EXAMPLES. 



irregular and tumultuous agitation of the surface of the sea after 

 a storm, and must arise from such a condition prevailing for the 

 moment in the igneous ocean upon which the solid crust of the 

 earth rests. 



22. A popular impression prevails that earthquakes are pre- 

 ceded by peculiar atmospheric phenomena, such as a profound 

 stillness of the air, a suffocating and oppressive heat, and a misty 

 horizon. Exact and extensive observations made in various 

 countries, and for long periods of time, have proved that this is 

 without any foundation in fact. Humboldt states not only as the 

 result of his own experience, but as that of those who have lived 

 for many years in regions where earthquakes are frequent, that 

 they take place indifferently in all weathers and in all states of 

 the atmosphere. He experienced them himself in clear and fair 

 as well as in rainy weather, and as often in a fresh east wind, 

 or in a storm, as in a calm. He observed no disturbance or 

 exceptional condition in the magnetic needle, the barometer or 

 thermometer, either on their approach or during their continuance. 

 His own observations within the tropics, and those of Adolphe 

 Erman during the earthquake of the 8th of March, 1829, at 

 Irkutsk, near Lake Baikal, lat. 53, were in this respect in 

 complete accordance. " . 



Nevertheless, the subterranean convulsion appears to have been 

 in some cases attended with atmospheric effects, which would 

 indicate some connection between the phenomena and the electric 

 state of the surface and of the atmosphere. Thus, for example, 

 during the long-continued trembling of the ground in the 

 Piedmontese valleys of Pelis and Clusson, considerable variations 

 of the electric tension of the atmosphere were observed, which 

 could not have arisen from any storm, the sky being at the time 

 quite serene and unclouded. 



23. From some statistical results, it would seem that there are 

 grounds for supposing a connection between the prevalence of 

 earthquakes and the season of the year. Numerical data, col- 

 lected with much care, by MM. de Hoff, Merian, and Hoffmann, 

 indicate the 'greatest frequency of these phenomena at the epochs 

 of the equinoxes. 



24. It is a fact not unworthy of remark, that notwithstanding 

 the backward state of physical science generally among the 

 ancients, and their total ignorance of the modern science of 

 geology, the true physical cause of earthquakes was indicated in 

 unequivocal terms by Pliny,* who denominated them " Subter- 

 ranean storms ;" and Senecaf gave the germ of all that was known 

 about their causes until a very recent date. 



* Pliny, ii., 79. t Nat. Qusest, vi., 431. 



