After having ſhown, by his ſublime Theory, the Courſe and Inequalities of the Planets, he ſubjects Comets to the ſame Law. The Orbit of theſe Fires (unknown for ſo great a Series of Years,) which was the Terror of Mankind, and the Rock againſt which Philoſophy ſplit; plac'd by Ariſtotle below the Moon, and ſent back by Des Cartes above the Sphere of Saturn, is at laſt plac'd in its proper Seat by Sir Iſaac Newton.
He proves that Comets are ſolid Bodies which move in the Sphere of the Sun's Activity; and that they deſcribe an Ellipſis ſo very eccentric, and ſo near to Parabola's, that certain Comets muſt take up above five hundred Years in their Revolution.
The learned Dr. Halley is of opinion, that the Comet ſeen in 1680, is the ſame which appear'd in Julius Cæſar's Time. This ſhows more than any other, that Comets are hard, opake Bodies; for it deſcended ſo near to the Sun, as to come within a ſixth Part of the Diameter of this Planet from It; and
