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and the distance of the surface from the centre, that is,the radius of the earth ; and, squaring these numbers,the inverse proportion would be that of the spaces fallenthrough, in one second, by the moon, and by a body atthe surface of the earth. If, then, this calculated resultagreed with the result actually observed, his conjecturewould be verified; and the very same force of gravitywhich causes bodies to fall near the earth, would bethat which causes the moon to fall, or, in other words,to be deflected from a rectilinear course, and to describeher orbit about the earth. In this calculation, Newton took for the radius of the earth that value which re-sulted from the measurement of the length of a degree,according to the determination of Norwood and others,before mentioned, the best known at that time. Theresult of the calculation did not fulfil Newton ’s anticipa-tion. Hence, with real, philosophic love of truth, hemodified his suppositions to accord with the result:and concluded that, though gravitation is in part thecause of the moon’s motion, yet, st^ie other, as yet un-known, cause conspires with it to produce the effect.
On his return to Cambridge, his attention, as wehave already seen, was fully occupied with other sub-jects; and, not having brought his speculation on gravityto any satisfactory termination, he carefully concealedhis ideas; and probably dismissed the subject from histhoughts for some time.
Meanwhile we have already noticed the approacheswhich had been made by several philosophers, but es-pecially by Hooke, towards the theory of gravitation.And it was by some speculations of this eminent man(now secretary to the Royal Society) that Newton ’s at-tention was recalled to the subject. In 1679, Newton addressed a letter to him, in reply to some queries pro-posed by the society, in which he suggested, that, if theearth be really in motion, it would be actually evinceby the circumstance, that a body let fall from a greatheight (since, by virtue of its elevation, it participatesin a velocity greater than that of bodies on the surface)y 2