—natrolite, stilbite, etc. We will now pass on to the Clays, which are a very important family of the aluminum group.

There are a number of hard minerals which, when disintegrated, form certain earthy masses. These we term clay, or clays, which possess various colours and receive certain names, according to the proportion of metallic oxides they contain. All clays have an affinity for water, and retain it to a very great extent. The earth has also a peculiar smell. Clay is used in various ways; pottery, for instance, we read in the Bible as having been an employment from very ancient times. One attribute of clays, the retention of water, is of the greatest use to the world in providing moisture for plants and seeds. We may mention other characteristics of clay. It absorbs oil very quickly, and therefore is useful for removing grease-spots. It cannot be burned, so we have fire-bricks and fire-clay in our stoves and furnaces. There are various clays—pipe-clay, for instance, which is white; potters’ clay is coarser. There is porcelain clay as well as porcelain earth, of which more below. Yellow ochre and sienna are clays used by artists. Bole is a reddish clay; and tripoli is employed for polishing. There are, besides, andalusite, or chiastolite and disthene, crystalline forms of clay.

* * * * *

Porcelain has been known to the Chinese for centuries. In 1701 it was discovered in Germany by Böttcher, a chemist, who while endeavouring to make gold by Royal command, found the porcelain, and was thereby enriched. Porcelain earth is frequently found; is known in many places as kaolin, and usually comes from the decomposition of felspar. But in Cornwall we find it as decomposed granite, and the filtering process can be viewed from the railway, while both gneiss and granulite have been known to yield kaolin. It is also found in China, Saxony, and France. It is free from iron, and when ground and mixed with silicic acid, it is handed to the potter or moulder. After the vessels have been dried in the air they are put into the kiln, and then become white and hard. After that they are glazed in a mixture of porcelain earth and gypsum, or ground flints and oxide of lead, made fluid with water in the glazing of earthenware. The vessel is then put into the furnace again, or “fired,” as the process is called, and then comes out white, hard, and partly transparent.

Earthenware utensils are made of a coarser material,—clay and powdered flints,—from which all the gross matter has been eliminated. Flint is not difficult to break, if made hot and thrown into cold water. A stamper is then used to break the flints. They are first ground in a mill and purified like the clay, then they are mixed and beaten, while moist, into “putty,” and turned, or forced, into moulds. The handles are fixed on afterwards. The ware is baked for two days and glazed. The various colours are obtained by mixing different clays and oxides—iron or manganese. Biscuit porcelain is made by pouring a creamy mixture of porcelain earth into plaster-of-Paris moulds, and when a thin case has formed within, the liquid is poured out again. It is then dried in the mould and shrinks. The mould is taken to pieces, and the thin biscuit porcelain is left.

FELSPARS are very like the zeolites, except that the former contain no water. Felspar crystallizes in a number of different forms. We annex illustrations of specimens. This spar is found in rocks, granite, gneiss, etc. One variety is the moonstone, of a peculiar lustre. Felsite is amorphous felspar. Albite contains soda instead of potash. Labradorite is nearly a pure lime felspar, and is remarkable for its colours, like a pigeon’s breast. Spodumene is like albite, and leucite, soda-lite, etc., belong to this family.

LAPIS-LAZULI is a felspar distinguished by its blue tint. It was used for ultramarine colouring at one time, which colour can also be made chemically. Lapis-lazuli is found in Siberia and China. It is a mixture of mineral species. Hauyne is something like it. Obsidian is a sort of black glass, and occurs in various colours in vitreous masses. It is derived from the fusion of rocks, and is employed in the manufacture of boxes, etc. Pumice stone bears a resemblance in composition to the foregoing, but is porous, and so called spongy. It contains both potash and soda in some quantities. Pearlstone and pitchstone also attach themselves to this family group.

The GARNETS possess many curious forms of crystals, which are coloured and used as gems. Tourmaline is a very particularly useful crystal, and is used in the investigations concerning the polarization of light. It is found of nearly all colours. The garnet and staurolite crystals are shown (figs. 470, 471).

The former is silicate of alumina with the silicate of some other oxide, which is not always the same. This change, of course, gives us a series, as in the case of alum above mentioned.

The red varieties, called almandine, or precious garnets, are distinguished from the duller, “common” species by their clear colour. Bohemia is the most productive soil for the garnets.

MICA includes, as we have already noticed, a group of minerals which have a peculiarly laminated structure. These layers are by no means all alike, but they present a smoothness to the fingers which is highly characteristic. The chief constituents are alumina and silica, occasionally with magnesia. Mica slate is very common, and is often used instead of glass in window-frames. Muscovite, lepidolite, and phlogopite are all micas of the “potash,” “lithia,” and “magnesia” varieties.

In the list of minerals associated with the lighter metals, we need only now mention the Gems, so well known. These stones are very hard in many instances, infusible, and exhibiting beautiful colours. Amongst them are diamonds, sapphires, and rubies, of which we have spoken; the topaz, noticed under corundum. The chrysoberyl (of a pale green, or occasionally reddish hue), of which the alexandrite of Siberia is a variety, is a compound of glucina with alumina; the beryl, a silicate of the same, and the emerald of beautiful green. Zircon is another gem, and “hyacinth” is its most valued form. The latter is found in basaltic rocks. The emerald crystallizes in the hexagonal system.

We may now consider the minerals formed by the heavier metals, such as Iron, Copper, Nickel, etc.

IRON. This well-known metal fills a very important place in our mineral arrangements, for the substances formed with iron ores occur in great variety of structure, and occasionally in very large masses. They are highly magnetic, and very hard. Were we here treating of iron as a metal, we could give some information respecting its extraction and manifold uses. All we need mention here is the fact that iron occurs in nature in various ores which are essentially composed of iron and oxygen. The iron is extracted in the blast furnace, in which the process is continued for years. The “slag,” or glassy scum, protects the molten iron from the air; its presence is necessary in all blast furnaces. The most important of the iron group of minerals are MAGNETIC IRON (magnetite), or loadstone. This mineral occurs in Sweden and North America, and is found in primary rocks, and in Scandinavia forms mountains. It crystallizes in the regular (octahedron) system, and often in the form in illustration in the margin. It is highly magnetic, as its name implies.

Native iron very rarely occurs, and then only in thin layers. The most extraordinary specimens are those termed meteoric iron, which fall from the atmosphere in great masses; and the meteoric stones, which contain ninety per cent of iron, together with other constituents in small quantities—viz., nickel, cobalt, copper, manganese, carbon, sulphur, arsenic, etc.

Red hematite crystallizes in the hexagonal system. It possesses much the same (chemical) constitution as corundum (q. v.). It is brightly metallic, and shows a red streak. It occurs in various forms, as iron glance or specular iron, which is found in Sweden and Russia; micaceous iron, bloodstone, clay, ironstone, etc.

Brown hematite has not been found in crystals, but brown ironstone (fibrous) is crystalline. The earthy brown, containing clay, gives us yellow ochre and umber. Pea-iron ore and “morass” or “bog” ore also belong to this class. Limonite is the name given to these more recent formations, of which yellow ochre is a pure specimen.

The combinations of iron with sulphur (pyrites) are also important. Iron pyrites and magnetic iron pyrites are two which may be mentioned. The latter first.

Magnetic iron pyrites (or pyrrhotin) crystallizes in six-sided prisms, and is attracted by the magnet. The composition of this mineral has not been exactly ascertained, and no chemical formula has been found for it.

IRON PYRITES (bisulphide of iron) is known as cubic pyrites, yellow pyrites, and mundic. It is generally found in the regular system of crystals, either as a cube or as a pentagonal dodecahedron. (See first system of crystals, ante.) Its colour is yellowish. It is known also as green vitriol when oxidised, and forms beautiful green crystals (copperas). This salt is used in the preparation of Prussian blue and violet dyes. With gallic acid it makes ink.

There are many other “ferruginous” minerals, such as vivianite, green ironstone, white iron pyrites, arsenical pyrites, or mispickel, etc.

A carbonate of iron, called chalybite, or spathic ironstone, is very abundant in nature, and forms obtuse rhombohedrons. It is very useful for the production of steel, as it forms the clay iron ore found in coal districts in combination. In a fibrous form it is known as sphærosiderite. It is a most useful mineral.

Chrome iron (chromite) is useful for the preparation of chromium compounds. It crystallizes in the cubic system. It is magnetic, especially when treated. Chromic acid forms scarlet “needle” crystals, and by its assistance chromate of lead, or chrome yellow, is prepared. (Chromate of lead is found in a native state as crocoisite). See Chromium.

MANGANESE is contained in several minerals. It usually occurs as an oxide. It colours minerals variously. In a pure state manganese is white and brittle. The chief minerals are—

Pyrolusite (the binoxide of manganese of commerce) occurs in crystals. It is black. It is used in the preparation of chlorine and oxygen. The other minerals are known as manganite, which is also found associated with pyrolusite, as are hausmannite and braunite, the other oxides.

NICKEL and COBALT are generally found together, both being similar, and the minerals are compounds of arsenic or sulphur, and occur under similar circumstances. The principal are of NICKEL and of COBALT—

Sulphide of nickel (ullmanite). Arsenical nickel (nickeline). Nickel glance (gersdorffite). Nickel pyrites (siegenite). Arsenical cobalt (smaltine). Cobalt glance (cobaltine). Cobalt bloom (erythrine). Cobalt pyrites (linnæite).

Nickel ores are used for extraction of the metal, which is used as a substitute for silver. The name is derived from the German, kupfernickel, or false copper. It was discovered in 1751.

COPPER, again, forms a number of minerals, and the chief is the red oxide of the metal, called cuprite. It crystallizes in the cubic system. Its colour is red, and tinges a flame green. Cuprite yields excellent copper, and is found in Cornwall, and in many places on the continent. The black oxide is rarely found. It is known as melaconite.

Malachite (carbonate of copper) is remarkable for its beautiful green colour. In Australia it is worked for copper. It is chiefly ornamental. Siberia yields the finest specimens, but the mineral is found in Cornwall and Cumberland, as well as on the continent. Chessylite (from Chessy, in France) is frequently found with malachite. It has been called blue malachite, or the azure copper ore. It is used as a paint.

Besides the above, copper unites with sulphur to form minerals, such as the needle ore (bismuthic sulphide of copper), antimonial sulphide, bournonite; purple copper, and copper pyrites, which is very abundant, and furnishes us with most of our copper. There is also the “grey” copper ore, which contains various metals; even silver is obtained from it at times.

BISMUTH gives us only a few minerals, of secondary importance. Native bismuth resembles antimony, but is reddish in hue. Bismuth ochre, bismuth blende, and bismuthine are the chief combinations.

LEAD is more important, and is obtained from galena, the sulphide of lead, which is very abundant, and the principal lead ore. It can be at once distinguished by its high specific gravity and metallic lustre; the “cubic cleavage” also is very easy. It frequently is found containing silver, and even gold, antimony, iron, etc. There are several suphantimonites of lead, such as zinkenite, geocronite, etc., and the salts, such as sulphate of lead and white lead ore, or carbonate of lead (cerasite). The chromate of lead is found in the Ural Mountains.

TIN is not found in a native state, but as tinstone, or binoxide of tin, named cassiterite. It is found largely in Cornwall, and the mines there have yielded great quantities for generations. Tin pyrites, a union of sulphides of tin, iron, and copper, is also found in Cornwall.

ZINC is produced from the ore called (zinc) blende, or sulphide of zinc (black Jack). Its colour is very variable, sometimes red, but when pure is greenish-yellow. It is also found black and brown. The red oxide of zinc (or spartalite) is also worked for zinc. The carbonate, or zinc spar, is common, and used to make brass, as is calamine, which is possessed of a remarkable lustre, and is even luminous when rubbed. It is a silicious oxide of zinc, and is found in the sedimentary rocks. When heated, it displays strong electric properties.

CHROMIUM occurs in very few mineral combinations; chromate of lead, chrome iron, and chrome ochre, or sesqui-oxide of chromium are the only important ones.

ANTIMONY minerals are very hard; the tersulphide is the most common, and from this the metallic antimony is produced. Red antimony, the oxide, is a rarer ore.

ARSENIC resembles antimony, and occurs in combination with many metals. White arsenic, or arsenious acid, is found in Bohemia, Alsace, Transylvania, etc. Orpiment and realgar are sulphides of arsenic, and are employed as colouring matters in paint and fireworks. Arsenic is very poisonous.

MERCURY is occasionally found native, but more generally as cinnabar. Chloride of mercury (or calomel) is found associated with the cinnabar, or hepatic ore. Cinnabar is easily volatilized, and possesses high specific gravity. The Californian mines are very rich. Spain also produces a large quantity. It is opaque, and carmine in colour.

SILVER occurs native, or in ores. The latter are as follows:—The sulphide, or the vitreous silver (argentite); antimonial silver; and the combined sulphides, of antimony and silver. There are many silver minerals, such as the chloride (horn silver, or kerargyrite), bromide, and carbonate of silver, bismuthic silver, etc. The bromide and iodide are bromargyrite and iodargyrite. Silver occurs most frequently associated with gold; natural alloys of these two metals are found, containing from 0·16 to 38·7 per cent. of silver, which causes considerable variations both of colour and density. In addition to this alloy, we may mention sylvanite (graphic tellurium), which contains, besides gold and silver, one of the rarer metals—viz., tellurium.

GOLD is our most precious mineral, and is generally found native. It exists in sand and in certain rocks. It crystallizes in various forms, and in Mexico it is found in companionship with silver and copper sulphides. Australia and California render the most valuable supplies of the metal.

PLATINUM is also found native, and rarely is crystals. It is often alloyed with other metals, chiefly with iron or gold; also with diamonds. We have already considered it as a metal. Little remains to be said about salts and resins, for with the exception of those we have referred to under Chemistry, they are of little value. The bitumens, rock oil, etc., which exude from the earth, are very useful, and as asphalt and petroleum play an important part in the civilized world, but scarcely come under the strict rule of minerals as we consider them, and with this reference we close our sketch of Mineralogy.

FOOTNOTES:

[23] A crystal should be held so that one of its axes is vertical to the spectator. This axis is termed the principal axis, and when there is inequality the longest axis is the principal.

[24] See “Strontium” in Chemistry.

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XXXII. NEW LOCOMOTIVE APPLIANCES

THE KITE—THE AEROPHANE—ICE YACHTS—SAILING TRUCKS—WATER VELOCIPEDES.

The kite, known from the earliest times, and constructed by a number of people, is a very familiar object, which we shall not describe; for we will now speak of some similar appliances of a more interesting and uncommon description.

M. Penaud has invented some appliances in which twisted india-rubber is the principal agent. Fig. 475 represents a sort of kite, which rises in the air if one twists and then looses the india-rubber round the central bow. Fig. 476 represents another kind of invention; it is an “aerophane,” with a screw at the back, so fixed that it receives no shock from striking against any obstacle. After having twisted the india-rubber, and loosened our hold of the apparatus in a horizontal position, it will first descend for an instant, then, acquiring increased speed, it rises seven or eight feet from the ground, and describes a regular movement in the air for a distance of about fifty yards; the motion lasts for several seconds.

Some models have also been constructed capable of traversing a distance of over seventy yards, remaining for thirteen seconds in the air, as lightly poised as a bird, and without any connection with the ground. During the whole time the rudder restrains with perfect exactitude the ascending and descending movements as they occur; and we can plainly observe the various oscillations like those of sparrows, or more especially woodpeckers. At last, when the movements are coming to an end, the apparatus falls gently to the ground in a slanting line.

M. Penaud has also succeeded in constructing a mechanical bird, that we have seen set in motion, which will continue flying for several seconds; we give an illustration of it in fig. 477.

Another scientist, M. Tatin, has also produced some remarkable results. His efforts have been unceasingly directed towards the reproduction of the flight of a bird by means of more or less complicated arrangements. He has endeavoured to discover in the small appliances made with indiarubber, and used by MM. Penaud and Hureau de Villeneuve, what were the best shapes in which to reproduce the wings, in order to adapt them to a large apparatus acting by compressed air. After several attempts, he decided on the employment of long, narrow wings. Wenham had previously proved that a wing may be equally effectual whether it be narrow or wide, and M. Marcy has also declared that birds with a quick, narrow wing-stroke have always very long wings. By means of these long, narrow wings (fig. 478) M. Tatin has reduced the time during which the wing reaches a suitable position for acting on the air when it first descends. Granted the fact, so long established, that a bird flies more easily if it rests its wing against a great volume of air, it will be understood that the maximum speed of movement will also be the most advantageous as regards the reduction of expended force. The inventor, finding that he could not prevent his mechanical birds from losing force in proportion as they attained considerable speed, remedied this defect by placing the centre of gravity in front. In consequence of this, the bird in full flight preserves the same equilibrium as the bird hovering in the air, and its speed is, to a certain extent, passive, the mass of air pressing of its own accord against the wings, all expenditure of force therefore being utilized for suspension. Thus has M. Tatin been enabled to increase the weight of his appliances, without increasing the motive power, and yet obtains a double course.

The movement made by the wing round a longitudinal axis, by means of which it always exposes its lower surface in front on rising, is obtained by the mechanism illustrated in fig. 478 a.

M. TATIN’S BIRD.

This apparatus, looked at sideways or from behind, is composed of a light wooden frame, on which are two small supports crossed by an axletree so as to form two cranks. This axle receives a circular movement from an india-rubber spring. The crank on the foremost plane causes the rising and falling of the wings, which move round a common axis, and pass the dead points as the cranks of a locomotive do—so the action is maintained.

But the wing does not only move as a whole; every part of it, particularly as it rises, shows a tendency to inclination, which is most marked towards the extremity; the part near the body alone preserves an invariable obliquity. M. Tatin was of opinion that it is with the screw that it is necessary to direct the twisting movement; and to obtain it with all its transitions, he has substituted for silk wings, which fold up, some wings composed entirely of strong feathers, arranged in such a manner that they slipped one over the other when in motion. The arrangement was perfect, but still not suitable for adaptation to the large bird. The inventor therefore again returned to the use of the silk wings, which he appears to have definitely adopted. By means of certain modifications which he has recently introduced in his larger apparatus—viz., a change in the shape of the wings, variation of the amplitude in the flapping, etc., M. Tatin has been enabled to make great progress. The bird, acting by means of compressed air, at first could only raise three-quarters of its own weight, but finally lifted itself entirely. And we must take into consideration that the apparatus has to struggle against the weight of the steering apparatus, which nullifying the vertical and horizontal reactions of the bird during flight, constantly fulfils the office of regulator.

We will now pass to the consideration of two ingenious appliances of a very clever inventor, M. Salleron.

SMALL ATMOSPHERIC BOAT.

The little boat shown in fig. 480, which is about the size of an ordinary plaything, is a very ingenious, if not a practical, application of the specific lightness of air acting as a propelling force. In this instance steam plays but a secondary part, which consists in carrying off the air that causes the moving of the boat.

The apparatus, as represented in fig. 481, is of extreme simplicity, as will be seen at a glance. A small cylindrical boiler, B, connected with a capillary tube, is placed on two supports over a spirit-lamp, in such a manner that the opening from which the steam issues is directly opposite the mouth of the tube, T. This tube, after forming a sudden inclination, terminates at the back of the boat in an inclined drain, R. The steam driven through the tube, T, carries along with it a certain quantity of air, which, forced under the water, propels the boat along. The little vessel soon reaches considerable speed, leaving a long track behind it. It will be seen that this is not a mechanical apparatus, capable of absorbing force or diminishing the action of steam by causing its condensation.

Let us now calculate the force engendered by this apparatus. We know that a litre of water at boiling point gives 1,700 times its volume. The steam, as it quickly issues from the opening of the boiler, carries along at least ten times its volume, or 17,000 litres of air, which, driven under the water, assumes an ascending force equal to the difference of the densities of water and air, or about the weight of the displaced water. Therefore in a litre of water transformed into 1,700 litres of the steam, which carries off into the water 1,700 × 10 = 17,000 litres of air, a force is developed represented by 34,000 kilograms. In fact, by reason of the inclined position of the drain on which the pressure of air acts, and its restricted dimensions, the quantity of force employed in the propulsion of the boat is but a fraction of the total force produced. Moreover, the resistance of traction increases with the size of the boat, and as the dimensions of the inclined pipe cannot be indefinitely enlarged, the result is that the propulsive action is soon insufficient, so that the invention is not, in its present condition, applicable to navigation on a large scale. Its superiority to the steam-engine cannot, therefore, be demonstrated; and we are only now discussing the contrivance in order to show that it is possible, with only moderately powerful generators and extremely simple mechanical appliances, to obtain considerable dynamic effects, susceptible of more serviceable application than is commonly believed.

CIRCULATING FOUNTAIN.

The apparatus given in fig. 482 is the subject of a very charming experiment, showing the influence of capillarity on the movements of liquids. Two glass balls, B B´, are connected by two tubes; one straight and of rather large diameter, the other extremely slender, and winding in and out in a more or less complicated manner. The large tube passes into ball B´, and forms a slender point, J, at the orifice of the narrow tube. At the lower end of the ball is a bulb, which is closed with a cork, and contains a coloured liquid. The apparatus is fixed to a board with a ring at each end, by which it can be hung on the wall. When commencing the experiment, it should be hung so that the ball B´ is uppermost. The liquid then flows through into the ball B, without presenting any particular phenomenon. The apparatus is then turned, and the liquid descends again with great speed, shoots through the opening, J, and rises into the twisted tube. The air displaced from ball B´ also rises, however, and mingles with the liquid, and it can be seen circulating through the winding tube in a number of air-bubbles, mingled with drops of liquid, gradually transmitting the pressure of the column contained in the upper ball and straight tube; so that by means of a similar phenomenon to that of the fountain of Nero, the liquid rises higher than the level of the reservoir, a part falling into ball B, which causes the experiment to be a little prolonged. This circulation of air-bubbles and coloured drops through the twisted tube of the apparatus has a very pretty effect.

THE PNEUMATIC PENCIL.

This ingenious invention is productive of results similar to Edison’s electric pen. It is the invention of an American gentleman, Mr. J. W. Brickenridge, of Lafayette, Indiana. The illustration (fig. 483) explains the mechanism of the pneumatic pencil. The whole apparatus is figured on the left side of the picture, while the longitudinal section of the pencil is shown on the right, the small cut at the top being a vertical section of a portion of the motive power. Compressed air furnishes the power of pressure, which is accomplished by means of a perforating needle.

If the treadle is put in motion, a backward and forward movement is imparted to a flexible diaphragm, as in the upper section in the centre of the illustration. By this movement the air is permitted to enter, and is compressed by the diaphragm into the flexible tube with which the diaphragm is connected. The air is thus brought into contact with another diaphragm at the end of the tube and presses on it. The pencil is fixed to the latter. When it is desired to use the pencil the apparatus is set in motion, and by a series of sharp, quick perforations, any writing can be traced, as by the electric pen. This indentation can be copied over and over again in a press, the writing acting as the negative; and if ink be first run over it, as in a stencil plate, by a proper “roller,” the latter will come out as plainly as possible.

TUBE WELLS.

The principle upon which the tube well depends is very simple. It is well known that in certain localities water lies a short distance beneath the surface of the ground, and a very little trouble would satisfy us upon the point, and render us quite independent of the water companies’ supply. On the supposition that the water exists underneath our garden at, say, twenty-five feet beneath the surface of the ground, we have only to drive into the soil a tube for that distance, and by the assistance of a common pump we shall obtain a pure supply of water.

We will now proceed to describe the manner in which these wells are sunk. The first step is to fix a platform firmly upon the ground and bore a hole, by which the tube is to enter the ground. This tube should be very thick, with an aperture of two inches or rather less, and three or four yards in length. The lower portion should be pierced with holes, as in the illustration, and terminating in a point of extremely fine-tempered steel. This tube can be driven into the ground by mallets, or by the suspended hammer, worked as shown in the illustration (fig. 484). This work will be easily accomplished, and when the first length of tube has been driven in, another can be fixed to it and hammered down in the same way.

When the tubes have been driven to the depth indicated it will be as well to let down a sounding line, a simple cord sustaining a pebble. If the stone be pulled up dry, another length of tube can be added, or the tubes can be pulled up, and another trial made. If, on the contrary, the pebble come up wet, the object is accomplished, and a small pump can be fixed to the upper end of the tube, as in fig. 485. At first the water will be found a little thick and muddy, in consequence of the disturbance of the soil and the particles adhering to the end of the first tube; but after an hour or so it will be found that the water has become quite clear. It need scarcely be said that if the water possesses sufficient ascensional force to rise to the level of the ground a pump need not be employed. An Artesian Well will, in that case, be the result.

The operation described on page 456 can usually be performed without any difficulty. Sometimes, however, the tube may come in contact with a large stone, and in that case the experiment must be tried elsewhere; but, as a rule, the pointed tube, in consequence of its small size and penetrative power, pushes any moderately-sized obstacle aside, readily turns aside itself, or passes between pieces of stone to the desired depth. Nine times out of ten the operation will be successful, and the experiment will not occupy more than an hour, under ordinary circumstances, and the tubing (and pump) may be obtained at a moderate price, which can even be diminished by arrangement. Ordinary wells are relatively very difficult to sink, and the soil thrown out from the pit is in the way, while a parapet is necessary to protect the opening. Besides, should water not be found after much work, the expense and trouble of digging will be uselessly incurred. Thanks to the tube system, we can search or probe for water anywhere with ease, and if we do not find it in one spot we can easily move on to another without incurring any serious trouble or expense.

We believe the idea of these “instantaneous wells” originated in the United States during the War of Secession, when some soldiers of the Northern army sunk rifle barrels into the ground, and obtained water in a barren land. To Mr. Norton the development of the idea is due, and in the Abyssinian Expedition the utility of the notion was fully demonstrated. Since that time M. Donnet of Lyons has modified and improved the tube-well, and arranged all the materials, including wider tubing and the hammers upon a carriage, thus giving greater facilities to the workmen and to those desirous of sinking such wells.

The general arrangement of M. Donnet, and the carriage with its equipments utilized, is depicted in fig. 484; the actual sinking of the well is carried out just as originally performed by Mr. Norton.

A NEW SWIMMING APPARATUS.

We have to mention a novel means of swimming, which may prove useful to those who distrust the natural buoyancy of water and their own powers of keeping afloat or swimming. The simple apparatus, shown in fig. 486, is the invention of an American named Richardson, a citizen of Mobile, U.S.

The machine consists, essentially, of a shaft, upon which a float is fixed, and at the end of the shaft is a small screw propeller. The shaft is put in motion by a wheel arrangement worked by the hands, and by a crank moved by the feet. The swimmer rests upon the float, with his head well above water. The float sustains him, while the propeller forces him through the water, without his feeling fatigued, at the rate of about five miles an hour. A certain amount of practice is necessary to obtain complete command of the machine, but when mastered the swimmer can proceed, without much exertion, at a rapid rate. The apparatus itself is not difficult to make, and persons who have tried it speak highly of its convenience and of the facilities it may afford. Captain Boyton’s swimming-dress is another useful invention, but the means of mechanical propulsion are wanting, while in this new apparatus the swimmer can drive himself through the sea with ease and expedition, and even a non-swimmer may thereby save life without danger to himself, or the person he wishes to rescue.

The NAUTICAL VELOCIPEDE, which also deserves some notice at our hands, is the invention of M. Croce-Spinelli, who tried it upon the great lake of Vincennes and also on the Seine, when it was the object of much curiosity; but when the Franco-German war broke out the experiments were discontinued, and the inventor did not live to perfect the apparatus. He fell a victim to his love for ballooning. But M. Joberts, a practical machinist, has lately taken up the idea broached by Croce-Spinelli, and has brought out a new water velocipede of very ingenious construction, with satisfactory results. The machine is described as follows. There are two hollow tin “floats” of cylindrical form, and tapered at the ends. These floats are joined together by a platform made of very light wood, on which the seat of the worker is raised, and underneath is the machinery for propelling the velocipede. The motive power is very simple, and corresponds to that employed to propel the bicycle on land, by the feet of the rider, the wheel being furnished with paddles in the water velocipede.

A rudder, which can easily be worked by cords, gives the velocipedist complete control of the machine, the steering being performed by a handle similar to that which the bicyclist uses to turn the machine he rides. In fact, the “water” velocipede is an adaptation of the “terrestrial” machine so familiar to all readers. This velocipede is equally adapted for sea or lake progression, the waves of the former being, under ordinary circumstances, no obstruction, for very little motion is imparted to the sitter. For those desirous to bathe in deep water the machine offers many facilities; and in the case of attack of cramp or faintness, rescue would not be difficult, as the swimmer could support himself upon the pointed cylinders of the water velocipede till assistance arrived. On the other hand, it is very necessary to know how to swim before attempting to work the machine.

Before describing the ice-yachts which are used in Canada when winter’s cold grasp lies on water and land, we will mention a very curious experiment in water locomotion made a year or two ago. The illustration explains itself. It is not an imaginary sketch, it is the record of fact.

This sagacious seal was exhibited in London, and was in the habit of performing certain tricks, one item of his performance being to draw the light canoe (as represented), and another accomplishment consisted in “striking the light guitar,” to the astonishment of the spectators, amongst whom was the writer. The instrument was placed between his fins, or “flappers,” and the seal twanged it more or less melodiously. He was very tame, and obedient to his master and trainer.

We all have heard of, even if we have not seen, the twin steamer Castalia, which, pending the opening of the tunnel beneath the Channel, was supposed to reduce sea-sickness to a minimum. The Castalia did not answer, however, but an American has planned certain double yachts, of which we give an illustration. The sailing-boats, as represented, have had much success upon the lake of Cayuga, and are quite seaworthy,—in fact, it is impossible to overturn them.

The weight of one of these yachts is about fifteen hundred pounds, and the draught six inches. Having two keels they answer the helm very readily. The boat, in the centre of the illustration, belongs to Mr. Prentiss, and is called the Pera Ladronia. It is a very fast “ship.”

From navigation in water, we now come to navigation on water. The ice-boats are much used in Canada, and their simple but effective construction will be readily perceived from the accompanying illustration. The Americans state that these ice-yachts can run before a good breeze as fast as an ordinary train. There are, or were, models of some such (Finland) yachts in the South Kensington Museum with two sails. The American yacht, as a rule, has only one sail, and the owners say—but we will not vouch for the truth of the allegation—that they frequently run far ahead of the wind that primarily propelled them!

SAILING ON LAND.

It is quite possible to sail upon land, although this statement may appear contradictory in terms. “The force of the wind upon sails,” says Bishop Wilkins in his work, “Mathematical Magic,” printed in London in 1648, “can be applied to vehicles on land as well as to ships at sea. Such conveyances,” he adds, “have long been in use in China and in Spain, as well as in flat countries, such as Holland, where they have been employed with great success. In the last-named country they are propelled with greater speed than are ships before a fair wind; so that in a few hours a boat containing several persons actually travelled nearly two hundred miles, with no trouble to any one on board except the steersman, who had little difficulty in guiding the boat.”

The astonishment expressed by the good bishop was quite justified, for, as a matter of fact, a carriage or boat on wheels, with sails, as shown in the illustration, achieved a distance of nearly thirty-eight miles in an hour. This pace was quite unknown at that time; such a rate of travelling had never entered the minds of people then. “Men running in front of the machine after a while appeared to be going backwards, so quickly were they overtaken and passed.” “Objects at a distance were approached in the twinkling of an eye, and were left far in the rear.” So it is evident that, had locomotion by steam not been adopted, the mode of sailing on land would have eventually become the most rapid mode of transit, and it is rather remarkable that it was never adopted as a mode of travel.

But Bishop Wilkins had not to reproach himself on this account, for he adapted the principle of the windmill to carriages, “so that the sails would turn and move his car, no matter in what direction the wind was blowing.” He proposed to make these sails act upon the wheels of a carriage, and trusted to “make it move in any direction, either with the wind or against it!” This suggestion has been lately adopted in the United States, and it is curious that after two hundred and fifty years no better mode for utilizing wind-power on land has ever been found. Perhaps the ice-boats already mentioned may be the forerunners of some new system of “land transport,” for which enormous kites have been made available.

It is somewhat remarkable that if the introduction of railroads quite “took the wind out of the sails” of any other mode of locomotion on terra firma, it is that very iron track which has led to the reintroduction of sails as a mode of progression upon the rails. In the United States at the present time there are many vehicles propelled by sails across the immense prairies at a pace, with a strong wind, which equals that of the trains. We are indebted to Mr. Wood, of Hayes City, Kansas, for the photograph from which the picture of the sailing-waggon, invented by Mr. Bascom, of the Kansas Pacific Railway, is copied. This carriage travels usually at thirty miles an hour, and a speed of forty miles an hour has been obtained when the wind has been high and blowing directly “aft.” The distance of eighty-four miles has been accomplished in four hours when the wind was “on the beam,” or a little forward of it, and on some curves with an almost contrary breeze.

The newest machine has four wheels, each thirty inches in diameter; it is six feet in length, and weighs six hundred pounds. The sails are carried upon two masts, and they contain about eighty-one square feet of canvas. The main, or principal mast, is eleven feet high, four inches in diameter at the base, and two inches at the top. As in the case of the ice-boats, it is claimed for the sailing carriage that it frequently outstrips the wind that propels it along the track. On the other hand, there is a difference between the best sailing points of the two kinds of vehicle. The ice-boat goes quickest with the wind “dead aft,” the carriage makes best time with the wind “on the beam”—i.e., sideways. The greater friction and larger surface exposed to the influence of a side-wind no doubt will account for the difference between the speed of the railway sailing-carriage and the ice-boat.

Mr. Bascom informs us that the carriage we have described is in frequent use upon the Kansas Pacific Railway, where it is employed to transport materials for the necessary repairs of the line, telegraph, etc., etc. It is a very cheap contrivance, and a great economizer of labour. We all have noticed the cumbrous method of “trolly-kicking” by “navvies” along the line. A trolly fitted with a sail would, in many cases, and on many English lines, save a great deal of trouble, time, and exertion to the plate-layers.

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XXXIII. ASTRONOMY

INTRODUCTORY—HISTORY OF ASTRONOMY—NOMENCLATURE.

Astronomy is the science which treats of the heavenly bodies and the laws which govern them. The term is derived from two Greek words, astron, a star, and nomos, a law. It may be included in the study of Physics, for the motion of the planetary bodies and equilibrium, gravity, etc., all have something to say to the arrangements and positions of the stars. The space in which they are set is infinite, and known as the “Firmament,” or “Heaven.” The number of the heavenly bodies must therefore be infinite also. We can see a few stars, comparatively speaking, and there must be numbers whose light has never yet reached the earth. When we calmly reason upon the immeasurable distances and the awful rapidity of motion, with the masses of matter thus in movement, we are constrained to acknowledge that all our boasted knowledge is as nothing in the wondrous dispensations of Him “who telleth the number of the stars, and calleth them all by their names.”

Astronomy, no more than any other of the physical sciences, cannot stand by itself. We have seen how heat, light, electricity, etc., are all, in a manner, inter-dependent. So astronomy is dependent upon mathematics, particularly geometry and trigonometry, for the wondrous problems to be solved. But in the following sketch we do not propose to plunge the reader in the slough of calculations. We only desire to put plainly before him the great phenomena of nature with regard to the heavens, and the glorious orbs which so thickly stud the space above us. We need not detail the laborious calculations by which philosophers have arrived at certain discoveries. We may refer to the results and explain general principles, thereby indicating the road by which the student may arrive at the more difficult bypaths in the fields of scientific discovery.

The history of astronomy is nearly as old as the world itself, or rather as old as the human race. From the earliest ages we can picture men gazing upon the “spangled heavens,” and the wandering tribes of the desert were always very careful observers of the paths of the stars. To the nomads of the East the planetary system served as compass and clock, calendar and barometer.

We shall find, therefore, that many observations of the heavenly bodies were made by the ancients, and have descended to more advanced generations, and this leads us to remark that the science of astronomy can be studied without any very special or costly apparatus. In other branches of science numerous instruments are indispensable before we can reveal to ourselves the desired results. In astronomy, a telescope—even a good field glass, such as possessed by any household, will reveal many interesting facts. We will, by means of more expensive instruments, and by the aid of large telescopes particularly, enjoy the sight of the moon and planets. But even with the naked eye a great variety of phenomena may be observed. With a celestial globe in our hands upon a fine starry night, we can easily find out the position of the constellations, and trace their forms in the firmament.

It is to the Chaldeans, Indians, Chinese, and Egyptians, that our knowledge of astronomy is primarily due. They did much to facilitate the observation of the stars; they named the planets, grouped the stars, and marked the sun’s track in the sky. Astrology was cultivated in very remote ages. The Jews practised it; and the astrologers of subsequent periods played very important parts in divining the future of individuals, and casting their horoscopes. Many of these so-called predictions came true, “because,” as was remarked by Pascal, “as misfortunes are common they” (the astrologers) “are often right,” as they foretold misfortune oftener than good fortune. Still the fact remains that occasionally a very startling prediction was made, and proved true; such, for instance, as the laying waste of Germany by Gustavus Adolphus, which was foretold by Tycho Brahé after his consideration of a certain comet, and the date of the king’s death was also correctly prophesied. Astrology, therefore, held a very considerable influence over the human race during the Middle Ages.

We can only give a very brief historical summary of the science. We know that the destinies of individuals and nations were at a very early period attributed to the influence of the stars. We read that “the stars in their courses fought against Sisera,” and many expressions surviving to the present time serve to remind us that the stars were at one time paramount in men’s minds. Thus we have the phrases—“unlucky star,” “born under a lucky star,” “mark my stars,” “moonstruck,” etc. Even the common term “consider”—to take counsel of the stars—is thus accounted for, and many men have a habit of looking up to the ceiling of a room or to the sky when thinking deeply—considering with the stars. “Contemplate” is another term signifying the same thing; for templum, a temple, was formerly a space marked upon the sky in imaginary lines, and traced on the ground in accordance with the supposed diagram. Thus temple became a place for heavenly “contemplation,” and by an easy transition to a place of worship. In our old poets’ writings we have many allusions to the influences of the stars.

“Now glowed the firmament With living sapphires; Hesperus, that led The starry host, rode brightest, till the moon, Riding in clouded majesty, at length Apparent queen, unveiled her peerless light, And o’er the dark her silver mantle threw.”—MILTON.

Although from Thales, who lived B.C. 610, the real science of astronomy may be allowed to date, there can be no doubt that the ancients were acquainted with many phenomena. The Chaldeans were, doubtless, the first to place on record the rising and setting of the celestial bodies and eclipses, and used the water-clock (clepsydra). A list of eclipses from 2234 B.C. is stated to have been found at Babylon by Alexander the Great. The Chaldeans also divided the ecliptic into twelve equal parts, and the day and night into twenty-four hours. The Chinese, again, have recorded astronomical phenomena as far back as 2857 B.C.; and the Egyptians also were well versed in the science, although no records of much importance remain to us, unless the zodiac signs were their invention.

Thales predicted the eclipse of the sun B.C. 610. Aristarchus and Eratosthenes also made important observations. Hipparchus (160-125 B.C.) discovered the precession of the equinoxes, calculated eclipses, determined the length of the year, etc., etc.

Ptolemy, of Alexandria, A.D. 130-150, was the founder of a theory called the Ptolemaic System, which recognized the earth as the centre of all—the sun, moon, stars, etc., all revolving in very complicated courses around it, as figured in the diagram herewith. Even though his theory turned out to be untenable, he paved the way for his successors in other ways, and left a valuable collection of observations on record. In this volume, called the “Almagest,” he reviewed the state of the science, and gave a catalogue of stars, as well as a description of the heavens. He discovered the lunar evection.

After his time astronomy, though it was not neglected, appeared to droop, and it is at a comparatively late period that we again open the records—viz., in 1543, the year in which Copernicus died. This philosopher, who was born in 1473, promulgated the true theory of the solar system. He placed the sun in the centre of the planets, and by this he explained their motion around the sun, though they appeared to be carried round the earth. The book in which he explained his theory, “De Revolutionibus Orbium Celestium,” was not finished till a day or two before he died.

The justly celebrated Tycho Brahé was the most important of the successors of Copernicus, but he opposed the Copernican theory, while other able philosophers agreed with it. Brahé was a Dane; he died in 1601. He adopted the theory that the sun and moon revolved around the earth, while the (other) planets moved around the sun. This theory did not gain much credence, but he, again, though he could not defeat Copernicus, and though he was wrong in his assumption, made many important investigations. After him came Kepler, whose observations upon the planet Mars cleared away many complications, and he laid down three laws, which are as follows:—

1. Every planet describes an elliptic orbit about the sun, which occupies one focus of each such ellipse.

2. If a line be drawn from the sun, continually, to any planet, this line will sweep over equal areas in equal times.

3. The squares of the periodic times of the planets are proportional to the cubes of their mean distances from the sun.

Kepler also remarked that gravity was a power existing between all bodies, and reasoned upon the tides being caused by the attraction of the moon for the waters.

It was about this time—viz., the beginning of the seventeenth century—that the telescope was invented, and logarithms came into use. The actual discoverer of this now almost perfected instrument is uncertain. Borelli, who wrote in the seventeenth century, ascribes the discovery to Zachariah Jansen and Hans Lippersheim, spectacle makers of Middleburg. Baptista Porta, also a spectacle maker, has had the credit of discovering the magnifying power of the lens, and, so far, the originator of the telescope.

But whoever invented it, the telescope did not penetrate into southern Europe till 1608-9. Galileo then made inquiries concerning the new instrument, and Kepler made some propositions for their construction. But Harriot had used the instrument so far back as 1611 or 1612, and had observed spots upon the sun’s disc. Galileo, in 1610, had also made observations with the telescope, and discovered the satellites of Jupiter. He thereby confirmed the Copernican theory;[25] and when Newton promulgated his immortal discovery of gravitation, after Picard’s researches, the relations of the sun and planets became more evident. His researches were published in the Principia, and then one-half the scientific world began to question the principle of gravitation, which was supported by Newton and his adherents. Subsequently the researches of Lagrange and Laplace, Adams and Leverrier, Sir J. Herschel, etc., brought astronomy into prominence more and more; and the innumerable stars have been indicated as new planets have been discovered. The spectroscope, which gives us the analyses of the sun and other heavenly bodies, has, in the able hands of living astronomers, revealed to us elements existing in the vapours and composition of the sun, etc. Stars are now known to be suns, some bearing a great resemblance to our sun, others differing materially. The nebulæ have been analysed, and found to be stars, or gas, burning in space—hydrogen and nitrogen being the chief constituents of this glowing matter. Instruments for astronomical observation have now been brought to a pitch of perfection scarcely ever dreamed of, and month by month discoveries are made and recorded, while calculations as to certain combinations can be made with almost miraculous accuracy. The transit of Venus, the approaches of comets, eclipses, and the movements of stars, are now known accurately, and commented upon long before the event can take place.

We will close this chapter by giving a brief explanation of the various definitions most usually employed in astronomy.

1. The Axis of the earth is an imaginary line passing through the centre (north and south); the poles are the extremities of this line.

2. The Equator is an imaginary circle passing round the globe, dividing it into northern and southern hemispheres. The equinoctial is the plane of the former circle extended to the heavens, and when the sun appears in that line the days and nights are of equal duration—twelve hours each.

3. The Ecliptic is the sun’s path through the heavens—though, of course, the sun does not actually move, and therefore the track, or supposed circle, is really the earth’s motion observable from the sun. When the moon is near this circle eclipses happen. The ecliptic cuts the equinoctial at an angle of about 23°. One half is to the north and the other to the south of the equinoctial.

4. The Zodiac is a girdle extending 8° on each side of the ecliptic, in which space of 16° the planets move. The zodiac is divided into twelve parts of 30° each, called the “Signs.” These names are as under written:—

NORTHERN SIGNS.

Spring.

Aries, the Ram, March. Taurus, the Bull, April. Gemini, the Twins, May.

Summer.

Cancer, the Crab, June. Leo, the Lion, July. Virgo, the Virgin, August.

SOUTHERN SIGNS.

Autumn.

Libra, the Balance, September. Scorpio, the Scorpion, October. Sagittarius, the Archer, November.

Winter.

Capricornus, the Goat, December. Aquarius, the Waterbearer, January. Pisces, the Fishes, February.

5. Colures are two circles dividing the ecliptic into four equal parts, and making the seasons.

6. The Horizon is the boundary line of our vision, and is called the sensible (apparent) horizon. The true horizon is the circle—as on a globe—dividing the heavens into two hemispheres. The sensible horizon is enlarged according as the eye is elevated above the ground. A man six feet high can see a distance of three miles when standing on a plain. We can always find the distance visible when we know the height at which we stand, or, inversely, we can tell the height of an object if we know the distance. We have only to increase the height one half in feet, and extract the square root for the distance in miles. On giving the distance in miles reverse the operation.

For instance, for the man six feet high, as supposed, add three feet, being half his height; that makes nine feet. The square root (or number multiplied by itself to give nine) is three, which is the number of miles the man can see on a plain. Or, again, suppose we can see a tower on the level, and we know we are twelve miles away from it. The square of twelve is one hundred and forty-four feet, one-third of that is forty-eight feet, which represents the half of the original height added to the whole tower in feet; so the whole tower is ninety-six feet high. Reversing, as in the former case, we can prove this by taking the tower at ninety-six feet high and trying to find the distance we can see from its summit = 96 + 48 = 144; the square root of 144 = 12, the distance required.

7. The Nadir and the Zenith are the poles of the horizon. The zenith is exactly overhead, the nadir exactly under foot. Circles drawn through these points are azimuth circles.

8. Meridians are circles passing through the poles at right angles to the equinoctial. Every place is supposed to have a meridian, but only twenty-four are upon the globe, and they represent the sun’s, or the planets’, “movements” every hour—15° being one hour, 360° being twenty-four hours (see fig. 500). One quarter of a degree equals one minute of time. Parallels of latitude are familiar circles parallel to the equator. Latitude in astronomy is the distance from the ecliptic at a right angle north or south. This will be explained as we proceed.

9. Declination is the distance of the heavenly bodies from the equinoctial measured as a meridian.

The Tropics indicate the limits of the sun’s declination.

10. Disc is the term applied to the apparently flat surface of a planet, such as the moon, for instance.

11. The Orbit is the path described by a planet revolving round the sun. The plane of the orbit is an imaginary surface cutting through the centre of the sun and the planet, and extending to the stars. The diagram shows the plane of the earth’s orbit. The circle, A B C D (fig. 501), is the ecliptic. The inclination of an orbit is the plane of the orbit with reference to the plane of the earth; and, supposing the shaded part of the illustration to be water, a hoop held inclined towards the earth, with one half in and the other half out of the water, will describe the planetary orbit.

12. Nodes are the opposite points of a planet where its orbit cuts the ecliptic or the earth’s orbit.

13. Apogee is the point of a planet’s orbit farthest from the earth. Perigee is the nearest point.

14. The terms Culmination, Conjunction, and Opposition require no special explanation. But planets are in conjunction with each other when in the same sign and degree. A planet with the sun between it and the earth is in conjunction with the sun. With the earth between it and the sun it is in opposition.

15. Latitude and longitude upon a celestial globe are known respectively as “Declination” and “Right Ascension.”

16. The Radius Vector is a line drawn from a planet to the sun, wherever the planet may be (see fig. 497).

FOOTNOTES:

[25] He was obliged to recant before the Inquisition, and to repudiate his researches. He was released on the condition of observing silence upon the theory he had supported, but again obliged to recant.

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XXXIV. ANGLES AND MEASUREMENT OF ANGLES

THE QUADRANT—TRANSIT INSTRUMENT—CLOCKS—STELLAR TIME—SOLAR TIME—“MEAN” TIME.

We must say a few words respecting the various instruments and aids to astronomical observation before proceeding, for astronomy requires very accurate calculations; and though we do not propose to be very scientific in our descriptions, some little idea of the manner in which observations may be made is necessary. The first thing to see about is the ANGLE.

Suppose we draw four lines on a piece of paper, ab and cd. These intersect at a point, m. We have then four spaces marked out, and called angles. The four angles are in the diagram all the same size, and are termed right angles, and the lines containing them are perpendicular to each other.

But by altering the position of the lines (see fig. 504), we have two pairs of angles quite different from right angles; one angle, a´ m´ c´, is smaller, while a´ m´ d´ is much larger than the right angle. The former kind are called acute, the latter obtuse angles. We can therefore obtain a great number of acute angles, but only three obtuse, and four right angles around a given point, m.

The length of the sides of an angle have no effect on its magnitude, which is determined by the inclination of the lines towards each other. We now may consider the magnitude of angles, and the way to determine them. For this purpose we must describe a circle, which is figured in the diagram. But what is a circle?—A circle is a curved line which always is at the same distance from a certain fixed point, and the ends of this line meet at the point from which the line started.

If we fasten a nail or hold a pencil on the table, and tie a thread to it, and to the other end of the thread another pencil, we can describe a line around the first pencil by keeping the thread tightly stretched. This line is at all points at equal distance from the centre point. Any line from the centre to the circumference is called a radius, and a line through the centre to each side of the circumference is the diameter, or double the radius. The circumference is three (3·14) times the diameter. Any portion, say k i l, is an arc, and the line, k l, is the chord of that arc. A line like m n is a secant, and o p is a tangent, or a line touching at one point only.

We may now resume our consideration of the angles by means of the circle. Let us recur to our previous figure of the right angles, around which we will describe a circle. We see that the portion of the circumference contained between the sides of the right angle is exactly one-fourth of the whole. This is termed a quadrant, and is divided into 90°—the fourth of 360 equal parts or degrees into which the whole circumference is divided. The angle of 45° so often quoted as an angle of inclination is half a right angle. To measure angles an instrument called a Protractor is used.

The Protractor, as will be seen from the accompanying illustration (fig. 507), is a semi-circle containing 180°. The lower portion is a diagonal scale, the use of which will be explained presently. The Protractor measures any actual angle with accuracy. If we put the vertical point of the angle and the centre point of the circle together, we can arrive at the dimensions of the angle by producing the lines containing it to the circumference. An angle instrument, figured herewith, may be assumed as the basis of most apparatus for measuring angles. An index hand, R R, moves round a dial like the hand of a clock, and the instrument is used by gazing first at one of the two objects, between which the angle we wish to determine is made—like the church steeples (fig. 508) for instance. The centre of the instrument is placed upon the spot where lines, if drawn from the eye to each of the objects, would intersect. The index hand is then put at 0°, and in a line between the observer and the object, A. Then the index is moved into a similar position towards B, and when in line with it the numbers of degrees passed over (in this imaginary case 20), shows the magnitude of the angle.

The simple quadrant is shown in the cut (fig. 510). This was so arranged that when any object in the horizon was being looked at through the telescope attached, a plummet line is at 0°. But if the telescope be raised to C S, the quadrant will move, and the line will mark a certain number of degrees of the angle which a line if drawn from the star makes with the line of the horizon. The “Astronomical Quadrants” are as shown in fig. 516, and consist of a quadrant of wood strengthened and fitted with a telescope. The circle is graduated on the outer edge, and a “vernier” is attached. The time is determined by the observation of the altitude of a star, and then by calculation finding out at what time the star would have the observed altitude. The quadrant is now superseded by circular instruments.

An ellipse is a flattened circle, or oval, and will be understood from the diagrams. Let us fix two pegs upon a sheet of paper, and take a thread longer than the distance between the pegs; draw with the pencil controlled by the thread a figure, keeping the thread tight. We shall thus describe an oval, or ellipse. The orbit of nearly all the heavenly bodies is an ellipse. The parabola is another curved line, but its ends never meet; they become more and more distant as they are continued. The comets move in parabolic curves, and consequently do not again come within our vision unless their direction be altered.

This figure has a long axis, ab (fig. 512); and perpendicular to this a short axis, de, passing through the centre, c. The two points, SS′, are called the foci of the ellipse; also, as is evident from the construction of the figure, any two lines drawn from the two foci, to any point of the circumference, for instance, S and S′m, or S_m′_ and S′m′, etc., which represent the thread when the pencil is at m or m′, are together equal to the larger axis of the ellipse. These lines, and we may imagine an infinite number of such, are called radii vectores. The distance of the foci, S or S′, from the centre, c, is called the eccentricity of the ellipse. It is evident that the smaller the eccentricity is, the nearer the figure approaches to that of the circle. The superficies of the ellipse is found by multiplying the two half axes, ac and dc, by each other, and this product by the number 3·14.

The Diagonal Scale is shown in the margin. It is used to make diagrams so as to bring the relative distances before the eye. The larger divisions represent, it may be, miles, or any given distance; the figures on the left side tenths, and the upper range hundredths of a mile. So a measurement from Z to Z′ will represent two miles, we may say, with so many tenths and hundredths.

The Transit instrument is due to Roemer, a Danish astronomer. It consists of a telescope so constructed as always to point to the meridian, and rotates upon a hollow axis, directed east and west. At one end is a graduated circle. The optical axis of the telescope must be at exactly right angles to the axis of the instrument; it will then move on the meridian. There is an eye-piece filled with two horizontal and five vertical wires, very fine, the latter at equal distances apart. The star appears, and the time it takes to cross is noted as it passes between each wire, and the mean of all the transits will be the transit on the meridian. For if we add the times of all the transits across the wires, and divide by five the number of them we shall get at a true result.

A good clock is also a necessary adjunct for astronomical observations, and the astronomical clocks and chronometers now in use record the time with almost perfect accuracy. The improvement in telescopes, the use of micrometers, etc., have greatly facilitated observations. In the transit clock we have a most useful timekeeper, for the ordinary clocks are not sufficiently accurate for very close observations. The sidereal time differs from solar time, and the twenty-four hours’ period is calculated from the moment a star passes the meridian until it passes it again. The sidereal day is nearly four minutes shorter than the solar day, and the sidereal clock marks twenty-four hours instead of twelve, like the old dial at Hampton Court Palace over the inner gate. The Chronograph has also been useful to astronomers, for by “pricking off” the seconds on a roller by itself, the observer can mark on the same cylinder the actual moment of transit across each wire of the instrument, and on inspection the exact moment of transit may be noted.

The Equitorial is another useful instrument, and by its means the whole progress of a star can be traced. The Equitorial consists of a telescope fixed so that when it has been pointed at a certain star a clock-work movement can be set in motion, which exactly corresponds with the motion of the star across the heavens, and so while the star moves from its rising to setting it is under observation. Thus continuous observations maybe made of that particular star or comet without any jerking or irregular movement.

We can thus see the uniform motion of the stars which go on in greater or lesser circles as they are nearer to or farther from the pole; and with the exception of the polar star, which, so far as we are concerned, may be considered stationary, every star moves round from east to west—that is, from the east of the polar star to the west of it, in an oblique direction. Therefore, as Professor Airy remarks, “Either the heavens are solid, and go all of a piece, or the heavens may be assumed to be fixed or immovable, and that we and the earth are turning instead of them.”

The Mural Circle is another very useful instrument, and is used by calling to aid the powers of reflection of quicksilver, in which a bright star will appear below the horizon at the same angle as the real star above the horizon, and thus the angular distance from the pole or the horizon of any star can be calculated when we know the inclination of the telescope. The Transit Circle is also used for this purpose, and is a combination of the transit instrument with the circle. In all calculations allowance must be made for refraction, for which a “Table of Refractions” has been compiled. From the zenith to the horizon refraction increases. The effect of refraction can be imagined, for when we see the sun apparently touching the horizon the orb is really below it, for the refraction of the rays by the air apparently raises the disc.

The clock and chronometer are both very useful as well as very common objects, but a brief description of the pendulum and the clock may fitly close our remarks upon astronomical apparatus and instruments. The telescope has been already described in a previous portion of this work, so no more than a passing reference to it has been considered necessary. We therefore pass on to a consideration of the measurement of time, so important to all astronomers and to the public generally.

Time was measured by the ancients by dividing the day and night into twelve hours each, then by sun-dials and water-clocks, or clepsydra, and sand-clocks. The stars were the timekeepers for night before any mechanical means of measurement were invented.

“What is the star now passing?— The Pleiades show themselves in the east, The eagle is soaring in the summit of heaven.”—EURIPIDES.

Sun-dials were in use in Elijah’s time, and the reference to the miracle of the sun’s shadow going back on the dial as a guarantee to Hezekiah, will be recalled at once by our readers. These dials were universal, and till sunset answered the purpose. But the hours must have been very varying, and on cloudy days the sun-dials were practically useless.

The water-clocks measured time by the dripping or flow of water, and they were used to determine the duration of speeches, for orators were each allotted a certain time if a number of debaters were present. This method might perhaps be adapted to the House of Commons, and speaking by the clock might supersede clôture. We find allusions to these practices in the orations of Demosthenes. Even this system was open to objection, for the vases were frequently tampered with, and an illiberal or objectionable person was mulcted of a portion of the water, while a generous or popular adversary had his clepsydra brimming full. Some of these water-clocks were of elegant design, and a Cupid marked the time with arrows on the column of the clock of Ctesibius, while another weeping kept up the supply of water. The motive power was the water, which filled a wheel-trough in a certain time, and when full this trough turned over, and another was filled. The wheel revolved once in six days; and by a series of pinions and wheels the movements were communicated more slowly to the pillar on which the time was marked for 360 days, or with other arrangements for twenty-four hours.

The repeating of psalms by monks also marked the time, for by practice a monk could tell pretty accurately how many paternosters or other prayer he could repeat in sixty seconds. At the appointed hour he then awoke the monastery to matins.

Nature also marks time for us—as, for example, the age of trees by means of rings—one for each year; and horses’ teeth will guide the initiated to a guess at the ages of the animals, while the horns of deer or cattle serve a like purpose. But man required accuracy and minute divisions of time. He had recourse therefore, to machinery and toothed wheels. Till the mechanical measurement of time was adopted, the sunrise and sunset only marked the day, and the Italians as well as Jews counted twenty-four hours from sunset to sunset. This was a manifestly irregular method. To this day we have marked differences of time in various places, and at Geneva we have Swiss and French clocks keeping different hours according to Paris or Berne “time.” This, of course, is easily accounted for, and will be referred to subsequently.

We have read that the first clock in England was put up in Old Palace yard in 1288, and the first application of the toothed-wheel clock to astronomical purposes was in 1484, by Waltherus, of Nuremberg. Tycho Brahé had a clock which marked the minutes and seconds. If we had had any force independent of gravitation which would act with perfect uniformity, so that it would measure an equal distance in equal spaces of time, all the various appliances for chronometers would have been rendered useless. In the supposed case the simple mechanism, as shown in the margin (fig. 517), would have sufficed. The same effect would be produced by the spring, were it possible that the spring by itself would always uncoil with the same force. But it will not do so: we therefore have to check the unwinding of the cord and weight, for left to itself it would rapidly increase in velocity; and if we likewise make an arrangement of wheels whereby the spring shall uncoil with even pressure all the time, we shall have the principle of the watch.

It is to Huygens that the employment of the pendulum in clocks is due, and the escapement action subsequently rendered the pendulum available in simple clocks, while the manner of making pendulums self-regulating by using different metals, has rendered timepieces very exact. Of course the length of a pendulum determines the movement, fast or slow; a long pendulum will cause the hands of the clock to go slower, for the swing will be a fraction longer. A common pendulum with the escapement is shown (fig. 518). Each movement liberates a tooth of the escapement. The arrangement of wheels sets the clock going. The forms of pendulum are now very varied.

But in watches the pendulum cannot be used. The watch was invented by Peter Hele, and his watches were called “Nuremburg eggs” from their shape. The weight cannot be introduced into a watch, and so the spring and fusee are used. The latter is so arranged that immediately the watch is wound and the spring at its greatest tension, the chain is upon the smallest diameter of the fusee, and the most difficult to move. But as the spring is relaxed the lever arm becomes longer, and the necessary compensating power is retained. Watches without a “fusee” have a toothed arrangement beneath the spring.

The Pendulum. A “simple” pendulum is impossible to make, for we cannot put the connecting line between the “bob” and the clock-work out of consideration, so “simple pendulums” are looked upon as “mathematical abstractions.” The most modern clocks have what is called a “deadbeat” escapement, and a compensating pendulum. Clocks are liable to alter by reason of the state of the air and varying temperature, and until all our clocks are placed in vacuo we must be content to have them lose or gain a little. There is a magnet arrangement by which the Greenwich Observatory clocks keep time by compensation, corresponding with the fall or rise of the syphon barometer attached to it. The description need not be added. We may here state that detailed descriptions of all the instruments used in the Observatory, together with full information as to their use, will be found in a very interesting work by Mr. Lockyer, entitled “Stargazing,” to which we are indebted for some corrections in our summary.

We have spoken of solar time and sidereal time, and no doubt someone will inquire what is meant by mean time—an expression so constantly applied to the Greenwich clock time. Stellar time, we have seen, corresponds to the daily revolution of a star or stars. Solar time is regulated by the sun, and this is the astronomical time generally observed, except for sidereal investigations. But the sun is not always regular; the orbit of the earth causes this irregularity partly. The earth moves faster in winter than in summer, so the sun is sometimes a little fast and sometimes a little slow. Astronomers therefore strike an average, and calculate upon a MEAN SUN, or uniform timekeeper. Mean time and true (apparent) time are at some periods the same—viz., on the 15th of April, on the 14th of June, on the 31st of August, and on the 24th of December. Twice it is after, and twice before it. The time occupied by this “mean” sun passing from the meridian and its return to it, is a mean solar day, and clocks and chronometers are adjusted by it.

Twenty-four hours represents a complete revolution of the heavenly bodies. The mean solar time is 23^h 56´ 4·091´´, while twenty-four hours of mean time are equal to 24^h 3´ 56·55´´ of sidereal time. The difference between the times is given by Dr. Newcomb as follows, and is called the Equation of Time:—

DIFFERENCES BETWEEN MEAN AND APPARENT TIME.

February 10th True Sun 15 minutes slow. April 15th “ Correct. May 14th “ 4 minutes fast. June 14th “ Correct. July 25th “ 6 minutes slow. August 31st “ Correct. November 2nd “ 16 minutes fast. December 24th “ Correct.

MEASUREMENT OF DISTANCES.

Before passing to consider the planetary system we must say a few words respecting the manner of ascertaining the distances of inaccessible objects, and by so doing, we shall arrive at an idea how the immense distances between the sun (and the planets) and the earth have been so accurately arrived at. To do this we must speak of parallax, a very unmeaning word to the general reader.

Parallax is simply the difference between the directions of an object when seen from two different positions. Now we can illustrate this by a very simple method, which we have often tried as a “trick,” but which has been very happily used by Professor Airy to illustrate the doctrine of parallax. We give the extract in his own words:—

“If you place your head in a corner of a room, or on a high-backed chair, and if you close one eye and allow another person to put a lighted candle upon a table, and if you then try to snuff the candle with one eye shut, you will find you cannot do it.... You will hold the snuffers too near or too distant—you cannot form any idea of the distance. But if you open the other eye, or if you move your head sensibly you are enabled to judge of the distance.” The difference of direction between the eyes, which is so well known to all, is ready a parallax. It can also be illustrated by the diagram herewith.

If two persons, A and C (fig. 523), from different stations, observe the same point, M, the visual lines naturally meet in the point, M, and form an angle, which is called the angle of parallax. If the eye were at M, this angle would be the angle of vision, or the angle under which the base line, A C, of the two observers appears to the eye. The angle at M also expresses the apparent magnitude of A C when viewed from M, and this apparent magnitude is called the parallax of M.

Let M represent the moon, C the centre of the earth represented by the circle, then A C is the parallax of the moon; that is, the apparent magnitude the semi-diameter of the earth would have if seen from the moon. If the moon be observed at the same time from A, being then on the horizon, and from the point B, being then in the zenith, and the visual line of which when extended passes through the centre of the earth, we obtain, by uniting the points, A C M, by lines, the triangle, A C M.

Therefore, as A M, the tangent of the circle stands at right angles to the radius, A C, the angle at A is a right angle, and the magnitude of the angle at C is found by means of the arc, A B, the distance of the two observers from each other. As soon, however, as we are acquainted with the magnitude of two angles of a triangle, we arrive at that of the third, because we know that all the angles of a triangle together equal two right angles (180°). The angle at M, generally called the moon’s parallax, is thus found to be fifty-six minutes and fifty-eight seconds. We know that in the right-angled triangle M C A, the measure of the angle, M = 56´ 58´´, and also that A C, the semi-diameter of the earth = 3,964 miles. This is sufficient, in order by trigonometry, to obtain the length of the side, M C; that is, to find the moon’s distance from the earth. A C is the sine of the angle, M, and by the table the sine of an angle of 56´ 58´´ is equal to 1652/100000; or, in other words, if we divide the constant, M C, the distance of the moon, into 100,000 equal parts, the sine, A C, the earth’s semi-diameter = 1,652 of these parts. And this last quantity being contained 60 times in 100,000, the distance of the moon from the earth is equal to 60 semi-diameters of the earth, or 60 × 3964 = 237,840 miles.

In a similar way the parallax of the sun has been found = 8´´·6, and the distance of the sun from the earth to be 91,000,000 miles.

Let us first see how we can obtain the distance of any inaccessible or distant object. We have already mentioned an experiment, but this method is by a calculation of angles. The three angles of a triangle, we know, are equal to two right angles; that is an axiom which cannot be explained away. We first establish a base line; that is, we plant a pole at one point, A, and take up our position at another point, B, at some distance in a straight line, and measure that distance very carefully. By means of the theodolite we can calculate the angles which our eye, or a supposed line drawn from our eye to the top of the object (C) we wish to find the distance of, makes with that object. We now have an imaginary triangle with the length of one side, A B, known, and all the angles known; for if all three angles are equal to 180°, and we have calculated the angles at the base, we can easily find the other. We can then complete our triangle on paper to scale, and find out the length of the side of the triangle by measurement; that is the distance between our first position, A, and the object, C. It is of course necessary that all measurements should be exact, and the line we adopt for a base should bear some relative proportion to the distance at which we may guess the object to be.

In celestial measurements two observers go to different points of the earth, and their distance in a straight line is known, and the difference of the latitudes. By calling the line between the observers a base line, a figure may be constructed and angles measured; then by some abstruse calculation the distance between the centre of the earth and the centre of the moon may be ascertained. The mean distance is sixty times the radius of the earth. The measurement of the sun’s distance is calculated by the observations of the transit of Venus across his disc, a phenomenon which will again occur on 6th December, 1882, and on 8th June, 2004, the next transit will take place; there will be no others for a long time after 2004.

All astronomical observations are referred to the centre of the earth, but of course can only be viewed from the surface, and correction is made. In the cut above, let E be the earth and B a point on the surface. From B the stars, a b c d, will be seen in the direction of the dotted lines, and be projected to e i k l respectively. But from the centre of the earth they would appear at e f g h correctly. The angles formed by the lines at b c d are the parallactic angles, f i g h and h l show the parallax. An object on the zenith thus has no parallax. (See fig. 524.)

Spotted a typo or a problem in this chapter? Tell us

XXXV. THE SOLAR SYSTEM

GRAVITATION—THE PLANETS—SIZE AND MEASUREMENT OF THE PLANETS—SATELLITES—FALLING STARS—COMETS—AEROLITES.

GRAVITATION is the force which keeps the planets in their orbits, and this theory, perfected by Newton, was partially known to Kepler. Newton brought this idea into practical shape, and applied it mathematically. We know that every object in the world tends to attract every other object in proportion to the quantity of matter of which each consists. So the sun attracts the planets, and they influence him in a minor degree. Likewise the moon and our earth reciprocally attract each other. But as the sun’s mass is far greater than the masses of the planets he influences them more, and could absorb them all without inconvenience or disturbance from his centre of gravity. We have, in a former portion of this work, spoken of the law of universal gravitation, which is the mutual attraction of any two bodies to each other, is directly proportioned to their masses (not size), and inversely to the square of their distances apart.

This law operates amongst the heavenly bodies, and it is to the never-changing action of gravitation that the planets are kept in their places. Let us see how this is effected. We have read of force, and motion, and rest. Every body will remain at rest unless force compels it to change its position, and it will then go on for ever in a straight path unless something stops it. But if this body be acted on simultaneously by two forces in different directions, it will go in the direction of the greater force. Two equal forces will tend to give it an intermediate direction, and an equal opposing force will stop it. The last axiom but one—viz., the two equal forces in different, not opposing directions, gives us the key to the curving line of the planetary motions. Were it not for the attraction of the sun the planets would fly off at a tangent; while, on the other hand, were not the impelling force as great as it is, they would fall into the sun. Thus they take an intermediate line, and circle round the centre of the solar system—the SUN.

The solar system consists of the sun and the planets which revolve in space around him. These stars are called planets because they move in the heavens. We shall see that they have certain motions—going from east to west, from west to east, and sometimes they appear to be quite motionless. This change of place, appearing now at one side of the sun and now at another, has given them their title of “wanderers” (planets). Besides the planets there are comets and meteors, asteroids and satellites, all circling round the sun in more or less regular orbits. And there must be families of comets, and whole systems of meteors that have not yet appeared to us, and which make up the comets, as has been lately suggested.

Five planets were known to the ancients, and were named after gods and a goddess: MERCURY, VENUS, MARS, JUPITER, SATURN. In later years a great number were discovered. We must, however, confine ourselves to the consideration of the principal ones, eight in number, including our own EARTH, URANUS and NEPTUNE completing the list. Of these VENUS and MERCURY are the inferior, or interior planets moving between us and the sun. MARS, JUPITER, SATURN, URANUS, and NEPTUNE are superior, or exterior, and pass quite round the heavens. All the planets are spheroids, and vary greatly in their magnitude, as will be seen by the illustration (fig. 528), the largest body being the sun. Mercury, Mars, and Venus, are not so large as the Earth. The other principal planets are considerably larger than our globe.

Mercury is the smallest of the planets, Venus being nearly as large as the earth. Then comes Mars, which, though smaller than Venus, is larger than Mercury. Jupiter is the largest of all—the giant amongst planets, as Jove was the ruler of the gods of mythology. Saturn comes next, though much smaller than Jupiter, but bigger than all the rest together. Next Uranus, then Neptune, larger than Uranus, but farther away from us. We shall speak more in detail about these in their order separately.

Taking the earth as 1, the comparative VOLUMES of the planets are as follows:—

Mercury 1/25, Venus 4/5, Mars 1/5, Jupiter 1300, Saturn 900, Uranus 80, Neptune 230. Sir John Herschel gives the following illustration of magnitudes and distances:—

“Choose any well-levelled field or bowling green; on it place a globe two feet in diameter; this will represent the sun. Mercury will be represented by a grain of mustard seed on the circumference of a circle 164 feet in diameter for its orbit; Venus a pea, on a circle 284 feet in diameter; the Earth also a pea on a circle 430 feet; Mars a rather large pin’s head on a circle of 654 feet; Juno, Ceres, Vesta, and Pallas grains of sand in orbits of 1,000 to 1,200 feet; Jupiter a moderate-sized orange on a circle nearly half a mile across; Saturn a small orange on a circle four-fifths of a mile; and Uranus a full-sized cherry, or small plum, upon the circumference of a circle more than a mile and a half in diameter”

From an inspection of the following table the relative distances of the principal planets from the sun, their diameters, and other information respecting them may approximately be obtained. The dates of the discovery of the more modern pair are added:—

+-------+---------+-------------+------------+---------+-----------+ |Names. |Diameters|Distance from| Sidereal | Time of | Date of | | | in | the sun | period of | rotation| discovery.| | | English | (about). |revolution. | on their| | | | miles | | | axis. | | | | (about).| | | | | +-------+---------+-------------+------------+---------+-----------+ | | | | d. h. m.| d. h. m.| | |MERCURY| 3,000 | 35,000,000| 87 23 16| 24 5 --| Antiquity | |VENUS | 7,500 | 66,000,000| 224 16 50| 23 21 21| do. | |EARTH | 8,000 | 91,000,000| 365 6 9| 23 56 4| do. | |MARS | 4,500 | 139,000,000| 686 23 31| 24 37 20| do. | |JUPITER| 85,000 | 476,000,000| 4,332 14 2| 9 55 21| do. | |SATURN | 70,000 | 872,000,000|10,759 5 16| 10 16 --| do. | |URANUS | 33,000 |1,753,000,000|30,686 17 21| 9 30 --| Herschel, | | | | | | | 1781. | |NEPTUNE| 37,000 |2,746,000,000|60,126 17 5| . . . | Leverrier | | | | | | | and Galle,| | | | | | | 1846. | |THE SUN| 850,000 | . . . . | . . . . | 25 7 48| | +-------+---------+-------------+------------+---------+-----------+

Altogether there are a great number of planets and asteroids, which latter are minor planets circulating outside the orbit of Mars. They have nearly all classical names, such as Juno, Ceres, Vesta, Flora, Ariadne, Pallas, Pomona, Thalia, etc., and are all at distances from the sun ranging between 200,000,000 and 300,000,000 of miles, the periods of sidereal revolution ranging from 1,100 to 3,000 days, so their years must be from four times to nine times as long as ours. Altogether about two hundred of the minor planets have been discovered, and they are all very much smaller than the earth; some, indeed, being very tiny—only a few miles in diameter, but very massive. They do not appear to possess any satellites—at least, none have been discovered, for such very small bodies as they must be, supposing they exist, would be quite invisible even with our perfected instruments.

SATELLITES, however, or “planetary moons,” as they are sometimes designated, are plainly perceived attending upon the great planets. There are twenty of these at present under observation. One we are all familiar with, and the moon, par excellence, lends a beauty to our nights which no other light that we can enjoy or command can ever do. It is remarkable that only this moon is specially mentioned in the Bible in connection with the sun. The stars are usually grouped, although, of course, the sun and moon are equally “stars” in the firmament. Mars possesses two moons and Jupiter four; Uranus also rejoices in the latter number; Neptune, like the Earth, has only one. It is reserved for Saturn to outstrip all the rest in his attendants, for no less than eight satellites wait upon that enormous planet. No doubt there are many more of these moons to be found, and every year will doubtless bring us further knowledge respecting them. Mars’ moons were only discovered very lately (in 1877), although they were known to exist; but being very small, unlike the others, they were missed. So we may conclude that the remaining satellites will remain for some time undiscovered, even if they actually are in existence. Jupiter’s moons are supposed to be as large as our own moon; Neptune and Uranus can boast of equally-sized attendants. But it is impossible to estimate the riches of astronomical lore which are beyond our ken. Millions of tiny planets are believed to exist, but their immense distance from us precludes all investigation. We are but mites in the scale.

METEORS, to which we have already referred, are small erratic bodies rushing through the planetary system, and getting hot in the process, appear in the atmosphere surrounding our earth as “shooting stars.” Some of these falling bodies have reached the earth, and several can be seen in the British Museum. Numbers, of course, are burnt up before they reach us, and who can tell what destruction such a catastrophe may represent, or whether it be or be not an inhabited world which has thus plunged to destruction by fire? They are of a metallic or stony nature. On certain nights in August and November it has been calculated that these meteors will appear. They fall from certain constellations apparently on these occasions, and are called after their names—as Leonides, from Leo, in the November displays.

The star-showers at times attain the dimensions of a very beautiful display of rockets. Millions of them rush round the sun; and when, as occasionally happens, our earth comes near them, we have (as in 1866) a grand display of celestial “fireworks.” But the individuals composing the mass of falling stars are very small. These meteors are very much like the comets we last year had an example of, and it has been lately suggested that there is a great degree of affinity between the comets and the meteors;—in fact, that a comet is merely an aggregation of meteors. They have been supposed to be bodies of burning gas. Their mass is very great, and their brilliant tails are many millions of miles in extent.

Comets are thus distinguished by their tails, and differ very much in their orbits from the planets. The latter are direct in their wanderings, but comets are most irregular and eccentric. The name bestowed upon comets is from the Greek Kome, hair; for when the comet recedes from the sun the “tail” may be said to come out of the head, and appear as a hair in front, so to speak. But though all comets have tails, there are many luminous bodies (classed with comets) which have no tails.

The comet which created the most excitement was Halley’s in 1456, of which we append an illustration (fig. 537). A comet had been observed in 1607, and Halley made a calculation that it would reappear in 1757. The calculation for its actual appearance was made by Claivant, and the expected visitor passed the perihelion in March 1759. This comet, on its appearance at Constantinople, is said to have caused much consternation, and Christians regarded it as a “sign,” for the Turks had just then captured Constantinople, and were threatening Europe. Pious people included it in their supplications for deliverance from their most dreaded enemies, and “Lord, save us from the Turks, the devil, and the comet,” was a common prayer.

There have been several very beautiful comets. Encke’s, Coggia’s, etc., and the comet of 1858 (Donatis) must be in the recollection of middle-aged readers. Others came in 1861 and 1874. In 1881 two comets appeared. Some comets of antiquity were very remarkable, and are reputed to have equalled the sun in magnitude. One tail is usually supposed to be the distinguishing mark of a comet, but in 1774 one appeared with six tails, arranged something like a fan. Sometimes the tail is separated from the head. Of the actual consistency of comets we cannot give any lengthened details. They apparently consist of elements similar to the meteors—namely, of solid masses, and have been supposed to be aggregations of meteors. Some appear at regular intervals, and their approach can be determined with accuracy. Of course we only see those which are attracted by the sun, or those which revolve in the solar system. There must be thousands of other comets which we never see at all.

The diagram (fig. 540 in the next page) represents a portion of the path of the comet of 1680. This visitor pursued its course for two months at a velocity of 800,000 miles an hour. The tail was estimated to extend 123,000,000 of miles, and a length of 60,000,000 of miles was emitted in two days. When this great comet was approaching the sun, or its perihelion, as such approach is termed, three minutes more would have seen it rush into the orb had its enormous pace been slackened, but as it was proceeding so rapidly, and being just then 144,000 miles away, it escaped. We can scarcely estimate the results of such a collision. This comet appeared B.C. 34, and again at intervals of about 575 years. It may be expected in 2255. It is to Halley that the discovery of the elliptical orbits of comets is due.

M. Biela’s comet was the cause of much anxiety in 1832, for a collision with the earth was apprehended. Fortunately a month intervened between the period at which the comet was expected at a certain place in the system and the earth’s arrival at that spot, so, as it happened, about 60,000,000 of miles intervened. We cannot say what the exact effect of such a collision would be, but some wonderful atmospheric phenomena and increased temperature would certainly result from the contact. Now the comet is supposed to have an effect upon the vintage, as “comet” wines are regarded with much favour. If comets, as is believed, do consist partly of solid particles, a collision might be unpleasant; but the weight is, as a rule, a mere nothing compared to their vapoury volume, which is enormous. That the tails must be of a very attenuated medium is evident, as we can see the stars through them, and we know that a very thin cloud will obscure a star. The “menacing” comet, mentioned in the Spectator February 1881, will not do much damage, so the scare was needless, as Mr. Proctor has explained.

AEROLITES, or “Meteorites,” are falling bodies (meteors), which reach the earth in solid form. The greater mass of falling stars are burnt up ere they reach us, or are dissipated in space. But many instances of aerolites descending might be adduced. They usually consist of metals, such as iron and nickel mixed with sulphur, magnesia, and silica. The theory concerning falling stars has been already mentioned.

We have thus far taken a brief general view of the solar system, with a few of the phenomena of the heavens. Our next step will be to consider the sun, the planets, and the asteroids, according to the order of magnitude. The asteroids we cannot consider separately, but the sun, moon, earth, and the principal planets will yield us much interesting information as we examine them more closely. We shall then, as far as possible, look into the domain of the fixed stars, constellations, and the nebulæ, commenting, as we proceed, upon the varied celestial and terrestrial phenomena connected with the movements of the heavenly bodies. As is due to the great centre of our system, we will commence with the SUN. But before proceeding to do so, we must say a few words about the motion of the heavenly bodies—that is, the apparent motion of the rising and setting of the sun and stars.

The attentive observation of the starry heavens, even during a single night, will convince us that all the visible stars describe circles which are the smaller, the nearer the stars are to a certain point of the heavens, P (fig. 542). In close proximity to this point there is a tolerably bright star, called the Pole Star, which has scarcely any motion, but appears to the eye as always occupying the same position. Hence a line, PP´, drawn from this star, through the centre of the earth, c, represents the axis around which all the heavenly bodies perform their apparent motions. The part of the celestial axis, PP´, passing through the earth, is the earth’s axis; the north pole, of which p is on the same side as the pole star, and the south pole, p´, is on the opposite side.

We have, therefore, by the aid of the stars, determined the position of the earth’s axis, and by this latter we can assign to the equator its proper place. For if pp´ be the earth’s axis, aq´ is the greatest circle drawn round the earth, equally distant from both poles, and the plane of which cuts the earth’s axis at right angles.

Furthermore, let us suppose the plane of the equator to be extended till it reach the celestial concave; we thus find the place of the celestial equator, A Q, or equinoctial, as it is generally termed in opposition to the equator, which always means the terrestrial equator. The equinoctial divides the heavens into the northern and southern hemispheres. We cannot actually describe the equinoctial and make it visible, but we can imagine its line of direction by observing those stars through which it passes.

We are now in a condition to assign to an observer different stations in relation to the earth’s axis on the earth’s surface, which will essentially modify the aspects under which celestial phenomena are represented. One of these stations may be supposed to be at one of the two poles, for example, at p, or at any one point of the equator, as at a, or, finally, on any portion of the surface of the earth which lies between the pole and the equator, as, for example, o.

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XXXVI. THE SUN

MOTION OF THE SUN—THE SEASONS—CHARACTER OF THE SUN—SUNSPOTS—ZODIACAL LIGHT.

Suppose that we rise early in the morning we shall, as the reader will say see the sun rise—that is, he appears to us to rise as the earth rotates. By the accompanying diagram (fig. 544) we can understand how Sol makes his appearance, and how he comes up again; not, it will be observed, after the manner stated by the Irishman, who declared that the sun “went down, and ran round during the night when nobody was looking.” The earth rotates from west to east, and so the sun appears to move from east to west. If we look at the diagram we shall see that after rising at O, the sun advances towards the meridian in an oblique arc to A, the highest or culminating point—midday. He then returns, descending to W; this path is the diurnal arc. At Q similarly, during his passage in the nocturnal arc, he reaches the lowest or inferior culmination. HH´ is the meridian.

On the 21st of March, this path brings the sun on the “ equinoctial” line mentioned at the close of the last chapter. Day and night are then of equal duration as the arcs are equal. So this is the Vernal (or spring) Equinox. Some weeks after the sun is at midday higher up at S´, so the diurnal arc being longer, the day is longer, (Z is the zenith, Z´ is the nadir, P P´ is the celestial axis). From that time he descends again towards the equinoctial to the autumnal equinox, and so on, the diurnal arc becoming smaller and smaller until the winter solstice is reached (S).

From what has been previously said, it is evident that the sun has a twofold apparent motion—viz., a circular motion obliquely ascending from the horizon, which is explained by the rotation of the earth, and by our position, o, to the earth’s axis, P P´, and also by a rising and setting motion between the solstitial points, S and S´, which causes the inequality of the days and nights. Independently of the daily motion of the sun, we observe that at the summer solstice, on the 21st of June, at midday, the sun is at S´, and one half year later—viz., on the 21st of December, at midnight, the sun is at s, from which he arrives again in the space of half a year at S´; so we are able to represent this annual motion of the sun by a circle, the diameter of which is the line, S´ s. This circle is called the Ecliptic.

The plane of the ecliptic, S´ s, cuts the plane of the equinoctial, A Q, at an angle of 23½°, and the axis of the ecliptic, S´´´ s´´, makes the same angle with the axis of the heavens, P P. The two parallel circles, S´ s´ and S s, include a zone extending to both sides of the equinoctial, and beyond which the sun never passes. These circles are called the Tropics, from τρέπω, I turn, because the sun turns back at these points, and again approaches the equinoctial. The parallel circles, S´´ s´´, and S´´´ s´´´, described by the poles of the ecliptic, S´´´ s´´, about the celestial poles, P P, are called the arctic and antarctic circles.

Whenever the sun crosses the equinoctial, there is the equinox; but the points of intersection are not invariably the same every year. There is a gradual westerly movement, so it is a little behind its former crossing place every year. (See diagram, fig. 547.)

[This is the “Precession of the Equinoxes,” because the time of the equinoxes is hastened, but it is really a retrograde movement. Hipparchus discovered this motion, which amounts to about fifty seconds in a year. So the whole revolution will be completed in about 28,000 years.]

It is obvious, then, that the sun is the most important star in the universe; and when we come to speak about the earth we shall consider the seasons, etc., more fully. Now we must endeavour to explain what the sun is like, and this can only be done with specially darkened glasses, for a look at the sun through an ordinary telescope may result in great, if not permanent, injury to the eye.

The sun is not solid so far as we can tell. It is a mass of “white-hot” vapour, and is enabled to shine by reason of its own light, which the planets and stars cannot do; they shine only by the sun’s reflected light. So we may conclude the sun to be entirely gaseous, but, thanks to the recent researches in spectrum analysis (already explained), by which the light of the sun has been examined by means of the spectroscope, and split up into its component colours, Mr. Lockyer and other scientists have discovered that a number of elements (metals) exist in the sun in a fused, or rather vapourous state, in consequence of the intense heat. Hydrogen exists in the sun, with other gases unknown to us here, and many metals, discovered by their spectra, which are the same under similar circumstances.

The sun is supposed to be spherical in shape,—not like the earth, flattened at the poles,—and to be composed of materials similar to what the earth is composed of, and what it would be if it were as hot as the sun is. Thus we can argue by analogy from the spectra of earthly elements, that, as the sun and star light gives us similar spectra, the heavenly bodies are composed of the same elements as our globe. We can thus form our opinion of the sun’s constitution. Mr. Neisen says:—

“With the aid, therefore, of the additional information given us by the spectroscope, it is not very difficult to form a true idea of the probable condition of the surface of the sun, which is all that we can see. It is the upper-lying strata of a very dense atmosphere of very high temperature—an atmosphere agitated by storms, whirlwinds, and cyclones of all kinds, traversed by innumerable currents, and now and then broken by violent explosions. Above the brilliant surface which we see is a less dense and somewhat cooler upper stratum, which, though hot enough to shine quite brightly, is quite invisible in the presence of the brighter strata beneath it.”

SUN SPOTS, as they are generally called, are hollows in the sun’s vapoury substance, and are of enormous extent; and there are brilliant places near those spots, which are termed faculæ. These spots have been observed to be changing continuously, and passing from east to west across the sun, and then to come again at the east to go over the same space again. Now this fact has proved that the sun turns round upon his axis, and although he does not move as we imagine, from east to west, round the earth, the orb does move—in fact, the sun has three motions: one on his axis; secondly, a motion about the centre of gravity of the solar system, and a progressive movement towards the planet Hercules.

If we examine the surface of the sun through a proper telescope, we shall find that the even surface we can plainly distinguish at sunset is marked, and the brightness is greater towards the centre of the orb. We can perceive various irregularities; we shall find spots, faculæ (little torches), etc. These spots were discovered by Galileo and other astronomers, and were, as we have stated, found to be surface markings, and not a series of bodies passing between the earth and the sun. The rotation of the sun was measured, and it was found that the orb revolved in about 25⅓ days, and in such a manner as to be slightly inclined to the plane of the ecliptic.

Herschel observed a spot at least 50,000 miles in diameter, which is more than six times the diameter of the earth. The sun spots are observed to be constantly changing, and are naturally observed differently as the revolution proceeds. The dark pole, or “nucleus” (umbra), as it is called, is surrounded by a less dark surface called the penumbra, but the umbra is not really dark; it is extremely bright when viewed alone, as has been proved by Professor Langley, while the heat is even greater in proportion. But the umbra of a sun spot must be below the level of the penumbra, for the shape changed as the sun revolved on its axis. The penumbra was wider on the side nearest the edge of the solar disc, and the umbra may be due to the uprushing or downpouring of gas or vapour like “whirlpools in the solar atmosphere.”

Near the sun spots the long streaks, or faculæ, are often observed by the borders of the disc, and a transition of the luminous part of the photosphere[26] into darkness has been observed, and bright bridges crossing the spots, and then gradually getting dark, were seen by M. Chacomac. The sun spots vary in direction, but the same general course is continued. Sometimes they describe curves, sometimes lines.

During solar eclipses the sun exhibits what are termed “red prominences,” which are the luminous vapours existing around the sun. When the orb is eclipsed, we can see the bright-coloured vapours shooting out from underneath the dark shadow, and this light is termed the “coronal atmosphere”; the vapours are called the sun’s chromosphere. In the coronal atmosphere are certain curious shapes of vapour thrown up, and frequently changing,—projecting, in fact, from the gaseous envelope. These red prominences were first observed in 1842, and in 1851 it was proved that they appertained to the sun, for the moon hid them as the eclipse began. Before the prominences were discovered, the red light surrounding the solar disc was known, and called the “sierra” (now chromosphere), or chromatosphere. “The luminosity of these prominences is intense,” says Secchi, “and they rise often to a height of 80,000 miles, and occasionally to more than twice that; then bending back, they fall again upon the sun like the jets of our fountains. Then they spread into figures resembling gigantic trees, more or less rich in branches.” We give some illustrations of the appearances of these prominences.

The zodiacal light is often observed. It is a glow, and frequently of a rosy tint. It may be seen in England in March or April before sunset, or in the autumn before sunrise; and it is doubtful whether it be a terrestrial or an extra-terrestrial light—a lens-shaped object surrounding the sun. Some philosophers maintain that the light is caused by multitudes of minute bodies travelling round the sun; but Herr Gronemann has lately fully discussed the observations, and the drift of his contention is under stated:—

There are valid observations against two items in the support of the old theory—viz., the affirmed connection of the evening and morning cones seen on the same night (if the corresponding sides be prolonged), and the participation of the cones in the daily motion of the heavens. The zodiacal light is sometimes seen when daylight has not yet disappeared; and, on the other hand, it sometimes fails to appear, though there is complete darkness. There would seem to be a real lengthening and shortening. It has been observed by Schiaparelli, that the light is much more difficult to make out when it passes through the meridian, than when it is only 30° above the horizon, and is less easily seen when the air is clearest than when a sort of mist is present. Indeed, the bright parts of the Milky Way may be seen to be weakened by mist, while the zodiacal light at the same height is unaffected. The zodiacal light has temporary variations of light intensity, and it shows from time to time remarkable changes of form and position, so sudden and short as to be hard to explain on the planetary hypothesis. The elongations of the cones show a half-yearly period, which is independent of the transparency of the air. The cone follows the observer northwards or southwards, so that there is no parallactic action; and this peculiarity (so adverse to the extra-terrestrial hypothesis) cannot be explained by reflection or absorption of light. As to spectroscopic observations, the author finds (1) that the zodiacal light consists partly of proper light; (2) that its connection with polar light is but secondary, temporary, and accidental; (3) that the cause of the second phenomenon is such that it may strengthen the zodiacal light and modify its spectrum; and (4) that the results of spectrum analysis rank with other arguments tending to find the source of the zodiacal light in the neighbourhood of the earth (like the polar light). Herr Gronemann, then, thinks the zodiacal light a terrestrial phenomenon, though he will not say that it cannot be influenced by cosmic action. He throws out the suggestion that the cone may be a kind of optical illusion, arising from some fine matter—gas or dust—being more accumulated near the observer in one direction than another. The apparent length of the cone might be conditioned by the conical shadow of the earth, and the changes of length be due to cosmic and electric influences. In any case, there is need of a more scientific theory than the old one.

We may conclude our brief notice of the great luminary to which we are indebted for everything, by a resumé of his distance from us, his diameter, and a few other plain facts and figures. In the first place the actual distance of the sun from the earth has never accurately been determined, but perhaps the next transit of Venus will assist the observers to a nearer estimate. It is quite sufficient for our purpose, however, to state that the sun’s distance from the earth is 92,000,000 of miles. The distance varies in winter and summer. In the former period the sun is nearer than in summer, and yet as the rays strike over us, and pass us, as it were, we feel less heat. When, as in summer, we are more in the focus of the rays, we feel the greater heat.

We have already spoken of parallax, and it is by finding the solar parallax that the distance of the sun from us is found. This parallax has not been exactly ascertained, or rather authorities differ, and as difference of 0·01´´ in the solar parallax means something over 100,000 miles of distance, it is evident that exactness is almost impossible. If 8´´·80 be settled as the solar parallax, 92,880,000 miles is the distance of the sun from the earth. If 8´´·88 be taken we have nearly 92,000,000 exactly.

The volume of the sun is 1,253,000 times that of the earth, and yet the density of the former is only about one-fourth of the latter, so the attraction of gravitation at the sun must be more than that of the earth’s surface twenty-seven times. A body dropped near the surface of the sun would fall 436 feet in the first second, and have attained a velocity of ten miles a minute at the end of the first second. The diameter of the sun depends in our calculations upon its distance from the earth. If we suppose that to be 92,880,000 miles, the diameter is 866,000 miles. If we take 91,000,000 of miles as the distance from the earth the diameter is 850,467 miles. The sun makes (apparently) the circuit of the heavens in 365 days, 6 hours, 9 minutes, and 9·6 seconds; the transit from one vernal equinox to the next being only 365 days, 5 hours, 48 minutes, 48·6 seconds, owing to the precession of the equinoxes already mentioned.

When we consider the power and grandeur of the sun we may well feel lost in the contemplation. The sun balances the planets and keeps them in their orbits. He gives us the light and heat we enjoy, and coal-gas is merely “bottled-up sunlight.” In darkness nothing will come to maturity. We obtain rain and dew owing to the sun’s evaporative power; and no action could go on upon earth without the sun; and yet we receive only about 1/2070650000 part of its heat and light.

As to the colour of the sun, Professor Langley states that it is really blue, and not the white disc we see. The whiteness is due to the effect of absorption exerted by the vapourous metallic atmosphere surrounding our luminary; and if that atmosphere were removed, his colour would change.

FOOTNOTES:

[26] The bright part of the sun.

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XXXVII. THE EARTH

FORM OF THE EARTH—MOTION OF THE GLOBE—RATE AND MANNER OF PROGRESSION—LATITUDE AND LONGITUDE—THE SEASONS.

We have learnt from our books on Geography that the earth is shaped like an orange,—that is, our globe is round and flattened slightly at the “poles,” and we can easily see that the earth curves away, if we only try the experiment mentioned in a foregoing chapter—viz., how far a person standing (or lying) on the ground can see on a level. Our power of eyesight is not limited to three or four miles, but a man of ordinary height standing on the plain cannot see more than three miles, because the earth is curving away from him.

We know that at the seaside we can see ships gradually appear and disappear. When approaching us the masts and top-sails appear first, then the main-sails, and then the ship itself. A sailor climbing up the mast can see farther than the captain on deck, because he can see over the curve, as it were. When the vessel is at a considerable distance we see her “hull down” as it is termed,—that is, only her sails are visible to us, and at last they disappear also. If we want any other proof that the earth is round we can see when an eclipse takes place that the shadow on the moon is circular. So we may be certain of one fact; the earth is round, it is a globe. So much for the rotundity of the earth.

But the earth appears to us, except in very mountainous districts, as being almost a plane. This is because of its extent; and even from very high mountains we can only see a very small portion of the earth, and so, on a globe sixteen inches in diameter, the highest hills would be only about 1/100 of an inch, like a grain of sand.

The motion of the earth is known to most people, though as everything upon the globe passes with it, and a relative fixity is apparent, this is, of course, not real rest. The earth is moving from west to east at a tremendous rate,—viz., nearly nineteen miles a second! We think a train at sixty miles an hour a fast train; but what should we think of an express going more than 68,000 miles an hour! Yet this is about the rate at which our globe whirls around the sun. Her fastest pace is really 18·5 miles a second; the least about one mile per second less.

That is one motion of the earth; the other is its motion on its axis. If we send a skittle ball rolling we perceive it turns round as it proceeds. So the earth rotates on its axis, N S, in the accompanying diagram; the extremities of the axis are called the poles. The line in the middle is the equator, which is divided into 360 equal parts, each being 69-1/10 miles in length; so there are 180 lines, or rings, drawn upon the globe from N to S, and these are meridians. In England the degrees are calculated from the Greenwich meridian. We can thus obtain the distance of localities east or west, as we may briefly show (fig. 554).

The distance of any meridian from the first meridian is termed the longitude, and it is employed in describing the situation of a place on the earth’s surface. Suppose L (fig. 554) a city, its longitude will be 30°, since it lies on a meridian which is 30° from the first. So, for example, the longitude of Oporto is 8° 37´ west, Paris 2° 22´ east, Vienna 16° 16´ east, Bagdad 44° 45´ east, reckoned from the meridian of Greenwich, and so on. At the 180th degree we have proceeded half round the globe, and reached the farthest distance from the first meridian, and are now on the opposite side of the earth, and proceeding in a similar manner in the opposite direction we get west longitude.

It will readily be perceived that a knowledge of the longitude alone is not sufficient to determine the situation of a place on the earth’s surface. When we say, for example, that the longitude of a place is 30°, it may lie on any point whatever of the line, N L S, on the whole hemisphere (fig. 554). This point must therefore be determined more accurately, and hence the first meridian is divided into 90 equal parts north and south of the equator towards the poles. These are called degrees of latitude, and the lines drawn through these round the globe, parallel to the equator, are called circles or parallels of latitude, and diminish as they approach the poles.

Hence, by the latitude of a place we mean its distance from the equator towards the poles, and we speak of north and south latitude according as the place is situated in the northern or southern hemisphere.

So, for example, the point L (fig. 554), which has 30° longitude and 60° N. latitude is in Sweden.

The latitude is also observable by ascertaining the altitude of the polar star above the horizon when in the northern hemisphere. The longitude is found by the chronometer; for if we know the time at Greenwich we can calculate how far we are east or west of it by seeing whether the local time be an hour (say) earlier or later, and that difference shows we are 15° to the east or the west as the case may be.

The earth’s rotation, according to sidereal time, is less than solar time, as we have seen, so we have 365 solar days and 366 sidereal days; so a person going round the world gains or loses a day as he travels east or west according to his reckoning, as compared with the reckoning of his friends at home. We can easily ascertain the earth’s motion by watching the stars rise and set. Now the path in which the earth moves is called an ellipse,—very nearly a circle,—but it does not always move at the same rate exactly. We will now look at the relations of the sun and the earth.

Let us take an example. Suppose we have a rod, at each end of which we fix a ball (see diagram), and let one ball be three times as large as the other, the common centre of gravity will be at c, at one quarter of the distance between the centres, and there the bodies will be in equilibrium. If these masses be set spinning into space they will revolve at that distance from each other, the attraction of gravitation and the force in opposition to it equalizing each other.

The earth, as we know, proceeds with a tremendous force around the sun, not in a circle, remember, but in an ellipse or oval track, from which it never moves year by year in any appreciable degree. Now what prevents this earth of ours from rushing off by itself into space? Why should not the earth fly away in a direct line? The reason is because the sun holds it back. The force of the sun’s gravitation is just sufficient, or we may say so enormously great, that it suffices to retain our globe and all the other planets in their various orbits at the very same distance, and to counteract the force which launches them through space. Therefore, as we have already noticed when considering the sun, it is to that ruler of the day that we are indebted for everything.

What would happen, then, if the earth were suddenly to increase her velocity, or the sun to contract his mass?—We should be flung into infinite space, and in a short time would be frozen up completely. Our present diurnal course would probably proceed, but all life and existence would cease as we whirled with distant planets through infinity.

Suppose, on the contrary, we were to stop suddenly. We have some of us read in a foregoing part of this volume that heat is the motion of molecules in ether, and that when a body strikes another heat is developed by contact and friction. If the earth were to be stopped suddenly, “an amount of heat would be developed sufficient to raise the temperature of a globe of lead of the same size as the earth 384,000° of the Centigrade thermometer. The greater part, if not the whole of our planet, would be reduced to vapour”, as Professor Tyndall says.

In the diagram (fig. 546, on page 497) we shall at once find the explanation of the constantly-recurring seasons, and the amount of our globe which is illuminated by the sun at various times. It will be easily understood that the poles have six months day and six months night. When the earth is at an equinox, one half of the surface is illuminated and the other half in shade, therefore the days and nights are equal. But when the north pole turns more and more towards the sun, the south pole is turning away from it in the same ratio,—the days and nights respectively are getting longer and longer, and at the north and south poles day and night are continuous, for the small spaces round the poles are, during a certain period, wholly in sunshine and shade respectively.

In March (in the diagram, fig. 546) we see that exactly one half of the earth is illuminated, and the other is darkened. So in September, when we have the opposite view. In June the earth is more inclined apparently to the sun, and more of the surface is exposed to it, so the days are longer in some parts. The opposite effect is visible in December.

The summer heat and winter cold are accounted for by the more or less direct force of the sun’s rays, for the more the angle of incidence is inclined the fewer rays reach the object; and if the rays fall at an angle of 60°, the heat is only half what it would be if they came vertically. When the days are shortest the sun is lowest, and therefore gives less heat to the earth at certain periods.

The wonderful precision which has adapted the position of the earth on its axis, will be apparent from the illustration (fig. 557). Here we have a table and some bottles, a candle to represent the sun, and a ball of worsted and a knitting-needle to represent the earth and its axis. Suppose we place the ball in the position at a, with its axis perpendicular to the plane of the orbit. As the earth would turn and go round the sun in this supposed case, we should find the days and nights equal, and the sun would quickly scorch up the tropics, and the other portions would have a never-changing spring or winter all the year for ever. This would not be so pleasant, for variety is the charm of nature, and the salt of life. So we may put a aside, as the earth would be scarcely habitable under the supposed conditions, and try b. Here we find the poles directed to the sun. The whole northern hemisphere would thus be illuminated one half year, and the southern similarly; such rapid changes from heat to cold and back again would not suit us. So we fall back upon c, the actual appearance of the position of the earth, and here we find all the most favourable circumstances existing for us. This inclination gives rise to all the varied phenomena of the pleasant gradations of heat and cold, summer and winter, the charming changes of season, and the wonderful results of the ever-recurring days and nights, months and years, as the earth spins round. So we see that the sun does not really rise and set upon the earth; the globe rotates, and brings us into view of the sun, and as we turn we lose his light.

In the foregoing brief description we have learnt some few facts concerning our earth. We have ascertained that the planet we inhabit is round; we have also seen that the earth moves around the sun and around its own axis, and also that it moves at a tremendous rate; we know that that rate is just counterbalanced by the attraction of gravitation, and the course round the sun gives us varying seasons, day and night. There are many subjects relating to the earth which will be more properly included under Physical Geography. We may here just add the diameter of the earth, and proceed to inquire concerning the moon. The polar diameter of our globe is 7,899 miles; the equatorial diameter 7,925 miles. It is distant from the moon 238,500 miles. We will close this chapter with the letters and characters of the Greek alphabet used in astronomical works to designate the stars.

Α α = Alpha Β β = Beta Γ γ = Gamma Δ δ = Delta Ε ε = Epsilon Ζ ζ = Zeta Ê ê = Eta Θ θ = Theta Ι ι = Iota Κ κ = Kappa Λ λ = Lambda Μ μ = Mu Ν ν = Nu Ξ ξ = Xi Ο ο = Omicron Π π = Pi Ρ ρ = Rho Σ ς = Sigma Τ τ = Tau Υ υ = Upsilon Φ φ = Phi Χ χ = Chi Ψ ψ = Psi Ω ω = Omega

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XXXVIII. THE MOON

WHAT IS IT LIKE?—MOON SUPERSTITIONS—DESCRIPTION OF THE MOON—PHASES—TIDES—ECLIPSES.

From the early days of childhood every man and woman has been familiar with the moon. This satellite of earth has been domesticated, so to speak, amongst us; and while the sun and other stars have been glorified in poetic and prose effusions, the moon has been always more tenderly addressed. The soft (reflected) light of our attendant moon is much more attractive than the brilliancy of the greater light “that rules the day.” The moon is regarded as our particular property, and has awakened an interest in our minds since the time that we could, as we fancied, see the “Man in the Moon.”

In ancient times the moon was supposed to possess some light of her own, and to be inhabited by immense creatures; and various theories continued to be promulgated respecting her, until the telescope came into use, and then astronomers began to find out many new things concerning Luna. Now, what has the telescope told us regarding our moon?—It shows us that there are mountains and craters, and numerous traces of volcanic action. At one time it was supposed that the dark masses apparent in the surface of the moon, and which can easily be distinguished by the naked eye, were seas, and maps of the moon were made, marking continents and craters.

If it were possible to reach the moon, as M. Jules Verne’s travellers did, we should find a very irregular and corrugated surface—plains and mountains without water. We should be able to see the stars in the daytime, because there is no atmosphere around the moon, and there is a silence that “might be felt.” The appearance of our earth from the moon, and the beauty of the stars in the unclouded and waterless space around the satellite, must be very grand, and has been, in a measure, depicted in the illustration (fig. 559) on the opposite page.

In this illustration (fig. 560) we have some idea what the moon is like. We see the rugged and cratered appearance of the disc; it is a desert waste, so far as we can ascertain, without inhabitants, and, in all probability, without vegetation. For there being no moisture amongst the plains and craters and mountains of our satellite, we must conclude that the moon is dead. It is a very interesting,—nay, a fascinating study. When we take up our telescope and look from the window at the heavens the most beautiful object within our small telescopic vision is the moon shining like a silver plate, and we wonder what is up there. With a small telescope even we can discern many interesting features in the moon at the full, which will assist us in verifying the diagrams in books and their explanations.

As the moon is only a few miles away, comparatively speaking, and as the large telescopes now in use bring us within a measurable distance of the surface, we are enabled to speak more positively about our changeable satellite than of any of the planets. When we look steadily at the full moon we perceive upon its surface dark and light tracts called “seas,” though they are dried up now. Thus we hear of the “Sea of Serenity,” the “Sea of Storms,” and the “Sea of Tranquillity”; and in the map upon a subsequent page you may see the names of the seas, mountains, and the general formations of the surface of the moon. Maps of the moon are now to be procured, though no personal visits can be made to the satellite. It is very interesting to observe or to read about the structure of the moon, for we may thus learn how similar the earth and her attendant are in formation; but one important agency—that of water—has made a considerable difference in the appearance of the formations. In the moon we have mountains, plains, and rugged craters; the surface is not level, because the sunlight is visible sooner at some points than others. The chief mountain chain is the Apennines, and has a great elevation; many traces of volcanic agency are discoverable amid the great desolation, and awful silence reigns throughout.

As is well known, water has a great erosive power, and its action disintegrates the surface of the earth with rapid persistency. So the physical appearance of the globe has become much changed in the course of ages: ravines exist where plains used to extend, and rivers cut their way through deep gorges to the sea. The sands and other deposits are overlaid, and thus the whole outward appearance has been altered. Not so the moon. With a very attenuated atmosphere without clouds or rain, there is no moisture, no lake, no water in the moon now. What may have been we can only conjecture. If there ever have been lakes or seas they have all been absorbed.

The heat upon one side of the moon must be very great at one period, and the cold on the opposite side intense, as one would think—yet upon this fact authorities differ somewhat. If the moon possess no atmosphere of any kind it would be fearfully cold and extremely hot at intervals, but a surrounding medium, even of very little density, would modify the extremes; and while we must accept the fact that the temperature varies very much we need not place it above 100° of heat, nor below 20° of cold. So from close observation and comparison we are enabled to form a very fair opinion of the “past” of the moon, and to ascertain that the same forces of nature which have moulded the planet we inhabit, have been at work in the moon also. When we study “Selenography,” therefore, we shall find a record of a history which may some day bear a parallel to the history of our physical world.

The moon, as all are aware, moves round the earth attendant upon us, but entirely under the control of the sun; our satellite, moreover, has been the subject of many superstitions. A great many rites and even domestic actions—such as the killing of fowls—were regulated by the moon; and in Scotland, Scandinavia, and other portions of Europe, she has always been regarded as effecting destiny. There are many interesting myths connected with the moon, and indeed with astronomy generally, and from a volume entitled “Notes on Unnatural History,” some very amusing extracts might be made. It will not be out of place to mention a few of these myths.

The Chinese have an idea that a rabbit exists in the moon, and is the cause of the shadows we see. The Buddhists think a holy hare is up there. In the Pacific Islands there is a belief of a woman in the moon; she was sent there because she wished her child to have a bit of it to eat; and Mr. Buchanan has versified the old Scandinavian myth about the two children kidnapped by the moon as they returned from a well with a bucket of water slung upon a pole. The Jews placed Jacob in the moon, and the Italians say that Cain inhabits the luminary with a dog and a thorn bush. In the Inferno of Dante this is referred to, and we know that in A Midsummer Night’s Dream we have the moon coming out to shine upon the loves of Pyramus and Thisbe with the dog and the thorn-bush; and in the Tempest the same idea is mentioned by Caliban. Readers of Longfellow will recall the lines how “the good Nokomis answered” Hiawatha, who asked about the moon—

“Once a warrior, very angry, Seized his grandmother, and threw her Up into the sky at midnight. Right against the moon he threw her, ‘Tis her body that you see there.”

But modern scientific research has exploded all these charming old myths, and laid bare the facts for us. We must now resume.

The moon moves around us in 27^d 7^h 43^m 11·461^s. Its diameter is about 2,160 miles, and it is much less dense than our earth, and so the force of gravity is less there than here. Its mean distance from us is 238,833 miles. The moon goes through certain changes or phases every twenty-nine days or so; and while rotating on its own axis our satellite goes round the earth, so that we only see one side of the moon, inasmuch as the two motions occupy almost exactly the same space of time. So we generally see the same space of the moon, though there is a slight variation at times. This movement or swaying of the central point is called the moon’s “libration,” and is an optical effect, due to the inequalities in the motion of the moon in its orbit, and to the inclination of its equator and orbit to the ecliptic.

We append a map of the moon, on which the mountains, seas, and craters can be perceived, according to the list. The hill ranges extend for hundreds of miles, and the elevation reaches 30,000 feet, and even more in places. The so-called craters do not resemble volcanoes when viewed closely, but take the form of basins or valleys surrounded by lofty hills. One great plain called Copernicus is more than fifty miles across. Respecting the appearance of the moon let us quote Mr. Lockyer.

“Fancy a world without water, and therefore without ice, cloud, rain, snow; without rivers or streams, and therefore without vegetation to support animal life;—a world without twilight or any gradations between the fiercest sunshine and the blackest night; a world also without sound, for as sound is carried by the air, the highest mountain on the airless moon might be riven by an earthquake inaudibly.”

PHASES OF THE MOON.

We have said that the moon revolves around the earth in the same time as she turns upon her own axis, and always presents one side to us when she appears. Any one can ascertain this by putting a candle upon a round table, and walk round it facing the candle. The experimentalist will find that he will turn upon his own axis as well as turn around the table. Thus we shall see how the moon changes, for to be as changeable as the moon is proverbial. These different aspects or phases we shall now proceed to explain.

The time intervening between one “new” moon and another is 29^d, 12^h, 44^m, 2^s, and is termed a synodic revolution. This is longer than the sidereal revolution, because the earth is also moving in the same direction and the moon has to make up the time the earth has got on in front, as it were. So the moon travels nearly thirteen times round the earth while the latter is going round the sun.

The revolutions of the moon have been a measurement of time for ages, and her varying appearances during lunation are always observed with interest. The illustration (fig. 567) will assist us materially. The sun’s rays fall in a parallel direction upon the earth and moon, and let us suppose that S is the sun in the diagram and T the earth; C at the various points is the moon, the capital letters, A, B, C, etc., indicating the planet as she appears from the sun, and the small letters show how she appears to us from the earth.

Let us suppose that the sun, earth, and moon are in conjunction—or in a direct line. The phases, C and G, are the moon’s “quadratures.” At A we see the sun shining on the moon, but we only have the dark side. It is then “new” moon; but by degrees, as she goes round in her orbit, we perceive a small crescent-shaped portion, lighted up by the sun at B and b. At c´ we have the first quarter or half-moon. When she is in opposition she is at full moon, and so on to the last quarter and conjunction again.

The moon’s phases may be easily shown by means of a medium-sized lamp to represent the earth, a smaller one to serve as moon, and a light to act as sun all at the same height. Colour the “lunar” globe white, and if we move it about the “earth” globe, we shall see the various phases of the moon in the sharp shadows.

THE TIDES.

The ebb and flow of our tidal waters depend upon the moon to a great extent. The phenomenon is so common, that we need only refer to it, but the cause of the tides may be stated. Twice every day we have the tides twelve hours apart, and the flow and ebb are merely examples of the attraction of gravitation, which is exercised upon all bodies, either liquid or solid. The tides are highest at the equator and lowest at the poles, because the tropics are more exposed to the influence of the lunar attraction.

By the small diagram (fig. 569) we shall be able to see in a moment how the moon acts. The moon being nearer to the earth at b, the water will be naturally attracted to the ball, m, and cause high water (a); and a similar effect will be produced opposite, because the earth is attracted, so the waves are higher than the ground which has been attracted away from the water, and the waters will flow in and cause a high tide at d, but not so high a tide as at the opposite point, a. It can then be understood that there will be low water at the other two sides, e and f, because the water has been taken away, so to speak, for the high tides at a and d. We shall learn more of this under Physical Geography.

The moon revolves round the earth in a changeable elliptical orbit, intersecting the ecliptic at certain points called Nodes. When the moon is nearest to the Earth she is said to be in perigee when farthest from us she is in apogee (the line uniting these points is the line of apsides), the difference in distance being about 4,000 miles. She passes the sun periodically, and so if the moon moved in the plane of the ecliptic there would be eclipses of the sun and moon twice a month; but as the orbit is inclined a little, she escapes by moving north or south. We will now endeavour to explain this theory.

ECLIPSES.

We have briefly considered the SUN and EARTH and the MOON separately. We are now about to regard the effects produced by them when they come in each other’s way and cause ECLIPSES, which are observed with so much interest. There are eclipses of the sun and of the moon. The former occur at the time of new moon, and the latter at full moon; and this will be at once understood when we remember that the sun is eclipsed by the moon passing between us and the sun; and the moon is eclipsed because the shadow of the earth falls upon her when she is opposite the sun, and therefore “full.”

Readers of the voyages of Columbus will remember that he managed to obtain supplies from negatively hostile Jamaica savages by pretending to cause an eclipse of the moon, which he knew was about to take place, and to the ancients eclipses were of dire portent. Even in enlightened Rome, to ascribe an eclipse to the causes of nature was a crime. The Chinese have an idea that great dragons are devouring the moon when she is eclipsed.

There are total, partial, and annular eclipses. The former terms speak for themselves; the latter name is derived from “annulus,” a ring; for a ring of light is left around the dark portion eclipsed, and is only seen in solar eclipses. In one sense the eclipse of the sun is really an eclipse of the earth, because it is caused by the shadow of the moon falling upon the earth.

If a bright body, A, be larger than the dark body, B, there will be two kinds of shadows—viz., the umbra and the penumbra. For instance, the umbra is the central dark part in the cut (fig. 572), and the penumbra is the lighter portion. As soon as the eye is placed on the umbra, it can perceive no part of the source of light, A, which appears to be eclipsed. On the other hand, the penumbra originates in that locality where only a portion of the light proceeding from a luminous object can fall; hence an eye in the penumbra would see a part, but not the whole of the illuminating body. This shadow also forms a cone, the apex of which, if extended, will fall before the opaque body. If we receive the shadows so projected at m n, for example, on a white sheet, we have in the centre a dark circle, which is the umbra, surrounded by the penumbra, which gradually decreases in intensity towards the exterior (see fig. 573). The farther we hold the sheet from the body producing the shadow, the umbra decreases, and the penumbra is enlarged. For where (in solar eclipses) the umbra falls there is totality; within the penumbra partial eclipse only.

Lunar Eclipse.—Let A (fig. 573) be the sun, and B the earth, the length of the umbra of the latter will exceed 108 diameters of the earth. Since the moon is only about thirty terrestrial diameters distant from the earth, and as the diameter of the earth’s shadow, at this distance, is nearly three times as large as the apparent diameter of the moon, it follows that when the latter enters this shadow, she must be totally eclipsed, for at those places where the moon’s shadow falls there is total eclipse. If the moon’s orbit were coincident with the ecliptic, or if both moon and earth moved round the sun in the same plane, there would be an eclipse at every conjunction, and at every opposition,—i.e., a solar eclipse would happen at every new moon, and a lunar eclipse at every full moon. But we have seen that the lunar orbit cuts the ecliptic only in two points; consequently an eclipse of the moon is possible only when, at the time of opposition, the moon is in one of her nodes, or in close proximity to it, which can only occur twenty-nine times in the space of eighteen years.

A lunar eclipse begins on the eastern margin of the moon, and is either total, when her whole disc enters the umbra, or partial, when only part of her disc is in the shadow. A total eclipse may last for two hours.

We shall understand this better, perhaps, with the diagrams.

Solar Eclipses.—When the moon and the sun are in conjunction, the moon’s place may be represented by M (fig. 574) between the earth, T, and the sun, S. If this conjunction occur when the moon is in one of her nodes, or within 16° of it, the shadow of the moon will fall upon the earth, and the sun will be eclipsed. At other places the sun will not be entirely covered; and if the moon be moved farther off, so that its shadow will not reach the earth, and so not cover the sun up completely, we shall have an annular eclipse, because a rim of the sun will be visible.

The lunar umbra extends from the moon by a space about equal to her distance from the earth, and hence only a small portion, d, of the earth’s surface enters the lunar umbra. To the inhabitants of this part of the earth the sun will be totally eclipsed, and the eclipse will be annular if only the margin of the sun’s disc remain uneclipsed by the lunar shadow. This is only possible when the moon is in her apogee, or greatest distance from the earth, where her apparent diameter is less than that of the sun, which it cannot in general exceed more than 1´ 38´´. Hence the duration of a total eclipse of the sun cannot be more than 3¼ minutes.

On the contrary, the penumbra of the moon is diffused over a much larger portion, n m, of the surface of the earth, since its section is five-ninths of the earth’s diameter. The inhabitants of this portion of the earth do not receive light from all parts of the sun, consequently a part of this luminary is invisible to them, and the eclipse is said to be partial.

Solar eclipses commence on the western margin of the sun, and advance to the eastern. On account of the proximity of the moon to us, an eclipse of the sun is, in all places above the horizon of which the sun appears, visible neither at the same time, nor is it of equal duration, nor of equal extent: in some parts it may not be visible at all. In favourable situations, the diameter of the umbra, where it reaches the earth, amounts to about 167 miles, and on this small strip of the earth’s surface only can the sun appear totally eclipsed.

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XXXIX. THE PLANETS AND ASTEROIDS

MERCURY.

Including our own globe there are eight principal planets—viz., Mercury, Venus, Mars, Jupiter, Saturn, Uranus, and Neptune. The two first-named being between us and the sun, are termed interior planets; the others are exterior. Mercury, Venus, and Mars are smaller than Earth. The other four are much larger.

We have already described the planets as bodies wandering through the zodiac, and reflecting the sun’s light. Their orbits are very different from the moon’s; for instance, planets take a retrograde motion as well as a direct one. The sun and the planets revolving around him constitute the solar system.

We will commence our brief consideration of them with Mercury, the planet nearest to the sun.

The distance of Mercury from the sun is 35,000,000 of miles, less than half the distance our earth is from him, and so receives much more heat and light than we do. The sun to the Mercurians, if there be any inhabitants upon the planet, must appear about seven times larger than he does to us. Mercury’s year is about eighty-five days in length, so the seasons must be shorter if they follow the same rotation as ours. It passes through space with an exceedingly rapid motion, and so probably the ancients called the swift planet Mercury after the winged messenger of Jove.

Mercury is not an easy planet to observe, owing to its proximity to the sun, yet the ancients managed to descry it. But it can be seen just before sunrise and sunset in autumn, and in spring if the weather be clear. It possesses phases similar to our moon. Some authorities have stated that Mercury has an atmosphere, but this circumstance, as well as its formation, is still shrouded in mystery. Mercury’s day is a few minutes longer than ours.

A transit of Mercury is represented in the accompanying illustration (fig. 577). This phenomenon took place in 1845, but there have been many others noticed. The first recorded took place in November 1631, and these transits always occur in May or November.

VENUS.

Venus is the planet next in order, and revolves about 66,000,000 of miles from the sun. It is the nearest planet to the earth, and is somewhat smaller than the latter. This planet is both a morning and evening star, and is very brilliant—so much so, that any close observation with the telescope is not possible; and when at her nearest point she is invisible as she passes between us and the sun, and of course when fully illuminated she is directly beyond the sun, and enclosed in his rays. But under other circumstances she is distinctly visible as a crescent in the evening, and nearly full as a morning star. Venus goes round the sun in 224 days, and her day is rather less than ours.

Venus has long been celebrated as the morning and evening star, as “Lucifer” and “Hesperus.” “Lucifer, son of the morning,” is mentioned by Isaiah. That Venus possesses an atmosphere denser than our own can scarcely be doubted. The observations made during the successive transits, particularly the last (1874), seem to have established the fact that aqueous vapour exists around, and water in, Venus. No satellite can be found, though the ancients reported such an attendant upon this planet.

The apparent diameter of Venus varies considerably in consequence of her varying distances at the inferior and superior conjunction. When nearest the earth, if she presented her fully illuminated disc to our gaze, we should see a miniature moon, and even under the circumstances Venus throws a shadow, so brilliant is her light.

The transits of Venus have been referred to, and, like those of Mercury, are simply a passing, or “transit,” of the planet across the illuminated disc of the sun. The transits afford means to ascertain the volume and distance, etc., of the sun, and this year (1882) the next transit is expected. There will not be another for more than one hundred years.

Whether Venus has a constitution similar to our globe is of course doubtful. The matter is less dense than the earth, and there is an atmosphere half as dense again as ours. Spots have been noticed crossing the planet, which may have been vapours or clouds, and the rotation of Venus on its axis was calculated from these spots as being 23^h 21^m 22^s. The seasons in Venus must be very different from ours, as her inclination is greater than our earth, and as the sun is so much nearer to her than to us her tropical and polar regions are close, and a vertical sun is scarcely enjoyed by two places for three successive days, and she may have two winters and summers, two springs and autumns!

MARS.

Having already considered the earth, we pass on from Venus to Mars. The orbit of the latter planet is exterior to the earth’s, as is proved by his never appearing “horned,” nor ever passing across the sun’s disc. Therefore no “transits” of Mars can take place as transits of Venus and Mercury.

Yet Mars is most favourably situated for astronomical observation by us, because it turns its full disc to us. Venus is nearer to us than Mars—but, as we have explained, when she comes nearest to us she is quite invisible. Astronomers have been enabled to ascertain a good deal concerning the planet of war—“the red planet Mars.”

Mars has been considered very like the earth. We perceive seas and continents, and the shape of Mars is like the earth. But our globe is larger than Mars, which is much less dense, so the force of gravitation is less also. Mars moves upon his axis in about twenty-four hours and a half, and takes rather more than 686 days to revolve round the sun. (See page 489.) Thus its days are a little longer than, and its years twice as long as our days and years. When in “opposition,” or on the opposite side of us from the sun, Mars is at his brightest. This happened in September 1877. He will come close again to us in 1892.

All planets are wanderers, but of all the wanderers Mars has the most eccentric orbit. He curls about, so to speak, in loops and curves in a very irregular manner, and therefore his distance from the earth varies very considerably; and this eccentric behaviour of the warlike planet must have, as we believe it did, puzzled the ancients very much. But—and here reason came to human aid—this very fact, this great eccentricity of the planetary motions, caused Copernicus to investigate the subject with great attention, and he at length explained the true reason of these irregular orbits from the hypothesis that it was around the sun, and not around the earth that the planets moved in regular orbits.

It is quite ascertained that Mars is very like our earth in miniature. We annex a diagram of the planet, and when it is examined with a good telescope the seas and continents can be quite distinctly perceived. At the poles there appears to be a white or snowy region at varying periods, which would lead us to the conclusion that the atmospheric changes and the seasons are similar to our own; and as the inclination of the planet is nearly the same as the earth, this supposition may be accepted as a fact.

Thus we see that Mars is the most like earth of all the planets, and its inhabitants—if, indeed, it is now inhabited—must have a beautiful view of us when the weather is fine, for we are so much larger. Mars is also attended by two satellites, or moons, as Professor Hall reported from Washington in 1877. These moons have been named Deimos and Phobos, and are both very small, their diameter being only about six miles; but late astronomers have reasoned that they must be three times this diameter.

There have been numerous theories concerning Mars being inhabited, and of course these suggestions made respecting life on one planet may, with varying circumstances, be applied to another. Each planet may have had, or may yet have, to pass through what has been termed a “life-bearing stage.” We on earth are at present in the enjoyment of that stage. So far as we can tell, therefore, Mars may be inhabited now, as he bears much the same appearance as our planet. Certain changes are going on in Mars, and all planets, just as they go on here in our earth, and as they did long, long ages before the earth was populated, and which will continue to go on after life on the earth has ceased to exist.

Mars is, as we know, much further away from the sun than earth is, and must receive less direct heat. When he was created, or formed, we can only conjecture, but in all probability he cooled before the earth did, as he is smaller. Here another theory concerning the state of Mars arises, and in support of it we may quote an American authority upon the planet.

“His mass is not much more than one-ninth of the earth’s, while his surface is about one-third of hers. Then, if originally formed of the same temperature, he had only one-ninth her amount of heat to distribute. If he had radiated it away at one-ninth of her rate, his supply would have lasted as long, but radiation takes place from the surface in proportion to the surface, hence he parted with it three times as fast as he should have done to cool at the same rate as the earth, and must have attained a condition which she will not attain until three times as long an interval has elapsed from the era of her first existence than has already elapsed. Geologists agree that the last-named period must be measured by many millions of years; hence it follows that twice as many millions of years must elapse before our earth will be in the same condition as Mars, and Mars must be three times as far on the way toward planetary decrepitude and death as our earth. Then assigning two hundred thousand years as the extreme duration of the period during which men capable of studying the problems of the universe have existed, and will exist on this earth, the theory holds that Mars would have entered on that stage of his existence many millions of years ago, and that the appearance of the planet itself implies a much later stage of planetary existence.”

Mars is a very interesting study, and the reddish hue which is so distinctive is perceived in certain spots when examined by the telescope’s aid. These red places were discovered by Cassini. Mr. Dawes made drawings of Mars, and Mr. Proctor has by their aid constructed a regular map of Mars, and a chart of the surface of the planet. There is much more land than water on Mars, as the bright surfaces which indicate land are much more extensive than the darker portions which betoken the existence of water. But these “markings” are not always visible, in consequence of something coming between us and the land on Mars, and this has been attributed to the production of vegetation, which a French savant declared was ruddy-coloured, and that this autumnal tint departed in the winter.

The seasons of Mars are not equal, in consequence of his wandering propensities, and winter is warmer up there than our winter, while summer is cooler than our summer. That there are clouds and an aqueous atmosphere surrounding Mars we learn from spectroscopic observation and analysis, and in fine we may look upon Mars as similar to our earth. Respecting the question of its habitation we take the liberty to quote Mr. Richard Proctor:—

“I fear my own conclusion about Mars is that his present condition is very desolate. I look on the ruddiness of tint to which I have referred as one of the signs that the planet of war has long since passed its prime. There are lands and seas in Mars, the vapour of water is present in his air, clouds form, rains and snows fall upon his surface, and doubtless brooks and rivers irrigate his soil, and carry down the moisture collected on his wide continents to the seas whence the clouds had originally been formed. But I do not think there is much vegetation on Mars, or that many living creatures of the higher types of Martian life as it once existed still remain. All that is known about the planet tends to show that the time when it attained that stage of planetary existence through which our earth is now passing must be set millions of years, perhaps hundreds of millions of years ago. He has not yet, indeed, reached that airless and waterless condition, that extremity of internal cold, or in fact that utter unfitness to support any kind of life, which would seem to prevail in the moon. The planet of war in some respects resembles a desolate battle-field, and I fancy that there is not a single region of the earth now inhabited by man which is not infinitely more comfortable as an abode of life than the most favoured regions of Mars at the present time would be for creatures like ourselves.”

THE MOONS OF MARS.

We must devote a few lines to the satellites of Mars, which during the last four years have proved a very interesting study for the astronomers, and some very interesting facts have been ascertained concerning the ruddy planet, which is now proved not to be “moonless Mars,” as the poet declared.

There are two satellites, which, in consequence of their distance from him, being so different, vary in apparent size. The outer one is twelve thousand, the inner one about three thousand five hundred miles from the planet, so the former would revolve in about thirty hours in a direction from west to east, and the inner moon goes round in the same way in about seven hours and a half. Mars revolves in twenty-four and a half hours from west to east. So the outer moon rises for him in the east, and the inner one in the west. This is accounted for by the fact that one travelling slower than Mars rises in the east, the other outruns him, and comes up in the west.

But if we suppose ourselves upon Mars we shall find that, after all, we have only one moon properly so called. The outer satellite is very small and very far away, so it is useless to give light—at most, it is no bigger than Mars appears to us on earth. So the Martians do not see two moons passing each other in the sky—that is, unless their eyes are of greater range and power than ours. Thus they have one moon rising in the west, appearing in all its phases every night, while our moon takes twenty-eight days to pass through her phases; for we must remember that Mars’ moon takes only seven hours and forty minutes to pass through its orbit, and therefore each quarter will not occupy quite two hours.

THE MINOR PLANETS, OR ASTEROIDS.

Passing onward from Mars towards Jupiter we arrive at a number of smaller planets, which will not concern us very much, as they are very small and scarcely visible without a good telescope. But a very interesting chapter in the history of astronomy was commenced when the discovery of these bodies was begun. In old times astronomers noticed a very considerable gap between Mars and Jupiter, which was remarkable when the regular progression of the distances between the planets was remembered. So Kepler was of opinion that some planet would be discovered having its orbit in that space between Mars and Jupiter. It is, however, to Piazzi, the Italian, that the discovery of the zone of asteroids is due.

Piazzi was surveying the constellation Taurus, where he fancied he had discovered a change of place in a star which he had observed on the 1st of January in that year (1801). He was quite sure of this change next day (the 3rd of January), and he expressed his opinion to Bode and Oriani. But letters took a long time to pass in those days, and when the other astronomers had received the advices the new star had been lost in the sun’s glory. But after a year, on the 31st December, 1801, the planet was again seen and the discovery was proved. The new planet was named CERES.

The discovery of Ceres led to other discoveries. For, while searching for her, Olbers found other minor planets, and so on to the present day. Now we have nearly two hundred asteroids, and more are probably to be found in the zone beyond Mars.

It would answer no purpose to give a list of the asteroids. We need only remark that the first four were discovered in quick succession, and then a lapse of thirty-eight years occurred before the fifth was found, thus—

CERES, discovered by Piazzi at Palermo, January 1st, 1801. PALLAS, “ Olbers at Bremen, March 28th, 1802. JUNO, “ Harding at Lillienthal, September 2nd, 1804. VESTA, “ Olbers at Bremen, March 20th, 1807. ASTRÆA, “ Hencke at Driessen, December 8th, 1845.

Since 1848 there have been numerous minor planets discovered every year.

The hypothesis that all these asteroids are fragments of one large planet which has been destroyed was started by Olbers; and in confirmation of this view it has been determined that the asteroids have essentially the same character. The orbits of these minor planets are different from the larger “wanderers,” and cross each other, as will be seen from the accompanying diagram, so that a collision may one day ensue.

Planetoids and extra zodiacal planets are titles which have been bestowed upon these bodies, of which VESTA is the first in order in the system, and revolves in 1,325 days, at a mean distance of 225,000,000 of miles from the sun. JUNO and CERES take each about four of our years to revolve in their orbits, at greater distances still, averaging 260,000,000 of miles. Pallas and Ceres are most alike in their periods and distance from the sun; the principal asteroids are only about 300 miles in diameter, while the smaller are very tiny indeed, and one certainly has quite disappeared.

JUPITER, THE GIANT.

Jupiter has been well named the Giant planet, since his diameter is eleven times greater, and he is thirteen hundred times larger than our planet. His inclination is very small, and you now know that under such circumstances he enjoys very small changes of seasons. Jupiter has four moons, or satellites, and an illustration of the “Jovian System” is herewith given.

Jupiter himself was well known to the ancients, but Galileo was the discoverer of the “moons.” His telescope was, of course, a very imperfect instrument, and while he was gazing at the planet he noticed three stars close by the bright disc, two on one side, but next day Galileo perceived them all at the same side. Next time he looked there were only two, and after many anxious observations he found out, not only that Jupiter had three attendant stars, but four!

These moons were found to revolve round Jupiter in times varying from nearly two days to nearly sixteen days, according as they were at a less or greater distance from him. They were found to have their times of eclipses and transits, etc., also. These moons act with respect to Jupiter very much as the inner planets act with respect to the sun, for observation showed Galileo that the satellites sometimes appeared on one side of the planet, and at other times on the opposite side.

From the diagram of the Jovian System we shall understand the orbits of the moons, which are all of nearly equal size,—two thousand miles in diameter,—and cause eclipses of the sun to Jupiter. If the earth be in the same direction as the sun the moons are lost to view. The satellites disappear into the shadow, and are eclipsed at 1′″, 2′″, 3′″, 4′″, respectively, but they do not always come into view again immediately they have passed through the planet’s shadow, because the earth is a little at one side of the sun. So when the satellite gets behind the edge of Jupiter, his shadow being on the opposite side to the satellite’s, it is said that the “moon” is in “occultation”; when it disappears in the shadow it is “eclipsed.” Cassini discovered the “transit” of Jupiter’s moons. The annexed diagram illustrates the eclipses, etc., very clearly. At the four points, A B C D, we have the earth; J is Jupiter with his moons; 1 2 3 4 is their orbits. At a moon No. 1 enters his shadow, and emerges at b. From the earth at D a will be visible, but not b, because Jupiter is in the way. So at B, the coming out, or emersion, will be visible, but not the entrance into the shadow, or immersion. At A the satellite is in transit d, on the disc of the planet, J.

From the observation of the eclipses of Jupiter’s moons the rate of the transmission of light was discovered by Roëmer in 1675, and its progressive motion was calculated. The eclipses were noticed to take place later than the calculated time, when the planet was approaching conjunction. Roëmer suggested that the delay was owing to the greater distance the light had to travel—a distance equal to the diameter of the earth’s orbit, or about 190,000,000 of miles. The time was about sixteen minutes. Light was found to travel at the rate of nearly 12,000,000 of miles a minute.

Let us now endeavour to picture Jupiter himself. Here we have an illustration of the planet. He is the biggest, and the brightest, except Venus, of all the planets. He revolves at a distance of 476,000,000 of miles from the sun, and his year is equal to nearly twelve of ours, while his day is scarcely ten hours long, showing a rapidity more than twenty times the rate of our earth. Jupiter, therefore, must have a very much greater diameter than the earth.

There is much less sunlight and heat found on Jupiter than upon Earth, because he is so much farther from the sun than we are, but at the same time the heat comes at less intervals than with us. And here the theory already noticed respecting the gradual cooling of the planets will be remembered. Jupiter, we can easily imagine, would take much longer to get cool than Mars or the earth, and, though his rapid rotation would assist him, he must be still in the midst of a glowing atmosphere without form and void—perhaps a furnace for cloud and vapour generation.

Now when Jupiter is examined with the telescope it will be seen that he is crossed by belts of vapour (see also page 489); and when we consider the results of the spectrum analysis of the planet, we may fairly assume that Jupiter is in a very heated state, and that we cannot really perceive the actual body of the planet at all yet. There is an immense quantity of water thus surrounding Jupiter, and he is still in the condition in which our earth was before geology grasps its state, and long ere vegetation or life appeared. The waters have yet to be “gathered together into one place,” and the dry land has yet to appear upon Jupiter, who is a very juvenile, if a very enormous planet. Under these conditions we can safely assume that there are no inhabitants of Jupiter.

The belts, or zones, of Jupiter vary in hue, and the continual changes which are taking place in this cloud region tend to show that disturbances of great magnitude and importance are occurring.

It is useless to speculate upon what will happen in Jupiter when the disc is eventually cooled. The planet, we know, has not nearly reached maturity; the earth is in the full prime of its life, and the moon is dead and deserted. What the millions of years which must elapse before Jupiter has cooled may bring forth we need not try to find out. The earth will then, in all probability, be as dreary as the moon is now, and we shall have returned to dust.

SATURN.

We now come to the most curious of all the planets—Saturn, which is an immense globe surrounded by a beautiful bright ring, or rather series of rings, and attended by eight moons. He appears to possess much the same constitution as Jupiter, but enveloped in an even denser atmosphere than the latter. Saturn’s diameter is about nine times greater than the earth; he revolves on an inclined axis in about ten hours, and has seasonal alternations of unequal length. His year is about thirty of ours (10,759 days). The most striking phenomena in connection with Saturn are his rings.

Saturn’s rings are supposed to be a close agglomeration of stars, or satellites, revolving around the planet and encircling him in a belt. The rings are apparently broad and flat and thin, resembling roughly the horizon of a globe.

The globe of the planet is not exactly in the centre of the rings, which have been measured, and are approximately as below:—

Diameter of exterior ring 169,000 miles. Diameter of interior ring 124,000 “ Diameter of innermost ring 100,000 “ Interval between innermost ring and Saturn 19,000 “ Intervals between the rings 18,000 “ Thickness of the rings 130 “ Breadth of the rings 37,000 “

The rings were first recognised as such by Huyghens in 1659, but Galileo had remarked the curious appearance the planet presented. Cassini confirmed Huyghens’ discovery, and found that the ring was duplicated, and Mr. Ball made the same discovery. The two outermost rings are very bright, the inner ring being darker and partially transparent, for the ball of Saturn can be perceived through it.

But the rings are not always so plainly seen as in the foregoing diagram. Sometimes they appear as a mere line of light on each side of the planet, as shown in the margin. This occurs at the time of the equinox (fig. 591). By degrees, however, as they become inclined, they appear broader (fig. 592). The inner ring may be formed of vapour, but the outer ones are of something more solid, as the shadows they cast upon the planet, and it casts upon them, at certain times (figs. 593 and 594).

Saturn possesses eight moons, seven of them revolving in orbits on the plane of the rings, but one is more inclined. These eight satellites have been named as follows:—

+-------------+-------------------------+-------------+-----------------+ | Name. | Distance from Saturn in | Time of | Discoverer. | | | radii of Saturn. Miles. | Revolution. | | +-------------+-------------------------+-------------+-----------------+ | | | d. h. min. | | | 1 Mimas | 3·36 (about) 120,000 | 22 37 | Herschel. | | 2 Enceladus | 4·30 “ 150,000 | 1 8 53 | Herschel. | | 3 Tethys | 5·34 “ 190,000 | 1 21 18 | Cassini. | | 4 Dione | 6·84 “ 240,000 | 2 17 41 | Cassini. | | 5 Rhea | 9·55 “ 340,000 | 4 12 25 | Cassini. | | 6 Titan | 22·15 “ 790,000 | 15 22 41 | Huyghens. | | 7 Hyperion | 26·78 “ 945,000 | 21 7 7 | Lassel and Bond.| | 8 Iapetus | 64·36 “ 2,250,000 | 79 0 8 | Cassini. | +-------------+-------------------------+-------------+-----------------+

But these eight moons are not so interesting as those belonging to Jupiter, because the great distance they are away precludes much examination of them. They vary much in size, Titan being the largest, and perhaps equal to Mars, Iapetus being next in magnitude. The light of these satellites and the rings is no doubt very great in the aggregate, and must have a magnificent appearance in the heavens (compare page 493). Very likely there are other attendants upon Saturn, but owing to the brilliancy of the rings it is impossible to distinguish them.

URANUS.

Uranus was discovered by Herschel in 1781, and has been called after its discoverer, and sometimes the “Georgium Sidus.” It revolves at an enormous distance from the sun—viz., 1,753,000,000 of miles. It takes about eighty-four of our years (30,686 days) to go round the sun, and possesses four moons. It is very much larger than the earth—about four times the diameter, and forty times its volume. We can only speculate concerning its physical constitution, which is assumed to be similar to that of Jupiter, while the changes of temperature and seasons must vary immensely. The names of the moons are Ariel, Umbriel, Titania, and Oberon. The outer pair can be seen without much difficulty.

NEPTUNE.

The existence of this planet was determined by calculation before it had been seen at all. Uranus was observed to be disturbed in his orbit, moving sometimes faster than at others; and even before Uranus had been discovered Saturn and Jupiter had been seen to be affected by some body in the system. M. Leverrier determined to ascertain the cause of this, and came to the conclusion that some other planet was influencing Uranus. The Newtonian theory here received a most convincing proof. While Leverrier was calculating, Mr Adams of Cambridge leaped to the same conclusion, and wrote the result of his calculations to Professor Airy, and the planet was seen, but not reported upon. Meantime Leverrier published his calculations, and the observers at Berlin detected the new planet in September 1846.

Very little can be said concerning Neptune, as its distance is too great for observation. It is at 2,746,000,000 of miles from the sun, and takes 164 years to go round it (60,126 days). It is about the same size as Uranus. It has one moon, which moves round the planet in 5^d 21^h, and is of great size.

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