(808)
which exhaleth from the Sun and the other Planets, not onely issues out of them in a straight way, but also, whil'st those Bodies, together with their Atmosphere, move round, turnes constantly about with them, till it desert the said Atmosphere. To which he adds, that the matter of those exhalations are of the like nature with our Terrestrial Vapors, in that they are apt to condense, and after a while to be dissolved, and to return again to their first principles. Having supposed this, he assumes farther, that Bodies turn'd round, the longer they move, and the greater the Circle is, they move in, the greater impetus and vehemence they acquire, tending alwayes to recede from the Center to the Circumference, and being separated and set at liberty from their vertiginous or circular motion, they continue to be moved (as long as no stop intervenes,) but no more in that circulary, but in a straight line, viz. in the Tangent of the Circle.
After this manner he concludeth, That as soon as vapors exhale or are expelled out of a Celestial Body, whether it be the Sun or other Planets, into the Atmosphere incompassing it (which that they do, he endeavours to prove) they still acquire more and more force to move from the Center or the Planet to the Circumference (by vertue of the swift circumrotation of the Atmosphere, which together with its Body or Planet is turn'd round about,) till at last, being ejected out of their Circle into the free Sky, and more and more supplyed with the accession of the like matter, they make up the Bodies of Comets, and then move in a straight line, obverting one side to the Sun, as Clouds do theirs to our Earth.
Having laid down this, he deduceth thence the explication of the Phænomena, and Questions, hitherto observed and raised in and about Comets, viz. How they increase in bulk, and commonly with much quickness near the Sun? Why they are made up of several Kernels? Why not all Cometick matter is moved towards the same quarter of the Heavens? How it comes to pass, that at one and the same time more Comets than one are produced, and that they meet one another, and by their mutual concourse change one anothers motion, or break into parts and so constitute several Stars? Why all Comets are not visible to us? Why there are more Comets in one Age, than another?
