Page:Popular Science Monthly Volume 12.djvu/241

Rh very great, and therefore not much deflected from a straight line, the waters are driven very little above the usual surface, no matter how rapidly the earth herself may move in this curve. The centrifugal force or original impulse felt by the whole earth is very great, but that felt by her waters is hardly visible or sensible in mid-ocean. For the tide-waves cannot get above the line tangent to the curve of the earth's orbit. The following illustration will show this:

Let A B C (Fig. 2) represent a part of the curve of the earth's orbit, in its motion around the central sun, and B D a line tangent



to the curve at the point B. Now it is very evident that no tide-wave produced by centrifugal force can get higher above the curve of the orbit than this tangent line, and the distance between the curve and the tangent, as at E, is very small. The part of the earth's surface most remote from the sun has indeed a greater tendency to continue moving on in the straight line of the original impulse than any other part. The particles of water have a small degree of cohesion, and they will therefore continue to move a short distance along this tangent, but only a little above the usual surface of the earth.

The curve in which the surface of the earth moves around the centre of gravity between herself and the moon is much more deflected from a straight line. Here also the tide-wave can rise no higher than to the line tangent to this curve. The distance of the point G (Fig. 3) from the curve is, however, much greater than the point E in Fig. 2 from its curve. The motion of the surface of the earth at H around