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(88.) An instance of the composition of motion, which is worthy of some attention, as it affords a proof of the diurnal motion of the earth, is derived from observing the descent of a body from a very high tower. To render the explanation of this more simple, we shall suppose the tower to be on the equator of the earth. Let EPQ, fig.18., be a section of the earth through the equator, and let PT be the tower. Let us suppose that the earth moves on its axis in the direction EPQ. The foot P of the tower will, therefore, in one day move over the circle EPQ, while the top T moves over the greater circle TT′R. Hence it is evident, that the top of the tower moves with greater speed than the foot, and therefore in the same time moves through a greater space. Now suppose a body placed at the top; it participates in the motion which the top of the tower has in common with the earth. If it be disengaged, it also receives the descending motion TP. Let us suppose that the body would take five seconds to fall from T to P, and that in the same time the top T is moved by the rotation of the earth from T to T′, the foot being moved from P to P′. The falling body is therefore endued with two motions, one expressed by TT′, and the other by TP. The combined effect of these will be found in the usual way by the parallelogram. Take T p equal to TT′; the body will move from T to p in the time of the fall, and will meet the ground at p. But since TT′ is greater than PP′, it follows that the point p must be at a distance from P′ equal to the excess of TT′ above PP′. Hence the body will not fall exactly at the foot of the tower, but at a certain distance from it, in the direction of the earth’s motion, that is, eastward. This is found, by experiment, to be actually the case; and the distance from the foot of the tower, at which the body is observed to fall, agrees with that which is computed from the motion of the earth, to as great a degree of exactness as could be expected from the nature of the experiment.