Chap. 2. PHILOSOPHICAL DISCOVERIES. 143

earth, is equal to that which, impressed in a horizontal direc-tion, would carry it in a circle about the earth for ever, ab-stracting from the resistance of the air; as appears from thetheory of gravity : and yet the first would overcome the re-sistance arising from the gravity of the body for a certain timeonly ; whereas the other would overcome that resistance forever, without any diminution of motion. In the first cafe,the gravity of the body would act directly against its force; inthe second, it would act in a line perpendicular to the directionof its motion : in the first cafe, the action of gravity is entirelyemployed in consuming the force of the body; in the other,in changing its direction only. The arguments drawn in favourof the new opinion from the resolution of motion, seem, atfirst sight, the most plausible of any that have been offered forit; but, from the considerations which we have suggested, itmay appear to an'impartial reader, that instead of overthrowingthe common doctrine, they rather confirm it. As, in other in-stances, Mr. Leibnitz % followers neglect the consideration oftime, in reasoning concerning the forces of bodies ; so here wefind that they have not due regard to the directions of motion?and forces, in estimating and comparing their effects ; which,however, in mechanical enquiries, are of no less importancethan the motions or forces themselves.

27. We have insisted on theso observations, because they setthe theory of motion in a plain and just light. We often ob-tain this advantage from disputes concerning the elementarypropositions of any science, that they are the more carefully en-quired into, and when sound just, are illustrated and the betterunderstood for having been disputed. We cannot, however,leave this subject without mentioning an experiment, made bythe 1 ingenious and accurate Mr. Graham , to whom tire me-chanical sciences arc so much indebted. He prepared a pen-

1 di ilous