326 but it will never happen that this equality lasts for any length of time whatever, since the one motion is continuously weakened but the other is continuously intensified: hence, necessarily, c will switch from one motion to its contrary, without the intervention of any rest. A third argument can be taken from a certain rectilinear motion that Nicholas Copernicus in his De Revolutionibus compounds from two circular motions. For they are two circles, one of which is carried on the circumference of the other, and when one is moved more swiftly than the other, a mark on the circumference (of the slower?) is carried along a straight line and runs back and forth along it continuously; and yet it cannot be said that it is at rest at the extremities, since it is continually carried around by the circumference of the circle. {1} [see also my notes below]
There is a fourth well-known argument concerning a large stone going down from a tower, which will not be sufficiently blocked by a pebble impelled upward by force, so as to permit the pebble to be at rest for any time: hence surely the pebble will not be at rest at the ultimate point of its upward motion, and Aristotle notwithstanding, it will make use of the ultimate point for the two limits, namely of upward motion and of downward motion; and the ultimate instant is taken twice, namely, for the end of one time and for the beginning of the other.
But the adversaries, in order to escape from this, say that the large stone is at rest, and thus they persuade themselves that they have done enough for the argument. But, so that in the future (unless they should be downright obstinate) they may not believe this, I shall add this to the argument: let these stones, which are moved by contrary motions, be carried, not upward and downward, but on a plane surface parallel to the horizon, one with great impetus, but the other more slowly, and let them be moved from contrary positions in contrary directions; and let them converge in the middle in an interacting motion: in that case the weaker will undoubtedly be thrust back by the stronger and will be compelled to be carried in the opposite direction; but how will they say that at that point of impact a rest intervenes? For if only once they were at rest, they would thereafter always be at rest, since they would have no cause for moving, as in the case of that large stone, coming from on high: if it were stopped by the pebble, still after the rest, they would both go down in concordance, moved by their proper heaviness; but when they are on a plane parallel to the horizon, after the rest, there is no cause for motion after the rest. Here is a last argument: before its explication let these two things be presupposed. First, I presuppose that a mobile can be at rest outside its proper place only when the force impeding its descent is equal to its heaviness, which exerts pressure downward: which surely is manifest; for if the impressed force were greater than the resisting heaviness, the mobile