407 with the medium through which it is carried, namely air, is very heavy; and since it goes away with an amount of impressed force as great as its heaviness, it assuredly goes away with a great impressed force, which the motion from the height of a tower is not sufficient to consume, so that the swiftness is always intensified all through the height of a single tower.Now if we were to take a certain heavy thing, whose heaviness would not so very far surpass the heaviness of air, assuredly we would then see with our own eyes that this thing, a little after the beginning of its motion, would always be moved uniformly, provided however that the air was very calm. And we would observe the same thing happen in the case of a stone, if it were let down from a very high place, and we were so placed as always to observe the line of motion according to the same disposition. {1}For even our position hinders us from apprehending the uniformity of the motion. For let there be a uniform motion from b to f, and let bc, cd, de, ef be equal distances; and let the eye of the observer be at a, and let lines of sight ab, ac, ad, ae, af be drawn: and since the motion is assumed to be uniform, and bc, cd, de, ef, are equal distances, therefore the mobile will go over them in equal times. Hence the time of transit from b to c will be equal to the time of transit from c to d: the motion from c to d, however, will appear faster to the observer, since distance cd also appears greater than distance bc (for it is seen under a greater angle {1}). And thus the motion from d to e will appear faster than that from c to d, since distance de appears greater than cd, and it is traversed by the mobile in an equal time: and for the same reason, the motion from e to f will appear faster than the motion from d to e. Hence the whole of motion bf will appear difform, and always faster at the end, although it is assumed to be uniform. It is thus necessary for distinguishing the uniformity and the difformity of motion that the distance be extended enough that the mobile is able to consume the resisting force in it, and that the eye be disposed in such a way that it is deceived as little as possible by the disparity of the angles.
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