Chap. 2. PHILOSOPHY.

Chap. II.

Concerning the cause, which keeps in motionthe primary planets.

S INCE the planets move in a void space and are freefrom resistance ; they, like all other bodies, whenonce in motion, would move on in a straight line withoutend, is left to themselves. And it is now to be explainedwhat kind of action upon them carries them round the fun.Here I shall treat of the primary planets only, and dis-course of the secondary apart in the next chapter. It has beenjust now declared, that these primary planets move so aboutthe fun, that a line extended from the fun to the planet, will,by accompanying the planet in its motion, pass over equal spa-ces in equal portions of time \ And this one property in themotion of the planets proves, that they are continually actedon by a power directed perpetually to the fun as a center. Thistherefore is one property of the cause, which keeps theplanets in their courses, that it is a centripetal power, whosecenter is the fun.

2 . Again, in the chapter upon centripetal forces b itwas observ’d, that if the strength of the centripetal powerwas suitably accommodated every where to the motion ofany body round a center, the body might be carried in

b Book I. Ch. j.

Z 2 an 7

* Ch. i. $7.