Page:A Treatise on Geology, volume 2.djvu/258
244 CHAP. IX.
A TREATISE ON GEOLOGY.
travel like a great wave of translation in water, with velocities corresponding to the elasticity of the rocks, so as to reach Lisbon, Loch Lomond, Italy, and the West Indies. Rocks, we know, are elastic in their parts, but very imperfectly so in their mass, owing to the numerous divisions which intersect them. Earthquakes cannot be compared to the vibrations of a string, or the pulsations of sound, gradually falling to rest; the motion observed is more similar to the undulation of a flexible lamina over an agitated liquid;—as when a long cloth is shaken in a particular manner, so that a wave of air travels below its parts successively to the end.
Mitchell, to whom physical geology is largely indebted, was the first to explain earthquakes by wave motion, and he employs for the purpose the mechanism of a fluid thrown into undulation, or vapour operating by expansion beneath or between the strata. He assigns 1750 feet per second for the velocity of the Lisbon earthquake. Professors H. D. and W. B. Rogers, following in the same track, make the phenomena of earthquakes depend on undulations propagated in molten rock below the solid crust, trace the path of some of these phenomena, and give measures of the rate of progress of the wave: viz., 27 to 30 miles per minute, or about twice as fast as the wave of sound in air. They find for the velocity of sea waves generated by the earthquake shock, 3½ and 5 miles an hour. They find the area agitated by earthquakes at any one epoch to be very long and narrow, corresponding to the great wave of translation, and trace the synchronous lines of movement for several hundred miles in length.
Mr. Mallet, in a paper communicated to the Royal Irish Academy, 1 followed by a Report to the British Association, has entered fully on the dynamics of earthquakes, and on the history of these phenomena; and has performed some capital experiments on the rate of movement of earth waves in incoherent sand, and in granite of perhaps the average degree of consolidation.
- Phil. Trans. 1760.
- Reports of the British Association, 1843.
- Ibid. 1846.
- Communicated to the British Association in 1851.