338 than before, it will now, undoubtedly, be moved more strongly by the force. But that which strives against something resists more than that which is either at rest or carried in the same direction: the force, then, is impressed more compactly in that which strives against it; those who play ball know this from experience; they want the ball to be thrown hard at them by someone, so that more motive force may be impressed in it, as it then opposes and resists more. But, as we have just said {1}, only those can accomplish this who are equipped with a strong and vigorous arm: but those with a languishing strudiness, and who are not able to strive against the impetus, put the ball in motion in the other direction when it is at rest or not moving in a contrary direction: but if the ball is moved in the same direction, as everyone knows, it is impelled only a little. As a matter of fact the cause of this effect is that a thing at rest that is struck with a very great force is moved before all the force is impressed, because its mobility {1} does not wait for such a great force to be impressed on it: this does not happen in the case of what is put in motion in the contrary directions {2} ; for, since its resistance has been increased by the movement of its heaviness {cf. my notes1below}, it resists more, and does not move back before all the force has been impressed. We all experience the same thing when we want to project a stone forward: for we first carry it swiftly backward with our hand so that, having been moved in a contrary direction, not only the stone but also the hand may resist more the force that must be impressed: but if, when it has been carried backward, we stopped the hand and that the stone, having been moved backward, came to rest, it could be thrown much less far, as is evident to everyone. It is thus necessary that the stone, to be thrown with a greater impetus, not come to rest at the turning point after it has been carried back.And the same thing is evident in the case of those who project a stone with a sling: for they first whirl the sling around two or three times in a circle, so that it may be moved faster; and finally they change this motion to a backward one, so that then a greater force may be impressed in the resisting stone.
These things having been noted, I bring forth a twofold cause for the present question: first I say that, even though the motive force in the cannon is the same, yet more of this same force is impressed in the iron ball, the more erect the cannon is set.The cause of this is that the ball then resists the force more: for it is moved with more difficulty in the ball chamber when it is to be impelled upward, as though on a more erect plane, than when it is more inclined; hence it also resists more the force that must be impressed on it. For when the cannon is low nearly falling, the ball then does not wait for the force that must be impressed, but it is shot out before the complete