148 Sir Isaac Newton’s Book I.

other portions oF matter, by which power the particles of fluidslike other bodies make resistance against being put into motion.

9. The resistance, which arises From the friction of thebody against the parts of the fluid, must be very inconsidera-ble; and the resistance, which follows from the tenacity ofthe parts of fluids, is not usually very great, and does notdepend much upon the velocity of the body in die fluid ;for as the parts of the fluid adhere together with a certaindegree of force, die resistance, which the body receives fromthence, cannot much depend upon the velocity, with whichthe body moves; but like the power of gravity, its effect mustbe proportional to the time of its acting. This the readermay find farther explained by Sir I s a a c Newton himselfin the postscript to a discourse published by me in the philo-sophical transactions, N° zy I. The principal resistance,which most fluids give to bodies, arises from the power ofinactivity in the parts of the fluids, and this depends upon thevelocity, with which the body moves, on a double accountIn the first place, the quantity of the fluid moved out ofplace by the moving body in any determinate space of timeis proportional to the velocity, wherewith the body moves;and in the next place, the velocity with which each particle ofthe fluid is moved, will also be proportional to the velocity of.die body: therefore since the resistance, which any body makesagainst being put into motion, is proportional both to the quan-tity of matter moved and the velocity it is moved with ; theresistance, which a fluid gives on this account, will be doubly in-creased with the increase os the velocity in the moving body;

that.