154 Prof. BischottV/t the Temperature of 



If the line DA, PI. II. Fig. 11. represent a part of tiio surface of the 



earth on the sea-coast or on the banks of a lake ; DAC the angle formed by 



the line AC, which runs along the bottom of the sea or lake to its deepest 



point C, and the line AD ; and if, further, the mean temperature at D or 



A = <°, at C = /° — p", at B=r — p°+l°, and consequently BC = w; we 



shall have 



DAB = DAC — BAC 



and BC being perpendicular to AC we find 



. „ ^ EC n 

 sinBAC = ^=^ 



Thus, then, we find the indination of the line AB, the 

 chthonisothernial of f, to the surface of the shore of the sea or 

 lake.* 



What will be the value of n, beneath the bottom of the sea^ 

 of lakes and of glaciers .'' As direct observations are here en- 

 tirely wanting, we must be satisfied with determining, by general 

 considerations, what will be the ratio of this value to that which 

 has hitherto been found beneath that part of the earth's surface, 

 which is not covered with water or ice. 



Let us imagine to ourselves a point at the bottom of a lake, 

 which, if the lake were not filled with water, but with the solid 

 substance of the earth, would have a temperature t°y a temperature 

 higher than 38°.75 : then, if by any revolution of nature the solid 

 matter above this point were suddenly to be removed and re- 

 placed by fresh water, this point would assume the temperature 

 of the water. But this temperature would, after some time, 

 change to that at which water assumes its greatest density, pro- 

 vided the temperature of the air at the surface had, during that 

 time, been reduced so low, namely, to 38°.75 (Chap. XI). 

 Under all these suppositions the said point would, therefore, 

 have been reduced in temperature from f to 38°. 75. But since 

 the difference of temperature between this point and n feet below 



* The deepest point of the bore at Prcgny near Geneva is C80 feet be- 

 low the surface of '.lie earth, and is nearly on the same level with the bot- 

 tom of the lake, near Geneva. The temperature of this point is 63°.05, that 

 of the lake in its depths, and consequently, also of its bottom, is, according to 

 De la Beche, 43°. If the temperature increases beneath the bottom of the 

 lake at the same rate as in the bore, the point beneath tiie bottom of the lake, 

 which has a temperature of ()3°.05, must be situated 97'J feet lower than the 

 {owest point of the bore. 



