Motion is sometimes distinguished into _absolute_ and _relative_. What “relative motion” means is easily explained. If a man walk upon the deck of a ship from stem to stern, he has a relative motion which is measured by the space upon the deck over which he walks in a given time. But while he is thus walking from stem to stern, the ship and its contents, including himself, are impelled through the deep in the opposite direction. If it so happen that the motion of the man, from stem to stern, be exactly equal to the motion of the ship in the contrary way, the man will be, relatively to the surface of the sea and that of the earth, at rest. Thus, relatively to the ship, he is in motion, while, relatively to the surface of the earth, he is at rest. But still this is not absolute rest. The surface itself is moving by the diurnal rotation of the earth upon its axis, as well as by the annual motion in its orbit round the sun. These motions, and others to which the earth is subject, must be all compounded by the theorem of the parallelogram of forces before we can obtain the _absolute state_ of the body with respect to motion or rest. CHAP. VI. ATTRACTION. (92.) Whatever produces, or tends to produce, a change in the state of a particle or mass of matter with respect to motion or rest, is a force. Rest, or uniform rectilinear motion, are therefore the only states in which any body can exist which is not subject to the present action of some force. We are not, however, entitled to conclude, that because a body is observed in one or other of these states, it is therefore uninfluenced by any forces. It may be under the immediate action of forces which neutralise each other: thus two forces may be acting upon it which are equal, and in opposite directions. In such a case, its state of rest, or of uniform rectilinear motion, will be undisturbed. The state of uniform rectilinear motion declares more with respect to the body than the state of rest; for the former betrays the action of a force upon the body at some antecedent period; this action having been suspended, while its effect continues to be observed in the motion which it has produced. (93.) When the state of a body is changed from rest to uniform rectilinear motion, the action of the force is only momentary, in which case it is called an _impulse_. If a body in uniform rectilinear motion receive an impulse in the direction in which it is moving, the effect will be, that it will continue to move uniformly in the same direction, but its velocity will be increased by the amount of speed which the impulse would have given it had it been previously quiescent. Thus, if the previous motion be at the rate of ten feet in a second, and the impulse be such as would move it from a state of rest at five feet in a second, the velocity, after the impulse, will be fifteen feet in a second.