Hot and Thermal Springs. 153 



that the highest mountains are almost nothing in comparison 

 with the diameter of the earth, and that where heat is forced to 

 extend to greater distances from its source, a communication of 

 heat takes place from all sides. 



In the same manner as the chthonisotherraal lines bend up- 

 wards in mountains, so will they, on the contrary, curve down- 

 wards under the sea, under lakes, and under glaciers.* The 

 bottom of the sea, between a certain latitude in the frigid zones 

 and the equator, has every where a lower temperature than it 

 should have in consideration of its latitude alone. If, therefore, 

 we draw a chthonisothermal line of f, from a continent or an 

 island, towards the bottom of the sea, since the temperature is 

 there more or less under <°, the line must sink more or less be- 

 low it. All other circumstances being the same, such a chtho- 

 nisothermal line will suffer a greater depression, the nearer we 

 approach the equator, and consequently the greater the value of 

 /°. The same thing takes place under lakes and glaciers ; and 

 such a chthonisothermal line will sink deeper under these, cateris 

 paribus, the nearer they are situated to the equator, and the less 

 their elevation above the sea. As, for example, according to Chap. 

 XI., the temperature of the sea in itsdepth,even in the torrid zone, 

 does not exceed 32°, the chthonisothermal of 81°.50 (the mean 

 temperature on the equator) will sink to a depth of about 2500 

 feet beneath the bottom of the sea, provided the value of n be 

 the same under the sea as under plains. Now if we take the 

 highest mountain on the earth as the measure of the greatest depth 

 of tlie sea, we should have two points under the equator of equal 

 temperature, = 8r.50, the one touching the surface of the earth 

 on terra firma, the other situated 26,500 feet below it. Similar 

 differences will occur, though of smaller extent, between two 

 points of equal temperature, one of which is situated on the 

 banks of a lake, or near a glacier, the other beneath the bottom 

 of the lake, or under the glacier. 



* That wliicli holds good for glaciers is also applicable to accunuilalioiu- of 

 snow and ice in narrow valleys, only in a much less degree. See Von Biich 

 in (Jilbert's Annal, vol. xli., p. 2. 



