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MOTION. The motion of a body is the change of its place in space. All changes in the material world consist of motion. The life of the organic creation, and the action of inorganic bodies, consists in motion : what we call rest, is only relative Experience alcne convinces us of the motion of bodies in space. Zeno of Elea, endeavored to prove this fundamental idea of motion to be contradictory to itself, in order to overthrow the testimony of experience. If we see that a body changes its external relations, we conclude that it moves: its continuance in the same reia tions is called rest. By a change of the situation or relation of bodies we are often deceived, and confound rest with motion. In some cases, it is easy to perceive the error; in others, it is so difficult, that many centuries have been necessary to dispel the illusion ; for instance, in relation to the earth and the sun.In motion,we must consider the cause, the moving body, the direction, the path described, the time, the velocity, and the quantity. The mass of the moving body must be taken into consideration, since the quantity of motion depends on the quantity of matter. To move twice as much matter, requires twice as much power. The direction of the motion of a body is the line along which a moving point proceeds, either for the whole or a part of the way. If all the points of a body move in the same direction, it is only necessary to observe the motion of a single point. The line described by this point is the path of the moving body. This path itself, if in a straight line, represents the direction of the motion ; if in a curved line, the direction at every point of the curve is determined by the tangent to this point ; that is, this tangent shows the direction of the moving body at that point in which it would continue to proceed, if it ceased changing its direction. If all the points of a body do not move in the same direction, the motion of each point, in particular, ought to be observed; and thus we may consider every motion as the motion of a point. By the space described, we understand the distance passed through by the moving point. Since we always consider the motion of points, this space is represented by a line ; and thus the observation of motion becomes geometrical. Time is necessary for motion, even for the smallest. By the comparison of the space described, and the time in which it is described, we find the velocity. One body moves quicker than another, if it decribes in the same time a larger space, or the same space in a less time. By the quantity of motion we mean the velocity combined with the quantity of matter. To move two pounds requires twice as much power as to move one pound with the same velocity. To move a body with the velocity 2, also requires twice as much power as to move the same body with the velocity 1. Hence it follows, that to move two pounds with the velocity 3, requires six times as much power as to move one pound with the velocity 1. Motion may be considered under several different views. With regard to change of position, by which it is ascertained, it is either absolute or relative. If a body passes from one place to another, this is called absolute motion; it is relative if we consider the objects to which we refer the motion of the observed body, whether in motion or at rest, as fixed points. With regard to change of position, the motion is, further, either common or proper; finally, either apparent or real. With regard to the powers or causes, which produce motion, it is either simple or compound; simple, if it is produced by a single power, or by several powers acting in the same direction; compound, if several motions meet, the various directions of which form angles with each other. With regard to the direction, the motion is either in a straight or a curved line; with regard to the velocity, either uniform, or accelerated, or retarded, and the accelerated motion again is either uniformly or variably accelerated ; and the retarded motion either uniformly or variably retarded. (See Mechanics.)