68

Sir Isaac Newt o n’s Book I.

hold it fixed at rest; the effect os the power os gravity on abody is the fame, as if that whole power were to exert itselfon the center os gravity only. Whence it follows, that, whenthe power of gravity acts on a body suspended by any point,if the body is so suspended, that the center of gravity of thebody can descend ; the power of gravity will give motion tothat body, otherwise not: or is a number of bodies are so'/ connected together, that, when any one is put into motion,the rest shall, by the manner of their being joined, receivestich motion, as shall keep their common center of gravity atrest ; then the power of gravity shall not be able to produceany motion in these bodies, but in all other cafes it will.Thus, ifthebodyAB (infig. 10,2.1.) whose center cf gra-vity is C, be hung on the point A, and the center C be per-pendicularly under A (as in fig. zo. ) the weight of the bo-dy will hold it still without motion, because the center Ccannot descend any lower. But if the body be removed in-to any other situation, where the center C is not perpendi-cularly under A ( as in fig. zi.) the body by- its weight will.be put into motion towards the perpendicular situation of itscenter of gravity. Also if two bodies A, B (in fig. zz.) bejoined together by the rod CD lying in an horizontal situa-tion, and be supported at the point E; if this point be thecenter of gravity common to the two bodies, their weightwill,not put them into motion; but if this point E is not theircommon,center of gravity, the bodies will move ; that partof the rod C JD descending, in which the common center ofgravity is found. So in like manner, if these two bodies wereconnected together by any more complex contrivance; yet

if