ON ITS EXTERIOR. THERMAL SPRINGS. 189 



coverings of our planet, which will he treated of more 

 particularly in the sequel, are here referred to in pass- 

 ing, in order that the influence of the vertical distri- 

 bution of temperature in the solid crust of the earth, 

 (the system of " geoisothermals,") may not be looked 

 at in too isolated a manner, but that it may be regarded 

 rather as a portion of the all-pervading movement of 

 heat, a true " cosmical activity." 



Instructive as are in many respects observations on 

 the unequal decrease of temperature with increasing 

 elevation of their point of issue of such springs as do 

 not vary with the seasons, yet the local law of such de- 

 crease ought not to be regarded, as it too often is, as a 

 general "geothermic" law. If we were assured that 

 the springs came from unmixed waters, resting on very 

 extensive horizontal strata, we might indeed believe 

 them to have gradually acquired the temperature of the 

 solid materials in the vicinity ; but the great network 

 of fissures by which the upheaved masses are inter- 

 sected must render this only a rare case. Colder waters 

 from higher levels mix with the lower ones. Our 

 mining operations, small as is the depth to which they 

 penetrate, are very instructive in this respect ; but we 

 shall not arrive directly at the knowledge of the geo- 

 thermal lines until observations are obtained at very 

 various elevations above the level of the sea by Bous- 

 singault's method, i.e. by sinking the thermometers 

 into the earth below the depth to which the influence 

 of the variations of the adjacent atmosphere extends.( 260 ) 

 From the 45th parallel of latitude to the parts of 

 the tropics nearest to the equator, the depth at which 

 the " invariable " stratum begins decreases from 64 feet 



