EFFECTS OF EARTHQUAKES ON LAND. 159 



temperature lowered by the additions of cold water from 

 another source, and, in a similar manner, waters may be 

 altered in their mineralisation. An important point to 

 be remembered in this consideration is the mutual de- 

 pendence of various underground water supplies, and the 

 area over which any given supply may circulate. A well 

 in the higher part of Lincoln Heath is said to be governed 

 by the river Trent, which is ten miles distant ; when the 

 river rises the well rises in proportion, and when the river 

 falls the water in the well falls.^ 



The change w^hich is usually observed in hot springs 

 is, that before or with earthquakes they increase in tem- 

 perature, but afterwards sink back to their normal state. 

 This increase in temperature may possibly be due to com- 

 munication being opened with new or deeper centres of 

 volcanic activity, or a temporarily increased rate of flow. 



That the water issuing from newly formed fissures or 

 springs should be hot, might be explained on the supposition 

 of its arising from a considerable depth, or from some 

 volcanic centre. It might also be attributed to the heat 

 developed by friction at the opening of the fissures. 

 These changes which earthquakes produce upon the 

 underground circulation of waters are phenomena deserv- 

 ing especial attention. Although we know much about 

 the circulation of surface water, it is but little that we 

 yet know about the movement of the streams hidden from 

 view, from which these surface waters have their sources. 

 Earthquakes may be regarded as gigantic experiments on 

 the circulatory system of the earth, which, if properly in- 

 terpreted, may yield information of scientific and utilitarian 

 value. 



The sudden elevations, depressions, or lateral shifting 

 of large tracts of country at the time of destructive earth- 



* Gent. 3fa.(jf. vol. xxi. 1751, 



