EARTHQUAKES. 165 



The effects of the impulse, and waves of commotion, may be 

 reduced to simple mechanical theories with more distinctness 

 than is furnished by the consideration of the nature of the 

 first impulse, which indeed may be regarded as heterogene- 

 ous. As already observed, this part of our knowledge has 

 advanced essentially in very recent times. The earth-waves 

 have been represented in their progress and their propaga- 

 tion through rocks of different density and elasticity;* the 

 causes of the rapidity of propagation, and its diminution by 

 the refraction, reflection, and interference^ of the oscillations, 

 have been mathematically investigated. Attempts have been 

 made to reduce to a rectilinearf standard the apparently 



orable ascent of Clrimborazo (Ascension au Cldmborazo k 16 Dec., 

 1831, loc. tit., p. 176), he says again : "Like Cotopaxi, Antisana, Tun- 

 guragua, and the volcanoes in general which project from the plateaux 

 of the Andes, the mass of Chimborazo is formed by the accumulation 

 of trachytic debris, heaped together without any order. These frag- 

 ments, often of enormous volume, have been elevated in the solid 

 state by elastic fluids which have broken out through the points of 

 least resistance ; their angles are always sharp." The cause of earth- 

 quakes here indicated is the same as that which Hopkins calls "a 

 shock produced by the falling of the roof of a subterranean cavity," in 

 his "Analytical Theory of Volcanic Phenomena" (Brit. Assoc. Report, 

 1847, p. 82). 



* Mallet, Dynamics of Earthquakes, p. 74, 80, and 82 ; Hopkins, 

 Brit. Assoc. Report, 1847, p. 74-82. All that we- know of the waves 

 of commotion and oscillations in solid bodies shows the untenability 

 of the older theories as to the facilitation of the propagation of the 

 movement by a series of cavities. Cavities can only act a secondary 

 part in the earthquake, as spaces for the accumulation of vapors and 

 condensed gases. "The earth, so many centuries old," says Gay- 

 Lussac, very beautifully (Ann. de Chimie et de Phys., tome xxii., 1823, 

 p. 428), "still preserves an internal force, which raises mountains (in 

 the oxydized crust), overturns cities, and agitates the entire mass. 

 Most mountains, in issuing from the bosom of the earth, must have 

 left vast cavities, which have remained empty, at least unless they 

 have been filled with water (and gaseous fluids). It is certainly in- 

 correct for Deluc and many geologists to make use of these empty 

 spaces, which they imagine produced into long galleries, for the propa- 

 gation of earthquakes to a distance. These phenomena, so grand and 

 terrible, are very powerful sonorous waves, excited in the solid mass 

 of the earth by some commotion, which propagates itself therein with 

 the same velocity as sound. The movement of a carriage over the 

 pavement shakes the vastest edifices, and communicates itself through 

 considerable masses, as in the deep quarries below the city of Paris." 



f Upon phenomena of interference in the earth-waves, analogous to 

 those of the waves of sound, see Cosmos, vol. i., p. 215 ; and Hum- 

 boldt, Kleinere Schriften, bd. i., p. 379. 



t Mallet on vorticose shocks and cases of twisting, in Brit. Assoc. 

 Report, 1850, p. 33 and 49, and in the Admiralty Manual, 1849, p. 213 

 (see Cosmos, vol. i., p. 204). 



