BRITISH EARTHQUAKES. f 



where rain scarcely ever falls, and where hail, lightning, and 

 thunder are unknown, these atmospheric explosions are replaced 

 by the subterranean thunder which accompanies the trembling of 

 the earth. From long habit and a prevalent opinion that dan- 

 gerous shocks are only to be apprehended two or three times in a 

 century, slight oscillations of the ground scarcely excite so much 

 attention in Lima as a hail-storm does in the temperate zone." 



43. Although we are in the habit of congratulating ourselves, 

 in this country, for our exemption from the -terrible visitations of 

 convulsions of the ground, it is certain that beneath the floor upon 

 which our dwellings are established, there exists a seat of dis- 

 turbance of a certain force and constancy. This is abundantly 

 proved by the fact, that not less than 256 or 257 slight earthquake- 

 shocks have been recorded, of which 139 took place in Scotland. 

 Yorkshire, Derbyshire, Wales, and the south coast of England, 

 have been the principal theatres of the remainder. 



44. In the cases of convulsions which have been noticed in the 

 preceding chapter, the effects of the phenomena have been generally 

 limited to derangement more or less violent of the surface. When 

 the internal forces acting outwards are exercised with greater 

 energy, or when the external strata of the earth's crust exercise 

 less resistance, disruptions - take place, and through the openings 

 thus produced the internal matter is ejected. The physical cha- 

 racter of the matter thus thrown out, and the state in which it is 

 found at the moment of its ejection, depend in a great degree upon 

 the depth of the strata from which it has proceeded. 



Of all the substances thus thrown out through the external 

 crust from the interior of the earth, the most frequent is water. 

 That liquid appears to be deposited in terrestrial strata, having 

 depths more or less considerable, and it necessarily acquires the 

 temperature of the strata in which it is thus confined. In ordinary 

 springs rising from inconsiderable depths within the limits of 

 the superficial strata, the temperature in warm seasons is gene- 

 rally lower considerably than that of the air at the surface, and 

 hence arises the coolness of common spring-water. But when 

 water rises from depths much more considerable, lying below the 

 stratum of invariable temperature,* it is found to have a higher 

 temperature than the air. 



, 45. It was first observed by Arago, that water rising in Artesian 

 wells had a temperature greater and greater as the depth of the 

 well augmented. This observation involved a physical principle 

 of high importance, supplyiDg, as it did, a thermal index to the 

 depth of hot springs. 



* See Tract on Terrestrial Heat. 



165 



