hence will ariſe a mighty reſiſtance to the paſſage of the Comets, and ſuch as can't but be very ſenſible; not to ſay, enough to put a ſtop to, and abſorb, their motions entirely. But now it appears from the perfectly regular motion of the Comets, that they ſuffer no reſiſtance that is in the leaſt ſenſible; and therefore that they meet with no matter of any kind, that has any reſiſting force, or, by conſequence, any denſity or vis inertia. For the reſiſtance of mediums ariſes, either from the inertia of the matter of the fluid, or from its want of lubricity. That which ariſes from the want of lubricity is very ſmall, and is ſcarce obſervable in the fluids commonly known, unleſs they be very tenacious like oil and honey. The reſiſtance we find in air, water, quick-ſilver and the like fluids that are not tenacious, is almoſt all of the firſt kind; and cannot be diminiſhed by a greater degree of ſubtilty, if the denſity and vis inertia, to which this reſiſtance is proportional, remains; as is moſt evidently demonſtrated by our Author in his noble theory of reſiſtances in the ſecond book.
Bodies in going on through a fluid communicate their motion to the ambient fluid by little and little, and by that communication loſe their own motion. and by loſing it are retarded. Therefore the retardation is proportional to the motion communicated; and the communicated motion, when the velocity of the moving body is given, is as the denſity of the fluid; and therefore the retardation or reſiſtance will be as the ſame denſity of the fluid; nor can it be taken away, unleſs the fluid coming about to the hinder parts of the body reſtore the motion loſt. Now this cannot be done
unleſs the impreſſion of the fluid on the hinder
