Chap. 8. PHILOSOPHICAL DISCOVERIES. 369

prevailed, that they were only meteors generated in the air,like to those we see in it every night, and in a sew momentsvanishing, no care was taken to observe or record their phe-nomena accurately, till of late. Hence this part of astronomyis very imperfect. The number of the comets is far from beingknown : many have been noted by historians formerly, andnot a sew of late observed by astronomers ; and some havebeen discovered accidentally by telescopes, pasting by us, thatnever became visible to the naked eye : so that we may con-clude their number to be very great. Their periods, magni-tudes, and the dimensions of their orbits, are also uncertain.This is a part of science, the perfection of which may be re-served for some distant age, when these numerous bodies, andtheir vast orbits, by long and accurate observation, may beadded to the known parts of the solar system. Astronomy will.appear as- a new seience, after all the discoveries we now boastof: but then it will be remembred, even in those flourishingdays of astronomy, that it was Sir Isaac Newton who dis-covered and demonstrated the principles by which alone suchgreat improvements could be made ; and that he begun andcarried this work so far, that he left to posterity little more todo, but to observe the heavens, and compute after his models.

Having this part of astronomy to deduce almost from itselements, he begins with shewing, against the scholastic phi-losophers, that the comets are above the moon ; because theyparticipate of the apparent diurnal motion, rising and settingdaily, as all things that are not appendent to the earth do, andthat without any sensible diurnal parallax. But, as they areall affected by the annual motion of the earth, appearing, likethe planets, sometimes direct, sometimes retrograde, he con-cludes that, when they become visible to us, they must be inthe regions of the planets. As they are all astected by the

B b b motion