68

Sir Isaac Newton’s Book I.

hold it fixed at reft; the effed of the power of gravity on abody is the fame, as if that whole power were to exert itfelfon the center of gravity only. Whence it follows,, that, whenthe power of gravity ads on a body fufpended by any point,if the body is fo fufpended, that the center of gravity of thebody can defeend ; the power of gravity will give motion tothat body, otherwife not: or if a number of bodies are foconneded together, that, when any one is put into motion,the reft fhall, by the manner of their being joined, receivefuch motion, as fhall keep their common center of gravity atreft ; then the power of gravity fhall not be able to produceany motion in thefe bodies, but in all other cafes it will.Thus, if the body AB (in fig. xo,xi.) whofe center of gra-vity is Cj be hung on the point A, and the center C be per-pendicularly under A (as in fig. xo.) the weight of the bo- 'dy will hold it {till without motion, becaufe the center Ccannot defeend any lower. But if the body be removed in-to any other fituation, where the center C is not perpendi-cularly under A ( as in fig. xi.) the body by its weight willbe put into motion towards the perpendicular fituation of itscenter of gravity. Alfo if two bodies A, B (in fig. xx.) bejoined together by the rod C D lying in an horizontal fitua-tion, and be fupported 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 D defeending, in which the common center ofgravity is found. So in like manner, if thefe two bodies wereconneded together by any more complex contrivance ; yet

if