Familiar letters of the physics of the Earth : treating of the chief movements of the land, the waters, and the air, and the forces that give rise to them

Front matter

FAMILIAR LETTERS

: ON THE

PHYSICS OF THE EARTH:

Letter V. Hot Springs And Jets Of Steam

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X11 CONTENTS.

LETTER VL.

JETS OF GAS, AND MUD-VOLCANOES ieee ese

LETTER VIL.

VOLCANOES AND EARTHQUAKES

LETTER VIIL.

THE TEMPERATURE OF THE OUTERMOST CRUST OF THE EARTH

LETTER IX,

THE TEMPERATURE OF THE LOWEST STRATUM OF THE ATMOSPHERE +a rr eee

LETTER X.

THE TEMPERATURE OF THE HIGHER LAYERS OF THE AIR

LETTER XI.

THE TEMPERATURE OF THE WATERS, AND THEIR INFLUENCE ON CLIMATE

LETTER XII.

THE CURRENTS OF THE SEA

Page a

LETTER XIV.

CONTINUED ; THEIR INFLUENCE a oa x LETTER XV. > e Aa | THE MOISTURE OF THE AIR, AND THE FALL OF WET

LETTER XVI.

TH f ELECTRICITY OF THE ATR, AND LIGHTNING ... 263

~

LETTERS

ON THE

PHYSICS OF THE EARTH.

LETTER LI.

GRAVITY, AND ITS EFFECTS.

THE ever-varying conditions of the earth and of its surrounding atmosphere, the great changes, which in the lapse of ages have passed over its surface, and the mysterious forces, which are at work within it, have occupied at. all times the thoughts of our race; giving rise, as they do, to numberless strikmg phenomena, and exerting a direct influence upon our physical welfare. Hence at all times thinking men have endeavoured to obtain an insight into the laws which rule the Life of our Planet. It is now no longer doubtful, that the primeval shocks and revolutions, through which the earth has evidently passed, and to which it owes its present form and state, were manifestations of the same natural forces, whose action, sometimes B x GRAVITY, AND ITS EFFECTS.

formative, sometimes destructive, we continue to witness at the present day. |

Two of these powers, which work throughout the entire universe, and bring under each other’s influence all the heavenly bodies— Gravity and Heat —stand out as the maimsprings both of the past revolutions, and of the present changes of the earth.

Gravity is the principle of attraction and solidity; Heat of expansion and mobility. All material things bear within them these two sources of action. On the one hand, Science points to gravity as an attractive force, put forth by all bodies alike—a force acting with a strength which, for any given distance, 1s always the same, but which is lessened as the distance is mereased. On the other hand, it reveals to us heat as a principle subtle m the extreme, which penetrates the substance of all bodies more or less easily, and accumulates m them to a greater or less amount; it © issues again from these bodies, when it does escape, im rays, that is, m straight lines, diverging from each other im all directions. The intensity of the heat-rays also is lessened as the distance from their source is mereased, according to the same law as that which governs gravity.

General Gravity (gravitation)—the mutual attracting power of all heavenly bodies—links the earth with its companion, the moon, and leads the

EARTHLY GRAVITY. 3 latter in its path round the former, and both in their yearly course around the sun. It maintains the equilibrium of our own solar system, and also that of the whole universe.

Earthly Gravity—the mutual attracting power of the atoms of the earth—tends to arrange them about a common central pomt. If the earth be supposed to be at rest, and covered with water, the attractions exerted upon any one particle near the surface by all the other particles, must together make up one compound force, acting perpendicularly to the motionless surface; for with any other direction of the force, the liquid would yield to it and flow over the surface, which would thus cease to be motionless. The sum of the attractions of all the other particles for any single particle, or for any body upon the earth, is what is called the weight of that particle or body. You will thus see why the direction of the pressure, which a body exerts by its weight, must of necessity be perpendicular to the surface of still water, quite idependently of the general form of the earth. This form, however, can be no other than a sphere, supposing for the present that the earth is covered with water, and at rest; for it is the only form in which all the parts of the surface keep a lke position with regard to each other, and therefore suffer an equally strong attraction. Hence it is the only form in which a state of rest is pos-

B 2 sible for the surface, the earth itself not mee in motion.

The earth, however, is not at rest; it turns round once every day on its axis; so that every point on its surface travels round a circle. The circumferences of these circles answer to the parallels of latitude of the globe. Their diameters are not equal, but imcrease from the poles to the equator, where their length is greatest. Now you are aware, that any body whirling im a aircle, like a weight hung by a string, is always trying to fly from the centre round which it moves, and does actually fly from it, if not kept back by a sufficient power. ‘This tendency is called the “centrifugal . Force.” Its strength mereases, as may be ascertained at once by the ‘simplest experiment, in proportion to the mass of the whirling body, and to the quickness of its motion.

If two bodies of equal weight accomplish their revolutions in equal times, as is the case with all - bodies on the earth, that one gains the greatest centrifugal force which travels m the greatest circle.

Now, by reason of the daily rolling of the earth round its axis, every particle of it must also get | this centrifugal tendency. This is greatest at the equator, and, diminishing as the latitude increases, vanishes at the poles.

Under the equator, the centrifugal force is di-

CENTRIFUGAL FORCE. 5 rectly opposed to that of gravity, which is however far greater than the former; otherwise the bodies would be thrown off from the earth. It has been calculated, that if the earth turned round seventeen times as quickly as it does, the centrifugal force at the equator would become exactly equal to the force of gravity. Bodies, m that part of the earth would then seem to have no weight; and anything that might be thrown up to a height, would not return to the earth, but would travel round it, ike the moon.

In fact, the moon is in the condition of a body whose centrifugal force is just balanced by its weight. Since the distance of the moon from the earth is known, the strength of the attraction between the two planets is easily calculated. It is found to be three thousand six hundred times less than the force of gravity at the surface of the earth. The centrifugal force of the moon, due to its revolving round the earth, is just equal to this ; so that the one of these forces always balances the other, and thus the moon is hindered from ever straying from its path around the earth. The earth and the other planets are in exactly the same condition as regards the sun.

Since the centrifugal force of bodies on the earth acts perpendicularly to the earth’s axis, this force cannot act in the direction of gravitation anywhere except at the equator.

For this reason the particles of water north and south of the equator are subject to a force or pressure, whose direction, for any given pomt, is that of the resultant of the centrifugal and gravitating forces at that pomt. But since, m order that the water may be at rest, itis necessary that this direction should be perpendicular to the liquid surface of the earth, it follows that the form of this surface cannot be that of a true sphere. Moreover, since the centrifugal force, which, at the equator, is greatest, has there also its whole effect directly opposed to gravity; and since, on the other hand, it not only gradually lessens in power as the latitude increases, but is also less and less directly . opposed to gravity, it follows from this, that the downward pressure of bodies at the surface of the earth is least at the equator, and increases towards the poles. The same result must therefore ensue as you may see happen to a blown-up bladder, or to a ball of imdia-rubber, when more strongly » pressed on two opposite sides than on any other part,—it must become flattened towards the points of greatest pressure.

On these grounds, Newton had conjectured that the liquid part of the surface of our earth was not quite, but only nearly, spherical; and that, towards the poles, it must be somewhat pressed together ; and this long before his reasoning could be confirmed by actual measurement. We

SHAPE OF THE EARTH. 7 now know, from the mean result of numerous measurements, that the flattenmg amounts to =1,, or that the half of the earth’s axis is 69,275 feet shorter than the half diameter of the equator, of which the length is 20,922,668 feet.

It is true that the solid part of the earth’s surface does not exactly take the curvature of the sea-level, but is in general uneven and slopmg. The loftiest mountains, however, do not reach much beyond twenty-eight thousand feet above the sea, a height far less than the amount of flattenmg at the poles. Besides, most of these he in the lower latitudes, that is, nearer to the equator than to the poles; while hitherto, only very little land, and that for the most part very low, has been found within the polar circles. The curvature then of the main land is the same in general, if we do not consider its partial unevennesses, as that of the sea; and, therefore, the form of the whole earth is that of a sphere somewhat flattened at the poles.

The deviation of particular parts of the land from this form, or the amount of slope at any place, is readily discovered from the angle which the slanting surface makes with that of stil water; for water must, as I have above explained to you, always set horizontally, that is, so that the direction of the earth’s attraction (or, rather, of that attraction as changed by the centrifugal force) shall be at right angles (perpendicularly) to its surface.

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Again, the direction of rest taken by a string to which a weight is hung (the Plwmé-lne), shows the position of the horizontal plane ; for this direction can be no other than that of gravity: because, so long as the string does not lie in the line of this force, the weight attached to it, which is so very easily moved, cannot come to rest. _

Heavy bodies, free to move, fall in the direction of the plumb-line; that is, their Ime of motion is perpendicular to the horizontal surface of the earth. They fall the more quickly, the greater the power which draws them down. They fall least quickly at the equator, more so as the latitude increases, and most quickly at the poles.

The increase, from the equator to the poles, of the speed of fall cannot be learned by the immediate observation of bodies freely falling, since this motion is everywhere too swift to allow of truly measuring the spaces fallen through im given times. However, we can make our observations | with the greatest certaimty and accuracy, by means of the Pendulum, or swmging plumb-line ; for it is nothing but the force of gravity that makes the pendulum fulfil its swings, or beats. It begins to move, and goes on swinging, because it has been removed from the position in which the cord hangs straight, and m which alone it can remain at rest. Stmce gravity is the cause of the beats of the pendulum, these must be faster or slower, or

THE PENDULUM. 9 the time of each swing must be shorter or longer, as the power of gravity increases or decreases. The beats of a given pendulum, whose length remains unchanged, mark, as you know, equal lengths of time, which may be measured with surprising accuracy. ‘The slightest changes, therefore, of the power which causes them, may be discovered by the alteration of the time of the swing.

Let then such an instrument—for instance, one that beats seconds, and which is so made as to set night the changes wrought in it by the rise and fall of temperature (called a compensating pendulum) —be put up at any place. It will, whenever and however often the time of its swing may be ascertained by a chronometer, always beat seconds. From this constancy of the swing-time at any given place, we must conclude that the force which causes these motions is equally unchangeable.

However, every change of place alters the swing-time of a given pendulum. Placed on a high mountain its beats are perceptibly slower, and they are quicker the nearer it is taken to the poles. The increase, therefore, of the swing-time on rising to a height, and its decrease by removal, on the level, from the equator to the poles, give us a sure proof of the decrease of gravitation as the distance from the centre of the earth increases, as well as of the flattening of the earth at the poles.

By means of the swing-time of the pendulum,

Wt Oe en vy ltt ’ .* aa Sins’ eyelet = ‘ - we certainly discover, m the first mstance, only the strength of the earth’s attraction at different points of the surface. But it is clear that, from the proportion which the decrease of this force bears to the increase of the upright height, the law must come out, according to which gravitation lessens as the distance of the pomts which attract each other becomes greater. And this law bemg once known, the distance of any pomt on the surface from the centre of the earth may be at once found, by reckoning from the strength of gravitation as observed at that pomt. This problem being solved, due regard bemg had to the effect of the centrifugal force, the amount of flattening is found. The curvature, as discovered by this means, agrees very closely with that found by direct measurements of arcs of the meridian, a process, to treat of which would be to enter the domain of astronomy.

I must not leave the subject of the pendulum, without reminding you that pendulums, differing | in bulk and in material, swing in equal times, if their length is exactly the same; thai is, they are equally subject to the force of gravitation.

Thus, then, the same plumb-line, which in common life we employ to find a horizontal level, or the true upright line for the walls of a house, and which, swinging as the pendulum, rules the goingrate of our clocks, becomes im the hands of the physical philosopher a most precious instrument.

FLOW OF WATER. 1)

By this same simple means, he settles by actual trial, and measures (what had been foreseen by theory) the deviation of the form of the earth from that of a sphere; and by its aid he values, both as to kind and as to amount, the attractive power that governs the particles of earth, and binds them together as a whole, which no force from without can sunder.

_ The same cause which gives rise to the swinging of the pendulum, forces every moveable body to strive for such a position as shall place its centre of gravity as low as possible, and as shall ensure as far as possible its remaining in a state of rest. First then we see that water runs down from mountains and sloping land, till it can find a basin which allows of no escape, in which it can take for itself a level surface, that is, one agreeing with the curvature of the earth, and in which it thus can come to rest. The surface of flowing water in brooks and rivers is not level; if it were so, that is, if the direction of the gravitation of the moving particles were perpendicular to the surface, the force that draws them onward would cease, and the motion which they had already would quickly be stopped by the resistance of the river’s bed. The slope of the surface of running water, or its fa//, agrees in general with that of the bed on which it moves, and is therefore usually greater

in highlands than in plains. The speed of the

12 GRAVITY, AND ITS RESULTS.

stream increases, if the fall becomes greater. Those rivers are therefore the swiftest that have the greatest fall. The latter, however, as I have already said, answers only 2m generad to that of the bottom. When the bed is narrowed for a certain distance, so that the water cannot get through with the speed at which it had been flowing, the fall at once mcreases; that is, the water is heaped up behind the narrow part of the channel, even when the slant of the bottom remains the same. On the other hand, the effect of the little unevennesses in the bed of the river, such as wave-like ridges, and hollows, and even steep slopes, which, when the water is shallow, may cause rapids for short distances, is more and more smoothed down as the level of the water rises. or the greater switftness with which the stream is now running at the steeper places, must necessarily heap up, as it were, the water 1n the lower parts of the river, where before the rise it had been running more slowly ; and thus, — the more the water rises, over the greater length is its fall spread. Besides, the swiftness of a stream does not depend on the fall alone; for, smce the water is always being robbed of part of its motion by the resistance of the ground, and since this loss must ever be made good again by the fall, it is clear that any lessening of the resistance of the ground must enhance the speed of the water, just as does any increase of the fall. In shallow rivers,

FLOW OF RIVERS. Ly.

the whole of the water touches the bottom, in its turn, in a shorter time, and thus meets with more | stoppage. This cannot happen so much with deep water, because the film that touches the bottom is much smaller in proportion to the bulk of water that is above it. This shows why the water taken across the breadth ofa stream, at any given place, runs more slowly in the shallow parts than in the deeper; and why again the rate of flow of brooks and rivers is greater when they are swelled by floods. And now you will see why, when, in very winding rivers, suitable cuttmgs are made, which shorten the channel, and for equal lengths increase the fall, the level of the water above must be lowered. The same cause, too, which in every case favours the draining of the water from above the cutting, will not fail, in case of sudden floods, to raise the level in the part below for a time; and this can only be prevented by deepening or widening the channel.

The greatest check which the speed of running water meets with is in large basms, such as lakes, through which it has to run, and in which its motion becomes almost nothing. If the size of a lake is very great in comparison with the width of a river that runs through it, any flood of the latter that soon passes off can but slightly raise the level of the former, and therefore cannot much increase the outflow of water from the lake. Large lakes are, therefore, truly governors of the water-level of ariver. Thus, the great lakes of Switzerland, all but one of which pour off their waters through the Rhine, gather durmg the spring a great part of the snow-water from the mountains, and letting it off slowly, usually keep plenty of water im the Rhine till the summer is pretty far advanced.

The stock of water in the mountains would be thus soon drained off, were it not that the heat returns it to the air in the state of vapour, which is then set down again on the high lands as rain and snow. Thus the waters travel m an everlasting round, which it seems must end at last in making smooth every unevenness on the face of the earth.

By the solvent power of-water, some parts of the solid surface of the earth are always bemg made liquid; others, laid open to the combined influences of water and air, are changed in their nature, and, becoming weathered, moulder away. Thus the higher parts of the earth are brought, by degrees, into a moveable condition, and are carried down, partly by their own weight, partly by the force of running water, to the valleys, the plains, and the sea. We see this levellmg gomg on everywhere on the solid land. Stones, and masses of rock, get loosened from the hill-sides, and are slowly fillmg up the hollows. Even the very mountain-peaks, slowly undermined by the trick-

LEVELLING EFFECT OF WATER. 15 ling water, and robbed of their supports, are tumbling headlong. Every brook, and every river, more or less, according to its speed, is bearing downwards earth and rolling stones, and, where its stream is slackened, is leaving them as a settlement of flood-soil. Where the rivers run but slowly, their bottom is raised; where they swiftly rush along, the sea-bed at their mouth is filled up by degrees. Thus have the Netherlands been won from the sea, step by step, by the burden set down by the Rhine at its journeys end; and thus has Lower Egypt been built up by the rubbish shot by the Nile. And so we see the coasts at the Mississtppi’s mouth stretched outwards almost year by year, by the huge bulk of earth and tree-trunks flooded down by the mighty river.

Letter I. The Tides

It is not without opposing efforts on the part of the other heavenly bodies that the earth maintains its shape: these efforts indeed are ceaseless, and are put forth most powerfully by the moon by reason of its smaller distance, and by the sun by virtue of its enormous mass. It is true that these forces are unable to draw the solid parts of the earth straightway from their places; but they are strong enough to set up some peculiar periodic movements in the readily.lifted masses of the air and of the sea. Those of the atmosphere are not easily seen, and are almost masked by other regular and irregular currents. Those of the sea, however, meet our view at once: they consist m an alternate rising and falling of its surface, each recurring regularly twice in every day, and are generally known as the edd and flow of the water, or, shortly, as the Tides.

On all sea-coasts, where ~he tides are felt, the waters are found at all times, even when the air is quite calm and clear, to be in restless motion. Great waves are ever rolling in, and breaking on rocky shores into foam and spray. All parts of

SEA-SHORE SCENERY. i by the coast-wall that can be loosed from their hold, are step by step torn off by their ceaseless attack ; and the firmest rocks are slowly worn and ground away by the sea-sand driven mm the water. Numbers too of plants and of animals living im the sea, such as shell-fish, with the remains of fishes, are thrown up and left upon the beach.

It is only on flat or gently sloping shores of narrow-mouthed gulfs, rannimg far up into the land, or on coasts where the force of the oceanswell is broken by sand-banks, or by reefs of,rock at some distance from the land, that this violence of the breakers is assuaged, or altogether calmed.

If you have ever been at the sea-side, at a place where the tides are strong, and where the shore slants gently to the water, you must have remarked that every wave, as it comes im, runs several paces upwards on the beach, and, immediately retreating, leaves uncovered again much of the surface which it had just overflowed, perhaps, by many feet, until the next wave arriving plays the same part again. If the tide is msmg you will observe that the retirmg water does not fall quite to its former level, and that almost every fresh wave rolls a little higher up than did the one before it. The advance is scarcely perceived at first, but step by step it becomes more evident. About three hours after the moment of lowest ebb the rise of the tide is strongest; then again it becomes slower

C and slower, till, after three hours more, the flood-. tide has reached its greatest height, at which it remains a short time without perceptible change. The fall of the tide begms as slowly as the flow, then the water retreats more quickly, and then again more slowly ; and once more, after six hours, the ebb has reached its lowest; and there, again, for a short time its level is kept unchanged.

The sea rises and falls mm this manner twice in the day, or, more rightly, twice m the time that passes between one epoch of the moon’s crossing the meridian and the next, which makes a period of 1:035 day, or twenty-five hours nearly. Thus the time of high water is later and later every day. For instance, if it is high water to-day exactly at noon, the flood will not be full to-morrow till one o’clock in the afternoon, and so on; so that after fourteen days we shall have high water again at noon, and, when the moon has fulfilled one entire journey round the earth, the third noon-day flood © comes up; and thus the tides go on.

The higher the sea rises during the flow of the tide, the lower it falls durmg the following ebb. The difference between the highest and lowest tidelevels 1s very unequal on different coasts. In the open ocean the mse of the tide is generally but little. Thus at Otaheite the Mmghest tides do not rise more than eleven inches, and at the Sandwich Islands, only two feet and a half;

AMOUNT OF RISE AND FALL. 19 and this, although, as you will soon see, it is chiefly in the Pacific Ocean that the tides are generated. At St. Helena they do not rise above three feet ; at Martinique and Porto Rico not above a foot and a half; and among the West Indies generally not four feet. Where, on the other hand, the tide meets with many hindrances to its advance, such as coasts which stop it, headlands, round which, or straits, through which, it must run, and particularly gulfs which narrow off towards thew upper parts, and mouths of rivers which are open to the direct line of its flow,—in such places, the flood may mount to an amazing height above low-water mark. ‘Thus, it is said, high up in the Bay of Fundy, between Nova Scotia and New Brunswick, the highest tides rise sixty or seventy feet, while at the same time, at the entrance of the bay, they do not reach a height greater than nine feet. On the coasts of Europe, the Bay of St. Malo and the Bristol Channel are especially remarkable for the great rise and fall of the tide. In the first of these it often rises forty and fifty feet. In general we find, on both sides of the Hnglish Channel, a rise and fall of about eighteen or twenty feet; while on the coast of Ireland, and on that of France to the south of Brest (both of which are open to the sea), it is usually not more than from four to six feet. On the German coast of the North Sea the difference between the high c 2 and low water-levels amounts to twelve feet; but round the neighbouring island of Heligoland, only to six feet. In the Baltic, and m the Black Sea, there is no ebb and flow. At the end, nearest to the ocean, of the Mediterranean there is a rise of a few inches or one foot. There are more considerable tides at the eastern end of this sea, and in the upper part of the Adriatic. At Venice and at Trieste they rise sometimes nearly four feet.

There are many places on the English shores where you may see the effect of these changes shown in striking contrasts of scenery; perhaps at none more beautifully than at [fracombe in Devonshire. But at St. Malo, on the French coast, you may witness this wonderful phenomenon even on a grander scale. ©

At low tide St. Malo itself seems to be surrounded on three sides with wild craggy rocks, which are covered with mussels and sea-weed, and among which start up the lofty walls of the town. © The level places between the cliffs are covered with a layer of fine sand firm enough for walking, and formed almost entirely of powdered mussel-shells. Here and there are pools containing water, which its taste, as well as the number of little crabs, mussels, and star-fish which may be found in it, show to be sea-water left behind by the tide. A fringe of sea-weed marks upon the rocks the line which must be reached by the sea,