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^ook IV. of Natural Philosophy <

^ is plain, that if there be any celestial Matter,

^d that it is in motion, it does not exert afusible Action on the Bodies of the, planetarysystem, which is also deduc’d from the smallResistance of such a Medium ; for by comparingshe most ancient Observations with the modern,

1|: does not appear that the Planets are sensiblyYarded in their Motions: Yet in Air the Resist-j^ce is sensible, wherefore the Density of the Me-[Rum, in which the Planets lhould move, must"e almost immensely less; therefore the planetary jhflem is not filled, unless it be by such a subtile^ “tedium .

But we may from the Divisibility of Matter^duce, that a Quantity of Matter, how smallsoever it be, may be dispersed ail over the plane-tary System, leaving but very small Interstices*. * tg

CHAP. XIII.

Concerning the Motion of the Earth .

B Esides the Question that has been handled inthe foregoing Chapter, there is also anothert0 be examin’d, before we proceed to the Expli-Ca don of the whole System.

* That no Doubt may be made concerning thelystem, which has been explained in the first^apter of this Book, we must here prove thepotion of the Earth, concerning which it is nounder that many have doubted ; for the cele-Motions cannot be determin’d by us, but by^ser various made by Observers on the Earth,

and