1(52

Sir I s a a c Newton’s Book II.

and to fhew from him what caufe keeps the heavenly bo-dies in their courfcs. But it will be necefiary for the ufe ofRich, as are not skilled in aftronomy, to premile a brief de-fcription of the planetary fyftem.

'l. T ii i s fyftem is dilpofed in the following manner. Inthe middle is placed the fun. About him fix globes con-tinually roll. Thefe are the primary planets; that whichis neareft to the fun is called Mercury, the next Venus,next to this is our earth, the next beyond is Mars, afterhim Jupiter, and the outermoft of all Saturn. Befides thelethere are difcovered in this fyftem ten other bodies, whichmove about fome of thefe primary planets in the famemanner, as they move round the fun. Thefe are calledlecondary planets. The moft confpicuous of them is themoon, which moves round our earth; four bodies move inlike manner round Jupiter ; and five round Saturn. Tholewhich move about, Jupiter and Saturn, are ufualJy calledfatellites; and cannot any of them be feen without a te-lefcope. It is not impoflible, but there mpyUae more fe-condary planets, befide thefe ; though our inftrumentshave not yet difcovered any other. This difpofition ofthe planetary or folar fyftem is reprefented in fig. 89.

3. The lame planet is not always equally diftant fromthe fun. But the middle diftance of Mercury is betweent and - of the diftance of the earth from the fun; Venusis diftant from the fun almoft £ of the diftance of theearth; the middle diftance of Mars is fomething more than