The wonders of the heavens, being a popular view of astronomy, including a full illustration of the mechanism of the heavens; embracing the sun, moon, and stars, with descriptions of the planets, comets, fixed stars, double stars, the constellations, the galaxy, or milky-way, the zodiacal light, aurora borealis, or norhtern lights, meteors, clouds, falling stars, aërolites, &c.

Front matter

C o V

^0 o ^

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Preface

In the preparation of this volume for the press, the main purpose, kept constantly in view, was to make the subject plain and interesting to the people. It has been heretofore too much kept from them, by the practice of mingling mathematics with it to such an extent as to alarm the neophyte at the very threshold of the temple of astronomy.

To succeed in my attempt, I have judged it best to select, from such books as have fallen into my hands, those parts that were least encumbered with the abstruse and unintelligible, rather than to trust to my own powers of making a subject simple and attractive that is in itself almost too difficult for the majority of minds. How far I have succeeded, I must leave, not to the learned or the critic, but to the people themselves for whose perusal these pages are intended.

The principal works used in the compilation were " Uranographie par Francoeur," "Manuel d' Astronomic par Bailly," "Astronomic Physique par Biot," "HerscheFs Astronomy," "Arago on Comets, translated by Farrar," Brewster's Ferguson," Silliman's Journal," Dick's Christian Philosopher," "Chalmer's Astronomical Discourses," "Forstei-'s Atmospheric Phenomena," ifec. From many of these I have largely drawn, and the least I can do is to make a general acknowledgment of the fact. Finally, I must rest my hopes of the success of this compilation, on the curiosity so natural to the human mind, that "awakes in us when free from cares a desire to learn what is going on even in the heavens."

DuivcAJv Bradford.

THE ENGLISH AND LATIN NAMES OF THE CONSTELLATIONS DESCRIBED

IN THIS VOLUME.

English Names.

Latin Names.

English Names.

Latin Names.

The Bull.

Taurus.

The Balance.

Libra.

Orion.

Orion.

The Serpent Bearer.

Serpentarius vel Ophiucus.

The Hare.

Lepus.

The Serpent.

Serpens.

Noah's Dove.

Columba Noachi.

The Northern Crown.

Corona Borealis.

The River Po.

Eridanus.

The Little Bear.

Ursa Minor.

The Charioteer.

Auriga.

The Scorpion.

Scorpio.

The Camelopard.

Camclopardalis.

Hercules.

Hercules.

The Lynx.

Lynceus.

Cerberus.

Cerberus.

The Twins.

Gemini.

The Dragon.

Draco.

The Little Dog.

Ca7iis Minor.

The Harp.

Lyra.

The Unicorn.

Monoceros.

The Archer.

Sagittarius.

The Great Dog.

Canis Major.

The Eagle.

Aquila.

The Ship Argo.

Argo Navis.

Antinous.

. Antinous.

The Crab.

Cancer.

The Dolphin.

Delphinus.

The Lion.

Leo.

The Swan.

Cygnus.

The Little Lion.

Leo Minor.

The Capricorn.

Capricornus.

The Sextant.

Sextans.

Andromeda.

Andromeda.

The Water Snake.

Hydra.

The Fishes.

Pisces.

The Cup.

Crater.

Cepheus.

Cepheus.

The Great Bear. ,

Ursa Major.

Cassiopeia.

Cassiopeia.

Berenice's Hair.

Coma Berenices.

The Flying Horse.

Pegasus.

The Crow.

Corvus.

The Little Horse.

The Virgin.

Virgo.

The Water Bearer.

Aquarius.

The Bear Driver,

Bootes.

The Southern Fish.

Piscis Australis.

The Greyhounds.

Aster ion et Char a ^ vel

Perseus.

Perseus.

Canes Venatici.

The Head of Medusa.

Caput Medusa.

The Centaur.

Centaurus.

The Ram.

Aries.

The Cross.

Crux.

The Sea Monster.

Cetus.

The Wolf.

Lupus.

Chapter I

Section I. — Appearances of the heaven — by day — by night — Diurnal motion — Celestial sphere — Its poles— Zenith — Nadir — Sensible horizon — Rational horizon — Meridian — Equator — Hemispheres — East,-west, north and south points — Circumpolar stars— Declination and right ascension — Equality of a star's successive revolutions — how made known — Use of this regular motion to astronomers — Sidereal day — Time-keepers used by the ancients — Chronometers — Illustration of the preceding — Deductions from appearances — Stars called Fixed — Idea of immensity impressed by the study of the stars — Annual parallax — Each star the centre of a system, and possibly the satellite of some more magnificent primary . 9

Section II. — Division of the stars according to their apparent magnitudes — Their number infinite — Impartial distribution over the heavens — Milky way — Distance of the stars — Their probable dimensions and nature — Periodical stars — Temporary stars — Double stars — Their revolution round each other — Subject to the laws of gravity — Colored stars — Proper motion of the stars — Compound sidereal systems — Clusters — Nebute — Nebulous stars — Stars are visible in the day 16

Section III. — Astronomy of the ancients — Their method of dividing the stars into constellations — Constellations easily distinguished — The division arbitrary, yet convenient — Anciently was important to the husbandman — Reason of the names of the constellations — The twelve signs of the zodiac — The origin of their hieroglyphic characters — Signs and constellations of the same name not coincident — Method of studying the stars — Fables, and descriptions of the constellations — ^Remarks — The heavenly bodies unequally distant from the earth — Earth comparatively but an atom 31

Chapter II

Section I. — Erroneous notions derived from appearances — Why the stars are not visible to the naked eye in the day-time — Fictions of poetry respecting the universe — Is the earth its centre ? — Does the earth rotate ? — Difierent constellations visible at different regions — Rotation of the earth consistent with appearances — Permanence of its axis — Precision of the ancients — Discovery by Copernicus — Causes of erroneous impressions — Consequences of considering the earth immovable — Centrifugal and centripetal forces — Pendulum a means of finding the force of attraction — Measures of gravity — Attraction not a simple force — Effects of the earth's rotation — Trade winds — Proofs of wisdom in the rotation of the earth — Consequences of a changeable axis — Advantages ■ of the existing law of attraction — Perturbations periodical 56

Section II. — -Sun's apparent motion — Ecliptic — Celestial latitude and longitude — Tropics — Does the sun really move in the ecliptic ? — Impulse requisite to produce the motions of the earth — Appearance of the motions of the planets as seen from the sun — System of Tycho Br3,he — Proofs of the earth's double motion — Annual parallax — Axis of the earth always points to the same celestial poles — Its inclination to the ecliptic — Radius vector — Cause of the change of seasons — Zones — "Winter at the poles — Illustration of the foregoing — Nature of the earth's orbit — Poetical rising and setting of the stars — Perihelion and aphelion — Designing wisdom apparent from the figure of the earth's orbit. 69

Section III. — The horizon and its dip — The earth at first supposed a limited plane — Early reasoning to the contrary — Proof furnished by na,vigation — Objections caused by our ideas of weight — Answered — Conical figure of the earth's shadow — Other proofs — Different aspects of the heaven according to the position of the observer — Time difierent in different places at the same absolute instant — Method of measuring an arc of the meridian — The earth an oblate spheroid — First meridian — Original constitution of the earth — Extent of the horizon proportioned to the height of the eye — Visible portion of the earth's surface — Temperature of the earth at its surface — Internal heat — Atmosphere — Reflections on the wisdom and knowledge of the Creator — Diversities of the globe 82

Chapter III

Section 1. — Sun — Comparatively stationary — Cause of twilight — Sun's mass — Gravity — Real diameter — Probable conclusions of the solar astronomer — Sun's disc as seen from the different planets — Sun the source

VI

CONTENTS.

of heat — What proportion of solar light falls on our globe — Divine wisdom — Solar spots — ^Variable in size and number — First authentic observations on — -Scheiner imagines they are planets — Their course and changes — Sun's revolution about its axis — Theories of different observers respecting the spots — Herschel's theory prevalent — Is the sun inhabited ? — Zodiacal light — Sun's progressive motion — Herschel's theory confirmed by a late experiment in France 99

Chapter IV

Section I. — Hori2ontal moon — An illusion — Mode of estimating distances — Variation of the moon's distance from the earth — Motion from west to east — Amount of motion per day — Per minute — Moon's nodes — Syzygies — Lunar Cycle — Golden number — The moon's periodic, synodic and sidereal revolution — Disturbance of the moon's orbit. . . . 119

Section II. — Phases of the moon^Law of their variation — The new holding the old moon — Earthshine — Earth new when the moon is old, and vice versa — Proportion of moonlight at different seasons and places — Harvest-moon — Libration in latitude — In longitude — Diurnal — Lunar mountains — Atmosphere 124

Section III. — Recurrence to the phenomenon called the new holding the old moon — Moon supposed by some to be phosphorescent — Leslie's explanation of the thread of light connecting the horns of the new moon — True explanation — Surface viewed through a telescope — Mountains and hollows — Lunar volcanoes — Arguments respecting the atmosphere — Is the moon inhabited? — Discovery of a fortification and of roads in the moon 138

Chapter V

Section I. — The system of the world — Limits of magnitude and minuteness — Theories respecting the world — Ptolemy's — Egyptian — Tycho Brahe's — Copernican — Des Cartes' whirlpools — Number and names of the planets — Origin of their symbols — Their comparative distances from the sun — Their division into inferior and superior — Their periodical times — Their secondaries — Origin of the planet's names — Heliocentric circle of a planet — Aspects, what — Venus both an evening and a morning star — Its phases — Why imperceptible to the naked eye — Inferior planets — Their geocentric motions in looped curves — Mars — The form of its disc — lis orbit without the earth's— Aspects and motions of superior planets — Mode of determining the position of the orbits — Elements of an orbit — Predicting a planet's return to the same situation — Venus sometimes visible at noonday 146

Section II. — Individual planets — Mercury — Period of its revolution — Method of finding its rotation — Motion in its orbit per hour — When visible— Its diameter and distance from the sun — Its telescopic appearances — Its mountains — Its atmo.sphere — Venus — Its distance from the sun, diameter, and rate of motion — Its period— Inclination of its orbit — Telescopic appearances — Atmosphere — Rotation— Mountains— Mars — How the earth and moon appear there— Its irregularities the subject of Kepler's studies — Peculiarities in its appearance — Effects of its atmosphere on the color of the planet — Its luminous zone, and the cause of the same— Mars supposed phosphorescent — Ultra zodiacal planets — Seemingly disturb the harmony of the system — Their feeble powers of gravitation — Peculiarities of Ceres— of Pallas — of Juno — of Vesta — Giber's theory respecting the small planets. 165

Section III. — Jupiter's form — Its situation — Length of its year — Rapidity of rotation— Small change in its seasons^Alternately morning and evening star — Its belts — Degree of oblateness — Cause of the belts — Jupiter's satellites — Particulars respecting them — Velocity of light — Saturn's size compared with the earth's — Surrounded by a ring — Singular form of Saturn — Its spots and belts — Why this planet is more oblate than Jupiter — Saturn's satellites — Position of their orbits — Theories concerning the rings — Their different appearances — Their revolution — How they are sustained — Herschel, particulars of — Its satellites — Their peculiarities — How kept in their orbits 175

Chapter VI

Section I. — Comets — Little known of their nature or purposes — Their number — Comet of 1680 — The tail not an invariable appendage — What are the essentials of a comet — How distinguished from a planet — Anciently considered as meteors — Proof to the contrary — Elements of a cometary orbit— Motions of a comet — How recognised — Halley's comet — Lexell's— Encke's — Biela's — Do comets affect the temperature of our seasons ? — Physical constitution — The envelope — Nucleus — Tail — Do comets have phases ? — Variation in the size of the envelope. . . 189

Section II — Chances against a comet's striking the earth — Do they finally fall into the sun? — Or into the stars? — Can the earth draw off the tail of a comet? — The effects — Were the fogs of 1783 and 1831 caused by a comet? — Was the cholera caused by a comet? — The deluge not caused by a comet— Has the climate of Siberia suddenly changed— Is the severe climate of North America owing to a comet ? — Is the depression in the centre of Asia owing to a comet? — Was the moon ever a comet? 207

Chapter VII

Section I. — Reflections on the system — Proofs from analogy that the planets are inhabited — Their magnitude — Their rotation — Their revolution round the sun — They have moons — Blountains and valleys— Clouds and snow — Our globe a small part of the universe— No limits to future discoveries — Rapid motion of the planets — Infinite power requisite to give them this motion — Immense spaces around the heavenly bodies — Their mutual influences — Astronomy an aid to religion. . . . 217

CONTENTS.

Vll

Chapter VIII

Section I. — Eclipses — All opaque bodies cast shadows — The moon visible during total eclipses — Explanation of this phenomenon — Lunar eclipses universal — The shadow conical — The penumbra, or partial shadow — Eclipses of the sun not universal — Breadth of the lunar shadow on the earth — Primaries never eclipse each other — Duration of total eclipses — Solar and lunar ecliptic limits— Detail of several eclipses— Eclipse of 585 B. C— of 434 B. C— of 383 B. C— of 201 B. C— Two mentioned by Dionysius — Chinese customs respecting eclipses — Eclipse of 1560 — of May, 1706— of April, 1715— of June, 1406— Annular eclipse of 1836 —Number of eclipses in a year— Particular explanation of eclipses- How affected by the position of the earth's axis— Use of eclipses in astronomy, geography, and chronology— Darkness at the crucifixion— Occultations. 224

Section II. — Universal gravitation — Dr. Hooke's suggestions and experiments Newton's successful investigation — All bodies tend toward each

other— Pressure and weight the effects of gravity— Heavy and light, relative terms — Weight varies at different parts of the earth — How discovered — Gravity diminishes as we recede from the centre — There would be no weight if but one body existed — Gravity retains the moon in its orbit — Explanation — The planets affected by the same force — Natureof this force inexplicable-^Attraction of mountains. . . 243

Section III. — Tides — Kepler's fanciful theory — Newton's theory — General course of the tides — Owing principally to the moon's attraction — In part to the sun's — Farther explanation of the tides^Cause of spring and neap tides — Priming and lagging of the tides — Declination of the sun and moon affect the tides— Establishment of a port— Exceptions to the general laws— Mediterranean and Baltic seas — Times of high water different in neighboring ports— Theory of Mr. Redfield. . . . 250

Section IV.— Solar and sidereal days — Equation of time — Inequality arising from the obliquity of the ecliptic — That caused by the unequal motion of the earth in its orbit — Deductions respecting the equation throughout the year — Mean and apparent time agree but four days in the year — Calendar — Standards of time — Their inconvenience — Gregorian method of correcting the calendar — Grecian calendar — Eoman Calendar — Its correction by Julius Cssar — Persian calendar — Subdivisions of the year — Cycle — Dionysian period — Dominical letters — Julian period. 259

Chapter IX

Section I. — Mensuration of the earth — Standard of measure — Astronomy teaches how to obtain the dimensions of the globe — Richer's observations on the pendulum — Their consequences — Laws of the pendulum — Oblate form of the earth confirmed by analogy — Early attempts to measure the earth — Riccioli's method — Snellius — Norwood — and others — Opposite theories of Newton and Huygens — Maupertuis measures a degree in

Sweden, and Godin in Peru— Result— Explanation of the method of measuring the earth 275

Section II. — Methods of finding the moon's parallax and distance — DiiRculty in finding the sun's parallax — Transits of Mercury and Venus — Sun's parallax found by the transit of Venus.— The sun's distance — Distances of the planets — Method of finding the diameters and magnitudes of the sun and planets — Method of finding their masses and densities 281

Section III. — Nature of light unsettled — Its properties known — Refraction — Experiments illustrating refraction — Cause of twilight — Advantages and disadvantages of refraction— Knowledge of the atmosphere important to the astronomer — Amount of refraction at the zenith, the horizon, and at intermediate points — Explanation of the " sun drawing water" — Terrestrial refraction — Aberration — Bradley's observations — Inferences — Motion of light — Aberration confirms the motion of the earth in an orbit — Precession of the equinoxes — Nutation — Obliquity of the ecliptic — Its cause and effects 287

Section IV. — The telescope — Its invention — Its simplest form — Mode of finding the magnifying power of telescopes — Common astronomical telescopes — Difficulties encountered in their early construction — Huygens' improvement — Newton's lenses — Reflecting telescopes — The Newtonian — The Gregorian — The Cassegrainian — W. Herschel's — Ramage's — General remarks on telescopes — Impossibility of minute discoveries in the moon 300

Chapter X

Section I. — Zodiacal light — Observed by Cassini in 1683 — By Childrey previous to 1661 — By others in 1707 — By Professor Olmsted in 1834 — Various theories — Aurora borealis — Appearances in the Shetland Islands — In Siberia — At Hudson's Bay — Sounds attending their appearance — Aurora seen by Capt. Ross — Southern lights observed by Forster — Northern lights as seen in England, March, 1716 — Supposed height of these meteors — Theories respecting their cause — Halley's — Mairan's — Euler's — Franklin's — Kirwan's — Appearances in August and September, 1827— December and November, 1835— April, 1836. . . 309

Section II. — Remarkable halos and parhelia seen in France in April, 1666 — In March, 1667 — Huygens' explanation of the causes of such phenomena — Mariotte's explanation of halos — Mock-moon seen by Forster — Extraordinary circles round the moon seen at New Haven in November, 1827 — Phenomenon seen at Green Bay in February, 1835 — Experiments illustrating the production of halos — Rainbows — Experiments producing colored bows — Cause of rainbows — Remarkable bows observed by Brewster— At Chartres— By Halley— In 1710— In July, 1824— Clouds— Their modifications — Curl-cloud — Stacken-cloud — Fall-cloud — Sonder-cloud — "Wane-cloud — Twain-cloud — Rain-cloud — Scud — The color of clouds — Their height — Their structure and buoyancy 327 vm

CONTENTS

Section III. — Division of falling meteors — Phenomenon witnessed at Leeds, England, 1710— In March, 1719, aU over England— In March, 1813, at New Haven, Connecticut — In Vermont, January, 1817, and March, 1822 — In various places, November 13th, 1833 — Olmsted's theory respecting its cause — Repetition of the meteoric phenomenon in 1834, 1835, 1836 — Arago's theory — Ignes fatui, or Will-o'-the-wisps. . . 348

Section IV. — Aerolites — Their resemblance to each other — A proof of their common origin — Direction in which they appear to move — No theory as to their origin satisfactory — Accounts of various aerolites — Their specific gravity— The substances of which they consist — They could not have been produced in the atmosphere — Do they fall from the moon ?— Or, are they fragments of an exploded planet ? . . . . 362

WONDERS OF THE HEAVENS.

Section I

Appearances of the heaven — ^by day— by night — Diurnal motion — Celestial sphere — Its poles — Zenith — Nadir — Sensible horizon — Rational horizon — Meridian — Equator — Hemispheres — East, west, north and south points — Circumpolar stars — Declination and right ascension — Equality of a star's successive revolutions — hffw made known — Use of this regular motion to astronomers — Sidereal day — Time-keepers used by the ancients — Chronometers — Illustration of the preceding — Deductions from appearances — Stars called Fixed — Idea of immensity impressed by the study of the stars — Annual parallax — Each star the centre of a system, and possibly the satellite of some more magnificent primary.

Take a man who had been blind from his birth, and who had imbibed no ideas of any existences but those that were immediately and intimately connected with his other senses, restore him suddenly to perfect and healthy vision, and place him in some extensive champaign, where there were no obstacles to confine his view ; would he not discover a scene the most brilliantly beautiful mortal eye ever beheld ? That azure dome sown broadcast, as it were, with living light ! How would he desire to understand the causes of the changes he would witness, the regular and constant succession of day and night; phenomena so different, yet each so interesting to his new-born and startled sense. Soon after the dawn of day, he would observe in one part of heaven a dazzling circle rising apparently out of the earth, and scattering about it in all directions numberless rays of brilliant light. It ascends slowly the gorgeous dome, the heat and light increasing as it ascends, until it attains a point nearly overhead, where it seemingly rests but a moment in its midway course, then descends as it arose, and disappears at last in a part of the horizon opposite to that where it first appeared.

The light soon fades away, and night comes on, bringing a new and more interesting, if not so brilliant a spectacle. During the day, a single body had attracted all admiration to himself, by his majestic motion and the splendor of his rays. Now there appear on all sides shining points, variable in size and brightness, decorating in countless numbers the vault of heaven, and increasing still in proportion as the darkness becomes more profound, till the whole celestial space seems to be filled. The motion of these bodies adds also to the beauty and interest of the scene. Some, moving in the same direction as the sun, disappear like him from our view in the west. Others are rising in or near the east, and ascending in their turn. Such are the general phenomena of the rising and setting of the heavenly bodies. Yet all do not thus disappear below the horizon. There are some which never reach that circle, and whose track can be followed during the whole night. Those stars which form the cluster called the Great Bear, or Charles' Wain, are among this number in our latitude ; and if one stations himself with his right hand toward the east and his left toward the west, he will see this group, so well known in our climate, assume successively different positions, while the individual stars of the group maintain the same mutual relations of figure and distance. Meantime the darkness diminishes, the dawn reappears, the light increases, the sun by his superior brightness dims the light of the stars, and the phenomena of morning are renewed in the same order as before. This general movement of the heavenly bodies from east to west, in the course of a day and night, is called the diurnal motion. In what we have said

WONDERS OF THE HEAVENS.

above, we have brought forward only those phenomena, which appear on a simple inspection.

The different motions that we have observed in the heavens may have led us to think that we were situated in a point of the universe, around which as a centre revolves a sphere sprinkled with shining drops. This sphere seems to turn on an imaginary line, which is called the axis of the earth, the points where this line touches the heaven being the celestial poles; the one that is elevated in our latitude being called the northern or Arctic pole, and that which is diametrically opposite, and which we cannot perceive, the southern or Antarctic pole. We must be careful not to confound these two points with that situated directly over our head and its opposite, the first of which is the zenith, the last the nadir. Every point on the surface of the earth has its zenith and its nadir; yet there are but two points whose zeniths coincide with the poles of the heaven, and these are the poles of the earth. Thus, let be the place of the observer, P and P' the poles of the heaven, p and p' the poles of the earth; Z is the zenith and N the nadir of the observer at 0. If the observer were at e, his zenith would be E and his nadir E'.

If now a plane, A B, be imagined tangent to the surface of the earth at 0, it will represent the sensible horizon of a spectator at that point, separating the parts of the heaven and earth visible, from the parts invisible to him. There is another horizon, called the rational. It is a plane, parallel to the sensible, and passes through the centre of the earth, which is also the centre of this circular plane; thus H H' represents the rational horizon of the observer at 0, Everyplace, then, must have a sensible and rational horizon peculiar to itself. These planes are quite important in astronomy. From them we measure the altitude of the heavenly bodies ; and these measures are the foundation of astronomy.

The great circle which passes through the poles of heaven and the zenith of a place is the meridian (north and south line) of that place.

The stars, supposed fixed to this celestial sphere, describe every day circles, smaller according as they are situated nearer the poles of the heaven. The largest of these circles, whose points are all equally distant from the poles, is called the equator, while the circles parallel to it are called simply parallels. The equator divides the sphere into two equal parts; one forms the northern, the other the southern hemisphere.

The east is that point in which a heavenly body (describing the equator) rises; the west, the point where the same body sets. The intersection of the horizon by the meridian determines two other points ; the south, where the sun is at noon in regard to us, and the north, where the sun is at noon in regard to those situated as far south, as we are north of the equator. These four points are known under the name of the cardinal points.

If we have observed the movements of the various stars, we must have noticed, that some of them never set, but are constantly above the horizon. Among these, a few experience but a very slight displacement, and scarcely appear to move at all. These, being near the axis about which all the heavenly bodies revolve, describe diurnal circles of so small a diameter, that they are scarcely discernible by the eye. Yet during their revolution they are seen to pass the meridian twice, once over the upper meridian, that is, between the pole and the zenith, and once over the lower meridian, that is, between the pole and the horizon. These heavenly bodies are denominated circumpolar.

The situation of the stars in reference to the equator is given by stating their declination and right ascension. The declination of a star is its distance from the equator measured on the arc of a great circle which passes through its centre and through the celestial poles, and is analogous to terrestial latitude, being north or south as the star is in the one or the other hemisphere. -Right ascension is the distance of a heavenly body from an arbitrary first meridian, (that which passes through the first point of Aries,) and is measured on an arc of the celestial equator, being analogous to celestial longitude, except that it is measured in one direction only, quite round the sphere.

Of all the phenomena which the diurnal motion of the stars presents, that, which has most fixed the attention of observers, is the constant equality of every revolution of the same star. If we direct a telescope toward any star, fix the instrument firm and immovable, and observe the time which passes until it again appears in the instrument, we may feel assured that we have the time of its revolution for all duration, as also that of every other star. At most, the variations are so small, as to be appreciable only in some stars near the axis of rotation. Astronomers make use of them to obtain with precision a measure of time. The interval between two consecutive returns of a star to the same meridian is styled a sidereal day. The instruments first used to measure this duration, called clepsydras, vessels filled with water, sand, or mercury, and used like our hour-glasses, were exceedingly rude and inexact. In later times, new discoveries in physics, and more ingenious mechanical skill, have caused the manufacture of nicer time-keepers, such as watches and clocks; and in later still, the chronometer, whose degree of exactness almost exceeds belief, since some of them have been said to vary less than one second per diem.