But if the impulse be received in a direction immediately opposed to the previous motion, then it will diminish the speed by that amount of velocity which it would give to the body had it been previously at rest. In the example already given, if the impulse were opposed to the previous motion, the velocity of the body after the impulse would be five feet in a second. If the impulse received in the direction opposed to the motion be such as would give to the body at rest a velocity equal to that with which it is moving, then the effect will be, that after the impulse no motion will exist; and if the impulse would give it a still greater velocity, the body will be moved in the opposite direction with an uniform velocity equal to the excess of that due to the impulse over that which the body previously had. When a body in a state of uniform motion receives an impulse in a direction not coinciding with that of its motion, it will move uniformly after the impulse in an intermediate direction, which may be determined by the principles established for the composition of motion in the last chapter. Thus it appears, that whenever the state of a body is changed either from rest to uniform rectilinear motion or _vice versa_, or from one state of uniform rectilinear motion to another, differing from that either in velocity or direction, or in both, the phenomenon is produced by that peculiar modification of force whose action continues but for a single instant, and which has been called _an impulse_. (94.) In most cases, however, the mechanical state of a body is observed to be subject to a continual change or tendency to change. We are surrounded by innumerable examples of this. A body is placed on the table. A continual pressure is excited on the surface of the table. This pressure is only the consequence of the continual tendency of the body to move downwards. If the body were excited by a force of the nature of an impulse, the effect upon the table would be instantaneous, and would immediately cease. It would, in fact, be _a blow_. But the continuation of the pressure proves the continuation of the action of the force. If the table be removed from beneath the body, the force which excites it being no longer resisted, will produce motion; it is manifested, not as before, by a tendency to produce motion, but by the actual exhibition of that phenomenon. Now if the exciting force were an impulse, the body would descend to the ground with an uniform velocity. On the other hand, as will hereafter appear, every moment of its fall increases its speed, and that speed is greatest at the instant it meets the ground. A piece of iron placed at a distance from a magnet approaches it, but not with an uniform velocity. The force of the magnet continues to act during the approach of the iron, and each moment gives it increased motion.