M. Gay-Lussac o?i Volcanos. 89 



to make us regard the ultimate or outermost stratum of the 

 earth as a crust of scoriae, beneath which exist a great many 

 furnaces, some of which are extinguished, while others are re- 

 kindled. It is well calculated to excite surprise that the earth, 

 which has endured through so many ages, should still preserve 

 an intestine force sufficient to heave up mountains, overturn 

 cities, and agitate its whole mass. 



The greater number of mountains, when they arose from 

 the heart of the earth, must have left these vast cavities, which 

 would remain empty unless filled by water. I think, however, 

 that De Luc and many other geologists have reasoned very 

 erroneously on these cavities, which they imagine stretching 

 out into long galleries, by means of which earthquakes are 

 communicated to a distance. 



An earthquake, as Dr. Young has very justly observed, is 

 analogous to a vibration of the air. It is a very strong sonorous 

 undulation, excited in the solid mass of the earth by some 

 commotion which communicates itself with the same rapidity 

 with which sound travels. The astonishing considerations in 

 tliis great and terrible phaenomenon are, the immense extent 

 to which it is felt, the ravages it produces, and the potency of 

 the cause to which it must be attributed. But sufficient atten- 

 tion has not been paid to the ease with which all the particles of 

 a solid mass are agitated. The shock produced by the head of 

 a pin at one end of a long beam causes a vibration thi'ough all 

 its fibres, and is distinctly transmitted to an attentive ear at the 

 other end. The motion of a carriage on the pavement shakes 

 vast edifices, and communicates itself through considerable 

 masses, as in the deep quarries under Paris. Is it therefore so 

 astonishing that a violent commotion in the bowels of the earth 

 should make it tremble in a radius of many hundreds of 

 leagues ? In conformity with the law of the transmission of 

 motion in elastic bodies, the extreme stratum, finding no other 

 strata to which to transmit its motion, makes an effort to de- 

 tach itself from the agitated mass, in the same manner as in a 

 row of billiard balls, the first of which is struck in the direc- 

 tion of contact, the last alone detaches itself and receives the 

 motion. This is the idea 1 have formed of the effects of earth- 

 quakes on the surface of the globe ; and I should explain their 

 great diversity, by also taking into consideration, with M. de 

 Humboldt, the nature of the soil and the solutions of continuity 

 which it may contain. 



In a word, earthquakes are only the propagation of a com- 

 motion through the mass of tlie earth, and are so far from de- 

 pending on subterranean cavities, that their extent would be 

 greater in proportion as the earth was more liomogencous. 



Vol. G2. No. aot. Au<r. 1823. M XVII. Ana- 



