Page:A Treatise on Geology, volume 2.djvu/260

 246 inclination, and other circumstances, that as appears in the case of the Lishon earthquake, in Mitchell's, Rogers's, and Humboldt's estimates, (⅓ to ½ a mile in a second,) the real velocity is much less. Mr. Mallet has ascertained it in the case of sand and granite to be even less than the above instances, and his experiments were so arranged in the sand at Killiney and the granite of Dalkey, as to give very accurate results. For his beautiful process the reader must be referred to the Brit. Assoc. vol. for 1852.

It appears very desirable, for the completion of this theory of earthquakes, to carry out the seismometrical observations recommended by the British Association, especially at the great public observatories.

Mr. Hopkins in treating this subject mathematically, has shown how, by proper observations of this kind, the local origin of the earthquake can be determined in depth, as well as in geographical position.

The force of an earthquake shock diminishing at points removed from its origin as the square of the distance increases, we see how great must have been the shock in the case of the Guadaloupe earthquake (1843), when, as Rogers has shown, an area not less than 2300 geographical miles in length by 770 in breadth was agitated. According to the observations made on this occasion, the shock was simultaneous in lines nearly north and south, and felt moving in opposite directions from a curved central axis, at the rate of 27 miles in a minute. This seems to indicate a linear subterranean fracture of great length—a fault geologically speaking—such as might occur over a cavity left by the withdrawal of a fluid support to the earth's crust.

By a mathematical investigation of this subject, founded on the phenomena of precession and nutation which arise from the action of the sun and moon on the unspherical mass of the earth, Mr. Hopkins has shown that whether the earth be partially fluid or wholly solid within, there would be no material difference in the