Chap. 8. PHILOSOPHICAL DISCOVERIES; 3,71

been generally esteemed two different comets. In Novemberswas descending towards the fun ; it passed very near the fun onthe i 2 th day of December ; where, having been heated to aprodigious degree, tho’ the light of the head or nucleus . wasduller, yet, while it ascended in the other half of its orbit,its tail was vastly greater than before, extending sometimes yo°in length, and continuing visible even after the head or nucleuswas carried out of sight.

Dr. Halley , to whom every part of astronomy, but this ina particular manner, is highly indebted, has joined his laboursto our author’s on this subject; nor is it necesiary for us to se-parate them. * Finding three observations of comets recordedin history, agreeing with this in remarkable circumstances, andreturning at the distance of 575 years from each other, he sus-pected that these might be one and the fame comet, revolvingin that period about the fun. He therefore supposed the para-bola to be changed into such an excentric ellipse as the cometmight describe in 5 7 5 years, and as should nearly coincide withthe parabola in its lowest part ; and, having computed theplaces of the comet in this elliptic orbit, he found them toagree so well with those in which the comet was observed tQpass, that the variations did not exceed the differences whichare found betwixt the computed and the observed places of theplanets, whose motions had been the subject of astronomicalcalculation for some thousand years. This comet may, there-,fore, be expected again after finishing the fame period,- aboutthe year 2255. If it then return, it will give a new lustre andevidence to Sir Isaac Newton's philosophy, in that distantage. And should the inconstancy of human affairs, and the per-petual revolutions to which they are subject, occasion any ne-glect of our philosophy in the intervening ages ; this cometwill revive it, and fill every mouth again with this great mans

B b b 2 name*