Chap. i. PHILOSOPHY. 167

periods. But it was the opinion of Des Cartes 1 , that theplanets might be kept in their courses by the means of a fluidmatter, which continually circulating round should carrythe planets along with it. There is one appearance thatmay seem to favour this opinion; which is, that the fun turnsround its own axis the fame way, as the.planets move. Theearth also turns round its axis the. fame way, as the moonmoves round the earth. And the planet Jupiter turns uponits axis the fame way, as his satellites revolve round him. Itmight therefore be supposed, that if the whole planetary regionwere filled with a fluid matter, the fun, by turning round onits own axis, might communicate motion first to that part ofthe fluid, which was contiguous, and by degrees propagatethe like motion to the parts more remote. After the famemanner the earth might communicate motion to this fluid, toa distance sufficient to carry round the moon, and Jupiter com-municate the like to the distance of its satellites. Sir I s a a cNewton has particularly examined what might be the resultof such a motion as this b ; and he finds, that the velocities,with which the parts of this fluid will move in different di-stances from the center of the motion, will not agree with themotion observed in different planets: for instance, that thetime of one intire circulation of the fluid, wherein Jupitershould swim, would bear a greater proportion to the time ofone intire circulation of the fluid, where the earth is; than theperiod of Jupiter bears to the period of the earth. But healso proves c , that the planet cannot circulate in such a fluid,

a InPrinc. philos.part. I 8cschol.

b Philos. princ. mathem. Lib. II. prop. 1. | c Ibid. prop.5-3.

so