Page:The Mathematical Principles of Natural Philosophy - 1729 - Volume 1.djvu/50

that may ſerve exactly to retain a body in a given orbit with a given velocity; and vice verſa, to determine the curvilinear way into which a body projected from a given place, with a given velocity, may be made to deviate from its natural rectilinear way, by means of a given force.

The quantity of any Centripetal Force may be conſidered as of three kinds: Abſolute, Accelerative, and Motive.

Definition VI.

The abſolute quantity of a centripetal force is the measure of the ſame, proportional to the efficacy of the cauſe that propagates it from the centre, through the spaces round about.

Thus the magnetic force is greater in one load-ſtone and leſs in another, according to their ſizes and ſtrength.

Definition VII.

The accelerative quantity of a centripetal force is the meaſure of the ſame, proportional to the velocity which it generates in a given time.

Thus the force of the ſame load-ſtone is greater at a leſs distance, and leſs at a greater: alſo the force of gravity is greater in valleys, leſs on tops of exceeding high mountains; and yet leſs (as ſhall hereafter be ſhewn) at greater diſtances from the body of the