Page:The Mathematical Principles of Natural Philosophy - 1729 - Volume 1.djvu/40

Rh hence will arie a mighty reitance to the paage of the Comets, and uch as can't but be very enible; not to ay, enough to put a top to, and aborb, their motions entirely. But now it appears from the perfectly regular motion of the Comets, that they uffer no reitance that is in the leat enible; and therefore that they meet with no matter of any kind, that has any reiting force, or, by conequence, any denity or vis inertia. For the reitance of mediums aries, either from the inertia of the matter of the fluid, or from its want of lubricity. That which aries from the want of lubricity is very mall, and is carce obervable in the fluids commonly known, unles they be very tenacious like oil and honey. The reitance we find in air, water, quick-ilver and the like fluids that are not tenacious, is almot all of the firt kind; and cannot be diminihed by a greater degree of ubtilty, if the denity and vis inertia, to which this reitance is proportional, remains; as is mot evidently demontrated by our Author in his noble theory of reitances in the econd book.

Bodies in going on through a fluid communicate their motion to the ambient fluid by little and little, and by that communication loe their own motion. and by loing 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 denity of the fluid; and therefore the retardation or reitance will be as the ame denity of the fluid; nor can it be taken away, unles the fluid coming about to the hinder parts of the body retore the motion lot. Now this cannot be done unles the impreion of the fluid on the hinder