Page:Popular Science Monthly Volume 47.djvu/321

 would yield to the attraction of the sun and moon as the oceans now do. Some effects of this pull may indeed be seen in the distribution of earthquakes, which are more frequent at full moon than at other times, as though the strain produced by the attraction of the moon helped to produce these shocks by the cracking and giving way of the earth. But if the earth as a whole were anything like as fluid as water, it would yield as a whole and assume the same shape, bulging about as much toward the moon as the watery envelope, so that the water would not be perceptibly deeper toward the moon than elsewhere; whereas, if it were perfectly rigid, it would retain its shape unaltered, and the water about it alone would be drawn by the moon. It would be pulled up into tidal waves. These two different cases and effects are illustrated in Fig. 1. As a matter of fact, we find that the heights of the tides are nearly as great as though the earth were absolutely rigid. The earth, therefore, must be exceedingly rigid; we may say solid, so far as these tidal strains are concerned. These are, however,

 —P, point of origin of earthquake shock; E (epicentrum), point on the surface directly over it; A, B, limits of the area of vertical, simultaneous, and earliest shock.

so varying in their application, shifting their direction through all the points of the compass every twenty-four hours, that if the interior of the earth is very viscous—and we know that hot iron is in just this viscous condition when at welding heat—the yielding to forces so rapidly changing direction might be no greater than that which is observed.

Another argument for the solidity of the earth is based on the