III.

SE C T. fyftem, may be traced in what is delivered down to us concerning the doctrine of Empedocles, of Archytas, of Timæus, and of Ocellus the Lucanian, the most renowned philofophers of the Italian school. The opinions of the two laft coincide pretty much; the one, with thofe of Plato; the other, with thofe of Ariftotle; nor do thofe of the two first seem to have been very different, of whom the one was the author of the doctrine of the Four Elements, the other the inventor of the Categories; who, therefore, may be regarded as the founders, the one, of the ancient Phyfics; the other, of the ancient Dialectic; and, how clofely thefe were connected will appear hereafter. It was in the fchool of Socrates, however, from Plato and Ariftotle, that Philofophy first received that form, which introduced her, if one may fay fo, to the general acquaintance of the world. It is from them, therefore, that we shall begin to give her history in any detail. Whatever was valuable in the former fyftems, which was at all confiftent with their general principles, they feem to have confolidated into their own. From the Ionian philofophy, I have not been able to discover that they derived any thing. From the Pythagorean fchool, both Plato and Ariftotle feem to have derived the fundamental principles of almost all their doctrines. Plato, too, appears to have borrowed fomething from

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two other fects of philofophers, whofe extreme s obfcurity feems to have prevented them from acquiring themselves any extenfive reputation; the one was that of Cratylus and Heraclitus; the other was that of Xenophanes, Parmenides, Meliffus, and Zeno. To pretend to rescue the system of any of those antefocratic fages, from that oblivion which at present covers them all, would be a vain and useless attempt. What feems, however, to have been borrowed from them, fhall fometimes be marked as we go along.

There was still another fchool of philofophy, earlier than Plato, from which, however, he was fo far from borrowing any thing, that he feems to have bent the whole force of his reafon to difcredit and expofe its principles. This was the philofophy of Leucippus, Democritus and Protagoras, which accordingly feems to have fubmitted to his eloquence, to have lain dormant, and to have been almoft forgotten for fome generations, till it was afterwards more fuccessfully revived by Epi

curus.

E C T.

III.

SECT.

SECT.
IV.

OF

SECT. IV.

The Hiftory of Aftronomy.

all the phoenomena of nature, the celeftial appearances are, by their greatnefs and beauty, the most universal objects of the curiofity of mankind. Those who furveyed the heavens with the most careless attention, neceffarily diftinguished in them three different forts of objects; the Sun, the Moon, and the Stars. These last, appearing always in the fame fituation, and at the fame diftance with regard to one another, and feeming to revolve every day round the earth in parallel circles, which widened gradually from the poles to the equator, were naturally thought to have all the marks of being fixed, like fo many gems, in the concave fide of the firmament, and of being carried round by the diurnal revolutions of that folid body: for the azure sky, in which the stars seem to float, was readily apprehended, upon account of the uniformity of their apparent motions, to be a folid body, the roof or outer wall of the univerfe, to whofe infide all those little sparkling objects were attached.

The Sun and Moon, often changing their distance and fituation, in regard to the other heavenly

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IV.

heavenly bodies, could not be apprehended to S E C T. be attached to the fame fphere with them. They affigned, therefore, to each of them, a fphere of its own; that is, fuppofed each of them to be attached to the concave fide of a folid and tranfparent body, by whose revolu tions they were carried round the earth. There was not indeed, in this cafe, the fame ground for the supposition of such sphere as in that of the Fixed Stars; for neither the Sun nor the Moon appear to keep always at the fame diftance with regard to any one of the other heavenly bodies. But as the motion of the Stars had been accounted for by an hypothefis of this kind, it rendered the theory of the heavens more uniform, to account for that of the Sun and Moon in the fame manner. The sphere of the Sun they placed above that of the Moon; as the Moon was evidently feen in eclipfes to pafs betwixt the Sun and the Earth. Each of them was fuppofed to revolve by a motion of its own, and at the fame time to be affected by the motion of the Fixed Stars. Thus, the Sun was carried round from eaft to weft by the communicated movement of this outer fphere, which produced his diurnal revolutions, and the viciffitudes of day and night; but at the fame time he had a motion of his own, contrary to this, from weft to eaft, which occafioned his annual revolution, and the continual shifting

of

IV.

SE C T. of his place with regard to the Fixed Stars. This motion was more eafy, they thought, when carried on edgeways, and not in direct oppofition to the motion of the outer sphere, which occafioned the inclination of the axis of the sphere of the Sun, to that of the fphere of the Fixed Stars; this again produced the obliquity of the ecliptic, and the confequent changes of the feafons. The moon, being placed below the sphere of the Sun, had both a fhorter course to finish, and was lefs obftructed by the contrary movement of the fphere of the Fixed Stars, from which fhe was farther removed. She finished her period, therefore, in a fhorter time, and required but a month, instead of a year, to complete it.

The Stars, when more attentively furveyed, were fome of them obferved to be lefs conftant and uniform in their motions than the reft, and to change their fituations with regard to the other heavenly bodies; moving generally eaftwards, yet appearing fometimes to stand ftill, and fometimes even to move weftwards. Thefe, to the number of five, were distinguished by the name of Planets, or wandering Stars, and marked with the particular appellations of Saturn, Jupiter, Mars, Venus, and Mercury. As, like the Sun and Moon, they feem to accompany the motion of the Fixed. Stars from eaft to west, but at the fame time.

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