SEC. XL] OF NATURAL PHILOSOPHY. 195 between the bodies. Now these distances revolve about their common term with an equable angular motion, because lying in the same right line they never change their inclination to each other mutually But right lines that are in a given ratio to each other, and revolve about their terms with an equal angular motion, describe upon planes, which either rest with those terms, or move with any motion not angular, figures entirely similar round those terms. Therefore the figures described by the revolution ot these distances are similar. Q.E.D. PROPOSITION LVIll. THEOREM XXI. If two bodies attract each other mutually with forces of any kind, and in the mean time revolve about the common centre of gravity ; I say, that, by the same forces, there may be described round either body un moved ajigure similar and equal to the figures ivhich the bodies so moving describe round each other mutually. Let the bodies S and P revolve about their common centre of gravity C, proceeding from S to T, and from P to Q,. From the given point s lot there be continually drawn sp, sq, equal and parallel to SP, TQ, ; and the ;urve pqv, which the point p describes in its revolution round the immovable point s, will be similar and equal to the curves which the bodies S and P' describe about each other mutually ; and therefore, by Theor. XX, similar to the curves ST and PQ,V which the same bodies describe about their common centre of gravity C • and that because the proportions of the lines SC. CP, and SP or sp, to each other, are given. CASE 1. The common centre of gravity C (by Cor. 4, of the Laws of Mo tion) is either at rest, or moves uniformly in a right line. Let us first suppose it at rest, and in s and p let there be placed two bodies, one im movable in s, the other movable in p, similar and equal to the bodies S arid P. Then let the right lines PR and pr touch the curves PQ, and pq ki P and p, and produce CQ, and sq to R and r. And because the figures CPRQ, sprq are similar, RQ, will be to rq as CP to sp, and therefore in a given ratio. Hence if the force with which the body P is attracted to wards the body S, and by consequence towards the intermediate point the centre C, were to the force with which the body p is attracted towards the Centre 5. in the same given ratio, these forces would in equal times attract