Sir I s a a c Newton’s Book II.

174

power, our author proceeds thus. He inquires what relationthere ought to be between the periods of the different planets,provided they were acted upon by the fame power decreasingthroughout in the forementioned proportion; and he finds,that the period of each in this cafe would have that very rela-tion to the greater axis of its orbit, as I have declared above ato be found in the planets by the observations of astronomers.And this puts it beyond question, that the disierent planets arepressed towards the fun, in the fame proportion to their distan-ces, as one planet is in its several distances. And thencein the last place it is justly concluded, that there is such apower acting towards the fun in the forefaid proportion at alldistances from it.

5. This power, when referred to the planets, our author•calls centripetal, when to the fun attractive; he gives it like-wise the name of gravity, because he finds it to be of the famenature with that power of gravity, which is observed in ourearth, as will appear hereafter b . By all these names he designsonly to signify a power endued with the properties beforementioned; but by no means would he have it understood, asif these names referred any way to the cause of it. In particularin one place where he uses the name of attraction, he cauti-ons us expressly against implying any thing but a power di-recting a body to a center without any reference to the causeof it, whether residing in that center, or arising from anyexternal impulse c .

• ch.1.5 7 .

Chap.f. § 8.

* Princ.pag. do.

6 . But