Page:Great Neapolitan Earthquake of 1857 Vol 2.djvu/386

Rh the lamellar form of the focal cavity to be, not rectangular, but circular, or approaching a circle in outline.

These last calculations must only be viewed, in the existing state of our data, as mere illustrations of a method, that a more complete state of experimental knowledge, as to the compressibilities of various rocks, wet and dry, and at different temperatures—some being high—would enable us to make available.

They are sufficient, however, as they stand, to one not unimportant deduction. They disprove at once the assumption very commonly made, and constantly repeated without any attempt to apply measures in test of the validity of the doctrine, that earthquakes are agents of direct permanent elevation of the surface of our globe, i.e., permanent, as contradistinguished from the transient and slight swell of the wave itself, at the moment of the transit of the shock. For it will be obvious that the amount of elevation possible from the greatest pressure we can assign, to the momentary force producing the impulse of the wave, is not capable, even right above the focus, of raising the level of the land, by a height, much more than instrumentally appreciable; and there is not the least evidence that any part of even this is permanent. On the contrary, everything tends to the conclusion, that after the passage of the shock, the surface is left again directly, at precisely the same level that it had before. That earthquakes occur along with, and as part of a train of other circumstances, that do produce permanent elevation occasionally, and that earthquakes are probably always the signals, that the forces producing elevation are operative, is another matter, with which the erroneous, or loosely expressed view, should not be confounded.

VOL. II.V