Chap. 2. PHILOSOPHY.

171

Chap. II.

Concerning the caufe, which keeps in motionthe primary planets.

S INCE the planets move in a void fpace and are freefrom reliftance; they, like all other bodies, when -once in motion, would move on in a ftraight line withoutend, if left to themfelves. And it is now to be explainedwhat kind of adtion upon them carries them round the fun.Here I fhall treat of the primary planets only, and dif-courfe of the fecondary apart in the next chapter. It has beenjuft now declared, that thefe primary planets move fo aboutthe fun, that a line extended from the fun to the planet, will,by accompanying the planet in its motion, pafs over equal {pa-ces in equal portions of time a . And this one property in themotion of the planets proves, that they are continually adtedon by a power directed perpetually to the fun as a center. Thistherefore is one property of the caufe, which keeps theplanets in their courfes, that it is a centripetal power, wholecenter is the fun.

l. Again, in the chapter upon centripetal forces b itWas obferv’d, that if the ftrength of the centripetal powerWas fuitably accommodated every where to the motion ofany body round a center, the body might be carried in

* Cb. 1. $ 7. b Book I. Ch. j.

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