Sir Isaac Newton*s

Book II.

230

C H A P. IV.

Of COMETS.

I N the former of the two preceding chapters the powershave been explained, which keep in motion those cele-stial bodies, whose courses had been well determined by theastronomers. In the last chapter we have shewn, how thosepowers have been applied by our author to the making amore perfect discovery of the motion of those bodies, thecourses of which were but imperfectly understood ; forsome of the inequalities, which we have been describingin the moon’s motion, were unknown to the astronomers.In this chapter we are to treat of a third species of the hea-venly bodies, the true motion of which was not at all ap-prehended before our author writ; in so much, that hereSir Isaac Newton has not only explained the causes ofthe motion of these bodies, but has performed also the partof an astronomer, by discovering what their motions are.

tl . That these bodies are not meteors in our air, ismanifest; because they rife and set in the fame manner,as the fun and stars. The astronomers had gone so far intheir inquiries concerning them, as to prove by their ob-servations, that they moved in the etherial spaces far beyondthe moon ; but they had no true notion at all of the path,which they described. The most prevailing opinion before

our