centuries to render them sensible; yet so powerfully have the great modern instrumental improvements aided the researches of the astronomer, that even in the comparatively short period since the invention of the telescope the greater accuracy of observations has compensated, as it were, for the shorter intervals of time at which the comparisons are made; and many of these most interesting points of enquiry have been cleared up, though many more probably yet remain in obscurity.

It was in this way that Hipparchus, comparing his catalogue with the observations of Aristillus and Timocharis of 150 years before, perceived that all the fixed stars, while they retained their latitudes sensibly unaltered, had advanced about two degrees in longitude; or, what amounts to the same thing, the equinoctial points appeared to have retrograded along the ecliptic by the same quantity. In other words, he made the first discovery of the precession of equinoxes, as a bare fact which the lapse of years had brought to light, but which received no explanation till the time of Newton.

He

To Hipparchus, also, we are indebted for the first attempt at fixing geography upon exact principles, by referring the position of places on the earth's surface to their latitudes and longitudes: he proposed to determine the latter by means of eclipses of the moon. also directed his attention to the more accurate correction of the calendar. He proposed to quadruple the period of Calippus, and then to subtract a day. Upon the whole, the praise we must ascribe to Hipparchus is unquestionably very great, even though we should consider the encomium of Pliny somewhat extravagant: "Hipparchus nunquam satis laudatus, ut quo nemo magis comprobaverit cognationem cum homine syderum, animasque nostras partem esse cœli.....ausus rem etiam Deo improbam, annumerare posteris stellas." (Nat. Hist. ii. 26.)—“ Hipparchus, never sufficiently to be praised; than whom no one more fully proved the kindred between the stars and man, and that our souls are a part of heaven..... who ventured also to do a

thing wrong in the sight of the Deity, to enumerate the stars to posterity."

With this indefatigable astronomer the zeal for prosecuting observations appears to have died away; and from various causes, among which perhaps the wars of the later sovereigns of Egypt may have had their share; and, notwithstanding the endeavours made by Ptolemy Physcon to stimulate the declining sciences (B. c. 137), astronomy was gradually reduced to a very low ebb. And though (about в. c. 50) Theodosius and Menelaus wrote on the sphere, and on spherical trigonometry, no advances of importance were made in these sciences for several centuries.

Physical Science.

From the school of Alexandria emanated the first optical treatise. The regularity with which the rays of light take rectilinear courses seemed naturally to make them a subject of study to the geometer. Euclid, perceiving this affinity, began to apply the science he had already cultivated with so much success to explain the laws by which the directions taken by the rays of light are regulated. This was probably some years previous to the labours of Archimedes in reducing other parts of physical science to the dominion of mathematical laws. Two treatises are extant, one on optics (in the more limited sense of the theory of vision), the other on catoptrics, both ascribed to Euclid, but as is now generally allowed, erroneously. In the optics, the principles are investigated by which we judge of the magnitudes of objects; but the reasoning proceeds too exclusively on the mere geometrical consideration of the angle subtended, without taking into account other causes. other respects also the investigations are faulty. In the catoptrics, the general principles of the place of images by reflection are also laid down, but not by any means correctly; and the proofs are obscure and defective.

In

These and other faults, so unworthy of the distin

guished author to whom these books have been ascribed, have led critics to deny the genuineness of both treatises. Some have considered them (but especially the catoptrics) as the compilations of some ignorant pretender to science; whilst others have supposed them to have been vitiated versions of an original treatise, which it is on all hands believed Euclid actually did compose, and of which these works are careless extracts or unskilful abridgments.

The mechanical enquiries begun by Archimedes were extended by Ctesibius and Hero in the school of Alexandria a century later, or about B.c. 150. They were the first who, by an analysis of all mechanical engines into their primary elements, reduced all their actions to 'some combinations of five simple principles, to which they gave the name of duvaμɛs, or mechanical powers; the same system which is retained at the present day.

In hydrostatics, after the developement (as we have seen) of some of the most essential principles by Archimedes, it does not appear that any very material advances were made; though several of his successors enriched the science by practical improvements.

It is uncertain when the common pump was first invented; but with the knowledge possessed long before this period it must have been an easy application. That since the pressure of the air acts on the surface of the water in the reservoir, but is removed from that in the tube, the latter ought to be forced up, was a consideration much too simple to accord with the prevalent notions of a philosophical theory; and nature's abhorrence of a vacuum had already been established as a more logical explanation of this class of phenomena. It is certain, however, that the principle must have been practically known before the more complex application of it in the forcing-pump, which was the invention of Ctesibius, perhaps the greatest mechanic of antiquity after Archimedes. It was produced almost exactly in the form of the modern fire-engine. To the same philosopher are also ascribed the clepsydra, or water-clock, and the air-gun.

His contemporary, Hero, was distinguished for inventions rather curious than useful; such as ingenious combinations of syphons, &c. forming various kinds of fountains and water-works. Such contrivances serve to illustrate the progress which was made in the knowledge of principles, however fallacious the theories to which those principles were referred.

Hero adopted the idea of nature's abhorrence of a vacuum, at least as extending through any considerable or sensible portion of space. He made this limitation because he conceived that in the insensibly small interstices of bodies a vacuum does exist; and by this means accounted for the compressibility of matter. He contrived a method for the exhaustion of air out of a vessel, very nearly resembling the air-pump, and practically applied it to the purpose of cupping instruments. But to explain the effect, he devised the long celebrated theory of suction.

According to this theory, the upper portion of the liquid or substance is drawn up in a tube or vessel, being attracted and supported, as it were, by some mysterious and occult power, applied in the act of suction, whilst the next portions in succession are sustained by those above them; and the extent to which such action can be carried is limited by the weight of the column suspended.

Posidonius, somewhat later than Hero, followed in the cultivation of the same sciences. He enquired into the nature of the tides*, and discussed the atmospheric refraction. In this he was followed by Cleomedes, who also compared the magnitudes of the earth and of the sun. +

Our knowledge of all these inventions is principally derived from the writings of Vitruvius. Some of the works of Hero, and others of this age, appear in the valuable collection published under the title of "Mathematici Veteres." Others exist in manuscript in the various public libraries throughout Europe.

*Cic. de Nat. Deor. ii.

Meteor. ci. i.

SECTION III.

THE STATE OF SCIENCE DURING THE AGE OF THE ROMAN EMPIRE TO THE PERIOD OF ITS DISSOLUTION.

Physical Science among the Romans.

A VERY slight acquaintance with the literature of the

Romans suffices to show that in that nation so eminent in war, in polite literature, and civil policy, there prevailed at all times a remarkable indisposition to the pursuit of mathematical and physical science. When the treasures of Greek literature were opened to the incipient curiosity of Rome, reposing from her earlier triumphs, the works of the poets, orators, and moral philosophers of that country, were sought with avidity and studied with ardour; whilst those of the geometers and astronomers were totally neglected: and these sciences, so highly estimated in the country which gave them birth, were not merely disregarded in Italy, but even considered beneath the attention of a man of good birth and liberal education: they were imagined to partake of a mechanical, and therefore servile, character. The practical results were seen to be made use of by the mechanical artist, and the abstract principles were therefore supposed to be, as it were, contaminated by his touch. This unfortunate peculiarity in the taste of his countrymen is remarked by Cicero. And it may not be irrelevant to enquire, whether similar prejudices do not prevail to some extent even among ourselves; and whether the exclusive attachment to classical studies, and the cultivation of the Roman literature, as the sole basis of the education of the higher classes, may not be the source from which such prejudices are too commonly imbibed.

Nevertheless it must in fairness be admitted, that there are instances in some of the Roman authors of better