and its use, called forth the strong language of Pliny, who describes it as a thing even hard for a god to perform-" Ausus rem etiam Deo improbam." Perhaps, had we an accurate catalogue of the stars, dating back four thousand years, some remarkable variations of position would be discovered, transpiring by slow and imperceptible degrees, which would open views of the universe, which will now require the observations and comparisons of future centuries to develope and confirm. After determining the places of the stars, Hipparchus made a representation of the heavens on the surface of an artificial globe, which appears to have been deposited at Alexandria; and with him also the happy idea originated of marking the positions of towns in the same manner, by circles drawn through the poles perpendicularly to the equator, or by latitudes and longitudes. When his brilliant career commenced and terminated, is unknown; but he was born at Nice, in Bithynia, made many of his observations at Rhodes, and was alive in the interval between 160 and 125 B. C. He amply merited the epithet applied to him by Ptolemy," the lover

at the destruction of the Alexandrian library; but the chief of the latter have been preserved by Ptolemy in his own works, and they have largely contributed to his fame. In a celebrated production which ruled the mind of Europe for fifteen centuries, the Great Collection, or Almagest, as it was called by the Arabic translator, he recorded the advances of past ages in astronomy, the state of the science in his own time, and developed a plan of the celestial movements. It recognised the earth to be a spherical body for reasons similar to those that are now alleged in proof of its convexity-to be the immovable centre of the universe- the sun, moon, planets, and fixed stars, prosecuting a daily revolution around it, in perfect circles, and with uniform velocities. This is in accordance with the appearance presented by the first blush of the universe to the physical eye. The sun is seen daily pursuing a course through the heavens in the segment of a circle from east to west. The same path is ap

parently described by the moon and stars; and on a calm autumnal or winter night, when the winds are lulled, and scarce a twig stirs, or a ripple is seen upon the waters, but the "river wanders at its own sweet will," how perfect the illusion in which the scene involves the senses-the quiescence of our terrestrial globe, and the ceaseless revolution of the firmament round it as a central point! These were dogmas firmly held by the Platonists and Aristotelians, who stoutly clung also to the idea of circular orbits and uniform velocities in relation to the celestial luminaries; because the circle was deemed the most

perfect of all geometrical figures, and on that account alone proper to represent their motions; and because of the fancy, that in such divine and eternal bodies no irregularity can exist.

We have seen, however, that the Greeks had a cultivated acquaintance with other movements besides the apparent diurnal revolution of the sphere, movements in an opposite direction to it. They had carefully traced the sun's path in the ecliptic, that of the moon in her orbit, and they had marked likewise the striking peculiarities of planetary motion. To an observer on the earth, the planets appear to pursue a course of the most irregular and unsystematic kind, which was a perfect puzzle to the ancient theorists. Sometimes they seem to go along with the sphere, but with greater celerity; then, to remain stationary; then, to retrograde, or make an angular movement, or describe a circuit like a loop in a knot of ribbons. The paths of Mars and Jupiter, as observed from the earth, described the courses roughly represented in the diagram, at the intervals stated. It seems surprising therefore, in

The Moon.

Venus.

JUPI

MARS

Холов 1720

1709

TER

EARTH

1708

б

Sun.

* Fixed Stars.

the face of these appearances, that the centrical station of the earth, and all the orbs of heaven moving round it in perfect circles, should have become so much the grand leading idea of the ancient mind. Hipparchus, indeed, was too well aware of the imperfection of observation, and of the importance of supplying data upon which to found a theory, to be given to theorising himself. But when he did systematise, he departed from one of the cardinal maxims of his compeers and successors, by supposing the sun to revolve round the earth in a circle, the earth not being at the centre. He had found the in

terval between the vernal equinox and the summer solstice 94 days, and that from the summer solstice to the autumnal equinox only 921 days, thus making the length of the summer half year 187 days, and that of the winter half year 178 days. To account for this unequal division of the year, and to reconcile it with the idea of the sun's orbit being a circle and his velocity uniform, Hipparchus dismissed the hypothesis of the centrical position of the earth. He supposed an eccentric circle, according to which, A B C D may represent the solar orbit, E the place of the earth, not coincident with F the centre of the orbit. EF will be the eccentricity, G H the line of the apsides, I the position of

DK

the sun at the summer, and J at the winter solstice, к his place at the vernal and L at the autumnal equinox. This theory provides for the circular orbit and uniform velocity of the sun; for the unequal division of the year, and an apparently unequal rate of motion to a spectator at the earth; for the two arcs formed between K and L are unequal, and it is perfectly obvious that if perceptible to a terrestrial observer the sun would appear to travel fastest at the nearest point of the apsis, H, and slowest at the farthest, G. This scheme is chiefly remarkable for its removal of the earth from the post of honour in relation to the solar orb, which the general voice of antiquity assigned to it with reference to the whole universe, that of being the centre of all the celestial revolutions.

BL

The idea of his great predecessor was not admitted by Ptolemy, who contended for the immobility and centrical situation of the earth, and attempted to reconcile with these positions, and with the dogmas of perfect circles and uniform velocities, the diverse and complex movements in the heavens. In explaining the irregularities of the planetary courses, he adopted largely the hypothesis of epicycles; a scheme based upon the general principle, that irregularities of movement may be resolved into a combined series of regular movements—a sagacious idea, not however his own, but proposed by some of the earlier Greeks, yet one which he was the first laboriously to apply to the phenomena observed. It is demonstrable that two motions, each uniform in itself, may, in combination, appear vexed and tortuous. In illustration, Dr. Nicol says, suppose that a ship were sailing evenly and uniformly along a coast; it is manifest that a passenger standing on deck would appear to a spectator on shore to move past him also in a regulated and uniform manner, with the same velocity as the ship; but if, instead of standing, the passenger should walk to and fro on the vessel's deck, likewise with a constant pace, his actual motion along the coast would clearly seem of the most irregular kind. While walking from stern to prow his motion would appear faster than the ship's; and when walking from prow to stern, it would, on the contrary, appear slower; nay, if in this latter case the passenger walked faster than the ship sailed, he would appear for a certain time to go along the coast the other way; so that if the vessel were invisible, and the observer fixed his attention solely on the passenger, he would see very irregular motions; sometimes accelerated, sometimes retarded, sometimes even retrograde—all, however, flowing from the mere combination of two uniform motions, the sailing of the vessel and the walking of the passenger. Now, Ptolemy's theory of epicycles combines two movements in the heavens, that of a centre and a circumference. Thus let E be the earth, and м an imaginary centre in the heavens: the planet is supposed to be moving around it in the circumference abcd all the while the imaginary centre is accomplishing the orbit A B C D. A little attention will soon discover that upon this hypothesis the planet's motion will appear at times direct, stationary, and retrograde to an observer on the earth. But though the assumption satisfies some of the conditions required by the problem of the planetary movements, it goes a very little way towards answering them all, and Ptolemy was obliged to crowd cycle upon cycle, and combine them with eccentrics, in a manner that would be difficult to make intelligible, and useless if attempted.

D

B

It was a blind adhesion to the physics of Aristotle-an unquestioning adoption of the false principle respecting circular motion being the law of nature and the earth the centre of the universe-that led to this complicated scheme to harmonise its apparent movements. These fundamental errors being admitted, it necessarily followed that the mind must go forth in quest of some plan, to bring into coherence with the maxims

adopted the observed facts of planetary motion. The acute intellect of the ancients is illustrated by the theories proposed; but the lesson is strikingly furnished by their laborious and abortive efforts, that it is folly to take for granted that which has not been proved. The later Ptolemaists occupied themselves for centuries in amplifying and mending his system, supplying an epicycle here where it was wanted, and an excentric there when required, and even converting his imaginary spheres into solid transparent wheels, revolving the one within the other, and each carrying a planet attached to it. The profound of space was crammed with a succession of huge globular forms, in which the planets were placed, all of the finest crystal, else the light of the stars had been intercepted; beyond these was the vast sphere of the fixed stars, crystalline likewise; beyond that was the primum mobile, incessantly rolling and giving motion by its friction to the spheres in its concave; while still farther out was the empyrean heaven, or paradise of blessed souls.

The illustrious Alexandrian is not, however, to be identified with those who bore his name in the middle ages. He would have repudiated at once their vain inventions. They were schoolmen chiefly, more familiar with the dreamy philosophy of the Academy and Stoa than with the experimental science of Hipparchus more skilled in logic than

in the examination of physical nature. The Ptolemy of the early Christian era was a practical astronomer, and his merits as such give him a title to fame, which his theory, now exploded and obsolete, ought not to obscure. He was the first to point out the effect of the atmosphere in changing the direction of the rays of light; and, though unable to appreciate the amount of its refractive power, he clearly perceived its influence on the altitudes of the stars, was aware of its increase with the distance from the zenith, and assigned this as the cause of the greater apparent magnitudes of the solar and lunar discs at the horizon—an explanation now generally held.

A scheme nearer to the truth than the Ptolemaic, commonly called the Egyptian, was in vogue when the former was broached, though there is no reason to suppose, with some, that it prevailed at an early period in that country. It regarded the inferior planets as revolving round the sun, and moving, in conjunction with the sun, round the earth. It is obscurely hinted at by Pliny; but explicitly announced by Vitruvius, who lived a short time prior to Ptolemy. Mercury and Venus, the architect remarks, are the planets nearest to the rays of the sun, and move round the latter as a centre, appearing sometimes progressive, sometimes retrograde, and occasionally stationary among the signs. But this system, the prototype of the Tychonic, never became popular. A valid objection to it was probably found in the fact that the inferior planets were never seen in phase, an appearance which would be exhibited if alternately beyond the sun and between it and the earth; a position strictly true, and had the eye at that period been aided by the telescope, Venus would have been seen in phase as at present. The superior planets were invariably arranged by the ancients in the same order-that which now obtains. They had obviously no guide as to their respective distances from the earth, but the indications discernible in their different brightness and velocity. Those which had the slowest apparent movement were concluded to be the most remote. Hence Saturn, a dull and sluggish traveller in space, was placed on the exterior; then Jupiter and Mars followed in succession towards the earth: the sun, moon, and inferior planets were placed within the orbit of Mars, and the moon was considered the nearest celestial object to our globe. Next to eclipses of the sun and moon, occultations of the stars and planets by the moon, or the approach of the moon or of a planet to any star, with the appearance of comets, attracted attentive observation. An occultation of Mars by the moon is mentioned by Aristotle, and one of Saturn is recorded by Ptolemy as having taken place in the year 228 B. C. In fact, we may find some notice, more or less distinct, taken by the ancients, of all the celestial phenomena which the unaided vision has observed in modern

times, with one exception, that of an annular eclipse of the sun, with reference to which the voice of antiquity is silent.

The instruments of observation known to the ancient world were of a simple and imperfect kind. The earliest of which we read is a vertical pillar employed to determine the sun's altitude by means of its shadow. It was in general use for this purpose among the Oriental nations. The lofty pyramidal stone which Diodorus Siculus describes as hewn out of the mountains of Armenia by order of Semiramis, and set up in a conspicuous part of the city of Babylon, is conjectured to have been erected with this design. A Chinese record of uncertain date, but undoubted antiquity, refers to an observation on the length of the meridional shadows cast at the times of the summer and winter solstices by means of a vertical pillar. It is scarcely to be doubted that such a simple and obvious way of approximating to the length of the year must have suggested itself at a very remote period. The greatest elongation of the shadow cast by the pillar at noon at midwinter, its gradual decrease towards the vernal equinox, its greatest declension at midsummer, and its gradual advance to its maximum elongation again, are points to which we may believe attention was early directed; and a series of observations, taken with

Obelisk at Heliopolis.

precaution, would lead to some results valuable in a rude state of society, and to an agricultural people. The same instrument was also employed to divide the day into equal parts, by means of lines traced on the pavement, which indicated the hour as the shadow of the

column fell upon them. There is strong reason to believe that these instruments originated the obelisks of a subsequent age, erected in open squares in the Asian cities, and near the entrances of the Egyptian temples, and that these were intended primarily to answer one or all of the purposes named. It has been objected to this idea that the form of the summit of the obelisk will not allow of the extremity of the shadow being accurately defined; but there is some foundation for the surmise that a ball crowned the summit, by which the end desired might be gained. Augustus removed two grand obelisks from Egypt to Rome expressly to be used as gnomons, which

conveys the impression that they had previously served that office; and Manlius placed a ball upon the obelisk erected by him in the Campus Martius, with a view to facilitate the accurate delineation of its shadow. The great importance in astronomy of precisely ascertaining the hour of any phenomenon occurring, or observation being made, led to various forms of the sundial, and to the construction of instruments for measuring time by night and by day when the sun was obscured. The one in general use was the clepsydra, or waterclock, as its name imports, an hour-glass, water being used prior to sand. The orations of the Greeks and Romans were regulated by this contrivance as to their time of speaking, which was called pleading by the clepsydra. The Tower of the Winds at Athens, otherwise styled the Tower of the Clock, an edifice of the age of Alexander, seems to have