Page:Popular Science Monthly Volume 11.djvu/287

Rh earth is drawn away from the waters, and that any tide is produced by the waters being left behind.

How, then, can we account for tides occurring on opposite sides of the earth at the same time? Let us see. In the first place, suppose the earth to occupy some place in space, and to be in a state of perfect rest. Then suppose the sun to come into position, and the earth to start on her journey of 68,000 miles an hour in her orbit around the sun; and suppose, too, that the earth rotates only once on her axis during one revolution around the sun. Then will the same side of her surface face the sun in every part of her orbit. Consequently, there will be a solar tide perpetually at a part of her surface, produced by centrifugal force, and at that part farthest from the sun. Night and solar tide will reign with unceasing steadiness at that one place; but there will be no motion of these piled-up waters. There they will stay, in a steady equilibrium, by the unceasing effect of centrifugal force, in the same manner as can be illustrated by swinging a hollow globe, partially filled with water, around the hand by means of a cord, or by swinging a bucket filled with the same liquid, and having for its bottom a piece of India-rubber, which bottom will bulge out when the bucket is swung around a centre, in the same manner as do the waters of the far side of the earth when she swings or sweeps around the central sun with a velocity of 68,000 miles an hour.

But there are always two solar tides occurring on opposite sides of the earth. The above explanation accounts only for the solar tide on the side of the earth farthest from the sun. How must we account for the fact that there is also one on her side facing the sun and occurring near noon? It is a well-known law of planetary motion that centrifugal and centripetal forces are precisely equal. By virtue of the first the earth seeks to fly from her centre of motion; by virtue of the second she has a tendency to fall into the central luminary; and everything on her surface is operated on in like manner. The particles of her water, moving very easily among one another, are therefore drawn readily away from her solid portions in opposite directions. On the one side the bulging out is caused by centrifugal, on the other by centripetal force. But, as these two forces are nearly equal in all parts of the earth's orbit, the tide-waves on opposite sides of her surface must also be nearly equal. The centrifugal force is produced by the revolution of the earth around the sun; the centripetal force is caused by the force of gravity lodged in the great central orb.

It must not be understood, however, that the earth in her orbital motion feels the effect of these two forces at her surface only. Every particle of the matter composing the earth feels both a centrifugal and a centripetal force while this planet moves around the central orb, and these two forces are precisely equal only at the centre of the earth. But the matter of her surface most remote from the sun feels