If any one ſhould be at the Diſſection of a Dog, and be there ſhewn the Intrails, the Heart, Stomach, Liver, Lungs and Guts, all the Veins, Arteries and Nerves; could ſuch a Man reaſonably doubt whether there were the ſame Contexture and Variety of Parts in a Bullock, Hog, or any other Beaſt, tho’ he had never chanc’d to ſee the like opening of them? I don’t believe he would. Or were we thoroughly ſatisfy’d in the Nature of one of the Moons round Jupiter, ſhould not we ſtraight conclude the ſame of the reſt of them? So if we could be aſſur’d in but one Comet, what it was that is the Cauſe of that ſtrange Appearance, ſhould we not make that a Standard to judge of all others by? [Arguments from their Similitude, of no ſmall weight.] ’Tis therefore an Argument of no ſmall Weight that is fetch’d from Relation and Likeneſs; and to reaſon from what we ſee and are ſure of, to what we cannot, is no falſe Logick. This muſt be our Method in this Treatiſe, wherein from the Nature and Circumſtances of that Planet which we ſee before our Eyes, we may gueſs at thoſe that are farther diſtant from us. [The Planets are ſolid, and not without Gravity.] And, Firſt, ’tis more than probable that the Bodies of the Planets are ſolid like that of our Earth, and that they don’t want what we call Gravity, that Virtue, which like a Loadſtone attracts whatſoever is near the Body to its Center. And that they have ſuch a Quality, their very Figure is a Proof; for their Roundneſs proceeds only from an equal preſſure of all their Parts tending to the ſame Center. Nay more, we are ſo skilful now-a-days, as to be able to tell how much more or leſs the Gravitation in Jupiter or Saturn is than here; of which Diſcovery and its Author you may read my Eſſay of the Cauſes of Gravitation.