in density, to extend itself every way as it recedes from the sun in the same manner as the breadth of a triangle increases from one of the angles to the opposite side; the other expression appearing to imply an opinion that, if the light or heat diminished in density as it receded from the centre, which would be the case if it issued from the sun in right lines like the radii of a circle, the heat near the sun must be so much greater than we find it to be in the region of the earth, that the moon and inferior planets would be destroyed.

In those times the physical principle by which the parts of the universe are connected together must have been very faintly apprehended; for though many passages in ancient authors seem to imply a knowledge of the existence of an attractive quality in nature, like that which Vitruvius supposes to reside in the solar light or heat, and daily observation must have shewn that a heavy body, if unsupported, falls to the earth; yet so little were the ancients able to discern the consequences which must result from such a tendency that, as far as we can discover, they never suspected the power by which all bodies near the earth descend towards its surface to be the cause which renders their rise from that surface impossible without the application of some superior. force. This we infer from the language of Pliny, in answer to the question, (which seems, then, to have been frequently proposed,) whether or not there were antipodes to the inhabitants of our hemisphere? and if so, why they do not fall from the earth? He observes, justly, that the latter question may as properly be made concerning ourselves by the inhabitants of the other hemisphere, but his reply is nothing more than an assertion that such a fall would be contrary to Nature, since she has refused a place to which terrestrial bodies may tend in that direction. The imperfection of the sciences was not, however, unfelt by many of the learned among the ancients; nor did it escape the penetration of one who most justly deserved the name of a philosopher, that they would, with the progress of time, receive those improvements which have actually taken place: Seneca, contemplating the state of astronomy, observes that it was only since fifteen hundred years before his time, the Greeks had given

Nat. Quæst. Lib. VII. cap. 25.

names to the stars, and that there were still many nations who only knew the heavens by sight; but, with a prophetic spirit, he says, "the time will come when posterity will be surprised that we could be ignorant of things the knowledge of which might be so easily acquired; and some one will at length arise who shall teach men the paths of the comets, their magnitude and number, and why they deviate so far from the routes of the planets. How many things" he adds, " are beyond the reach of human intelligence; and how small is the part of the universe which is accessible to our knowledge; even the Deity himself is no better known to us." Let it be observed here, that the fifteen hundred years mentioned above, would carry us back to a period more remote than the origin of the Greek astronomy; and it is probable that Seneca confounds the science of the Greeks with that of the Egyptians and Chaldeans.

CHAPTER XIV.

WORKS OF THE ARABIAN ASTRONOMERS.

Decline of Astronomy after the destruction of the Roman Empire.-The Arabians adopt the hypotheses of Hipparchus and Ptolemy.-Their improvements in pure mathematics. – Destruction of the Alexandrian library.— Measurement of an arc of the terrestrial meridian in Syria. The hypothesis of a libration of the equinoctial points supposed to have been conceived in the age of Ptolemy.-The opinion adopted by the Arabians.-The hypothesis of material spheres adopted by the same people.-Manner of representing the effects of the supposed libration.-Determination of the obliquity of the ecliptic by Albategnius.-The length of the year and the elements of the solar orbit determined.-The elements of the lunar orbit adopted from Ptolemy. Opinions concerning the distances of the planets.--Researches of Ibn Jounis.-Values assigned by the Arabians to the general precession.— Astronomical tables computed by the same people.-The correction for refraction introduced.-Observatories established at Maragah and Samarcand.

THE science which we have seen glimmering through the obscurity enveloping the ancient history of the Egyptian and Assyrian empires, and shining with a steady lustre over the territories subject to Grecian influence, was marked in its progress by just such steps as might be expected to be taken by an intelligent and industrious people, who persevered in their application to the subject till they had brought it to the highest degree of perfection which their means permitted; and it is highly probable that, had the political existence of the Grecian states been extended to a later period, the moderns would have been anticipated in the discovery of that law which has enabled them to unfold all the mechanism of the universe. But the iron hand of Roman despotism crushed the intellectual energies of the people, and annihilated the sciences both of Greece and Egypt: then, instead of a nation ennobled by the culture of a sublime philosophy, and adorned with the elegant arts, history reveals to us a people of soldiers, originally occupying only a narrow district of Italy, but, subsequently, becoming the conquerors of the most powerful nations of antiquity and rulers of that vast portion of

U

the earth which extends from the Atlantic to the Indian Ocean; who, in ignorance of that which constitutes true glory, sought theirs only in warlike achievements; or rather, who, in accordance with the precepts of the poet, left to others the care of explaining the form of the heavens and the courses of the stars, and devoted the energies of their minds to the arts of subjugation and government. During the existence of a nation influenced by such precepts it was impossible that astronomy or the other sciences should flourish, and the destruction of the giant empire was followed by a long night of barbarism and ignorance. This continued till the ninth century of the Christian era, when the successors of Mohammed had carried their arms and their religion over all those provinces of Asia and Africa which, during so many ages, had been ruled by the sovereigns of the "eternal city;" and till a new race arose in the west, which, reviving the smothered fires of ancient learning, produced a flame whose steady brilliancy seems destined to enlighten every region of the earth.

But to what can be ascribed this change, which caused so dark a veil to overspread the sun of ancient science, literature, and art, in countries where these had previously been so highly cultivated? The only plausible answer to this question is, that while the empire of Rome subsisted and held those countries in subjection, that portion of the revenues of the states which should have been consecrated to the support of the public seminaries was expended in foreign wars; and that, among the rich and powerful, a relish for merely sensual luxuries had superseded the taste for pleasures of an intellectual nature: when learning ceases to be patronized by a government, or when it is no longer esteemed a qualification for rank in society, it soon ceases to be cultivated; and the painful application of the mind which is necessary to attain proficiency in it is too often gladly dispensed with by persons whose interest it does not immediately promote.

Before the invention of printing, books were rare and costly, therefore not generally diffused; and it must have been in the power of those persons only who were in possession of considerable wealth to place themselves under the tuition of some philosopher

a Eneidos, Lib. VI. ver. 850.

who, perhaps, resided in a distant country; hence the number of students could never have been considerable in the ancient world, and this number would naturally be diminished when the stimulus to exertion was withdrawn. It is certain, also, that an ambitious priesthood, from a love of the power which superior science has always conferred on those who possess it, withheld from the laity the instruction it was its duty to communicate, in order to preserve a monopoly of learning within its own establishments; and, as might be expected from such policy, it happened that, in proportion as knowledge declined among the people, it declined also among the clergy till, at length, all were enveloped in one common cloud of ignorance and superstition.

The immediate successors of Ptolemy appear to have been too well satisfied with the works of that illustrious astronomer to attempt any alteration in the theory he had so extensively developed, and ages undistinguished by any originality of conception or brilliancy of talent, succeeded the publication of the Almagest. The second Theon, Pappus, and Simplicius are justly celebrated as commentators of Ptolemy, Aristarchus, and Aristotle; but the other persons who held the chair of philosophy in the east, from the second to the ninth century, are chiefly known as compilers of mathematical elements, in which occur only some occasional notices of astronomy: the low state of the science within this period may be concluded from the language of Isidorus of Seville, who, in the seventh century, and in a work intended for the learned, gravely states that the rising sun is seen at the same moment in Britain and India; that the stars are sustained and guided by angels, and that the universe turns on a material axis which is prevented from taking fire, in consequence of the friction, by water descending on it from heaven.

The Syntaxis of Ptolemy still remained the text book of the schools, and the Arabians, who rekindled the science, did it from the lamp which had been burning, though with continually declining lustre, in the Institution of Alexandria; but it is not to be expected that, during the short period in which the Khalifs were able to afford patronage to the learned, the foundation of astronomy should be renewed; in fact the hypothesis concerning