PHYSICAL AND MATHEMATICAL SCIENCES. 265
ciety an account of a series of experiments for deter-mining whether bodies experience any variation in theirweight at different distances from the centre of the earth.His first “experiments (as he himself confesses) werevery unsatifactory ; but they led him to the ingeniousidea of measuring the force of gravity by observing therate of a pendulum at different altitudes. How far heput this in practice does not appear. He also gave asort of mechanical illustration of motion in an orbit, bya weight freely suspended by a string, and tlms madeto oscillate either in an elliptic or circular course.
In l6'74, Hooke resumed the subject in a dissertationentitled “An Attempt to prove the Motion of the Earth from Observation,” in which the following remarkablepassage occurs:-
“ I shall hereafter explain a system of the worlddiffering in many particulars from any yet known,answering in all things to the common rules of mecha-nical motions. This depends upon three suppositions.1st, That all celestial bodies whatsoever have an at-traction or gravitating power towards their own centres,whereby they attract not only their own parts, and keepthem from flying from them, as we may observe theearth to do, but that they also do attract all the othercelestial bodies that are within the sphere of theiractivity, and consequently, that not only the sun andmoon have an influence upon the body and motion ofthe earth, and the earth upon them, but that Mercury,Venus, Mars, Jupiter and Saturn also by their attractivepowers have a considerable influence upon its motion,as in the same manner, the corresponding attractivepower of the earth hath a considerable influence uponevery one of their motions also. The 2d suppositionis this ; That all bodies whatsoever, that are put into adirect and simple motion, will so continue to moveforward in a straight line, till they are by some othereffectual powers deflected and sent into a motion des-cribing a circle, ellipsis, or some other compoundedcurve line. The 3d supposition is, That those attract-