191. =Absolute and Relative Motion.=--The motion of a body is said to be _absolute_ when it is considered without relation to the position of any other body. Its motion is said to be _relative_ when it is moving with respect to some other body. Absolute rest is unknown, for no body in the universe is known to be without motion. But a body may be relatively at rest, that is, in a fixed relative position to other bodies. Every body is in a state of absolute motion, and yet it may be in a state of relative rest. All objects that appear to us to be at rest have a very rapid absolute motion. They appear to be at rest merely because they have the same rapidity and direction of absolute motion that we have ourselves. And all the motions which are apparent to the eye are only slight differences in the common absolute motions, of which, though they are so exceedingly rapid, we are entirely unconscious. Thus, if I stand still, and another at my side walks at the rate of three miles an hour eastward, we both of us have a common absolute motion of 1000 miles in every hour, and he merely adds three miles to his thousand--I move 1000 miles, and he 1003. So if I sit still in my parlor, and my friend travels eastward at the rate of 20 miles an hour, I move every hour 1000 miles, and he 1020. And if he travel westward at this rate he really travels slower than I do--he has an absolute motion eastward of 980 miles, and I of 1000. At the same time we are both whirling on in our annual journey around the sun at the rate of 68,000 miles an hour. 192. =Obstacles to Motion.=--As motion is naturally disposed to continue (§ 49 and § 184), whenever it is stopped it does not spend itself, but is stopped by obstacles. The principal of these obstacles are: gravitation; the resistance of opposing substances--solids, liquids, and gases; and friction. When a stone is thrown into the air its upward motion is gradually destroyed by the attraction of the earth and the resistance of the air. Observe, now, why it descends. It is from the action of one of the causes which arrested its upward flight--the attraction of the earth. In its descent it is retarded by the resistance of the air, as it was in its ascent. This retardation is very obvious in the case of substances which present a large surface to the air, as a feather. A small piece of lead will outweigh many feathers, and therefore, as its quantity of matter is so much greater in proportion to its surface than that of a feather, it will fall to the ground much more quickly. That this is owing wholly to the resistance of the air can be proved with the air-pump. [Illustration: Fig. 128.]