160 COSMOS. 



a. Earthquakes. 

 (Amplification of the Picture of Nature, Cosmos, vol. i., p. 204-217.) 



Since the appearance in the first volume of this work 

 (1845) of the general representation of the phenomena of 

 earthquakes, the obscurity in which the seat and causes of 

 these phenomena are involved has but little diminished ; but 

 the excellent works* of Mallet (1846) and Hopkins (1847) 

 have thrown some light upon the nature of concussions, the 

 connection of apparently distinct effects, and the separation 

 of chemical and physical processes, which may accompany 

 it or occur simultaneously with it. Here, as elsewhere, a 

 mathematical mode of treatment, such as that adopted by 

 Poisson, may have a beneficial effect. The analogies between 

 the oscillations of solid bodies and the sound-waves in the or- 

 dinary atmosphere, to which Thomas Young f had already 

 called attention, are peculiarly adapted to lead to simpler 

 and more satisfactory views in theoretical considerations upon 

 the dynamics of earthquakes. 



Displacement, commotion, elevation, and formation of fissures 

 indicate the essential character of the phenomenon. We 

 have to distinguish the efficient force which, as the impulse, 

 gives rise to the vibration ; and the nature, propagation, in- 

 crease, or diminution of the commotion. In the Picture of 

 Nature I have described what is especially manifested to the 

 senses ; what I had myself the opportunity of observing for 

 so many years on the sea, on the sea-bottom of the plains 

 {Llanos), and at elevations of eight to fifteen thousand feet ; 

 on the margin of the craters of active volcanoes, and in re- 

 gions of granite and mica schist, twelve hundred geograph- 

 ical miles from any eruptions of fire ; in districts where at 

 certain periods the inhabitants take no more notice of the 



* Robert Mallet, in the Transactions of the Royal Irish Academy, 

 vol. xxi. (1848), p. 51-113, and First Report on the Facts of Earth- 

 quake Phenomena, in the Report of the Meeting of the British Associa- 

 tion, 1850, p. 1-89; also Manual of Scientific Inquiry for the Use of the 

 British Navy, 1849, p. 196-223. William Hopkins, On the Geological 

 Theories of Flevation and Earthquakes, in the Report of the British As- 

 sociation for 1847, p. 33-92. The rigorous criticism to which Mr. Mal- 

 let has subjected my previous work, in his very valuable memoirs {Irish 

 Transactions, p. 99-101, and Meeting of the British Association at Edin- 

 burgh, p. 209), has been repeatedly made use of by me. 



f Thomas Young, Lectures on Natural Philosophy, 1807, vol. i.. p. 

 717. 



