Hot and Thermal Spr'ingfi. 155 



it was originally f -f 2°.25 — i^ = 2° 25, after it became covered 

 with water this difference must have risen to ^+ 2.°25 — 38.°75 =z 

 i° — 36°.50. So that the higher the original value of f was, the 

 greater will be the difference after the filling up of the lake 

 with water. Now, as in general, a difference of temperature of 

 2°.25 occurs in n feet, a greater difference of temperature than 

 2°.25 for the same depth cannot long exist. The cooling of the 

 bottom of the lake, which at the moment of its filling with water 

 is only superficial, will soon extend to a greater depth. But 

 however deep this cooling may extend, yet there must be a 

 point at which it becomes equal to nothing, or at least imper- 

 ceptible. The increase of temperature beneath lakes, must, 

 therefore, follow a more rapid progression, that is to say, the 

 value of lb must there be smaller than under the solid earth. 



The same will be the case under seas and glaciers : under the 

 sea, where it has a great depth, even in high latitudes, provided 

 only that the temperature of the bottom of the sea do not reach 

 that degree which it would have, were it completely filled up 

 with the solid matter of the earth, and under glaciers also, only 

 so long as the original mean temperature of the soil beneath 

 them is not reduced to 32°. (Chap. IX). Under the deepest 

 parts of the sea, the value of n will be considerably diminished ; 

 for, according to Chap. XI. if the depth of the sea, under the 

 equator, for example, is equal to 6658 feet, the original tempe- 

 rature of the bottom of the sea would almost reach the boiling 

 point of water. The reduction of this to 35°.96, the lowest 

 temperature observed in the depths of the sea in low latitudes, 

 would consequently require a very considerably more rapid 

 increase of temperature towards the interior. 



As the solid masses of our globe are very bad conductors of 

 heat, the reduction of temperature consequent on the creation of 

 a lake, or the filling up of an alpine valley with a glacier, must 

 certainly ensue very slowly. We may obtain a measure for the 

 time necessary for the progress of this reduction of temperature 

 towards the interior, from the time which the temperature of the 

 air, as it goes through the successive changes of the seasons, re- 

 quires in the temperate zones, to pcnclraie to a certain depth 

 into the crust of the earth. From the observations communi- 

 cated in Chap. VIII. and XVIII, wc find that at a depth of 



