196 KEACTION OF THE INTERIOR OF THE EARTH 



are found in the narrow tract (Ust-Urt) intermediate 

 between two great inland seas, the Caspian and the 

 Aral. In order, at some future time, to elucidate fully 

 phenomena so complicated, it will be needful to employ 

 exclusively methods which lead directly to the know- 

 ledge of the internal temperature, such as deep borings, 

 and not to trust merely to observations of the tempera- 

 ture of springs, or of the atmosphere in caves ; for these 

 give results as insecure as does the temperature of 

 the atmosphere in the galleries and chambers of mines. 

 In comparing a low flat land with mountain-ridges 

 rising abruptly many thousand feet, or with high table- 

 lands, the law of increasing and decreasing temperature 

 does not depend simply on the relative heights of the 

 points compared. If we should compute, according to a 

 determinate scale, the variation of temperature for a 

 given number of feet taken in proceeding upwards from 

 the plain to the summit, and in a line downwards from 

 the summit to a stratum in the interior of the mass of 

 the mountain on a level with the surface of the plain, 

 we should find, in the one case, the summit much too 

 cold, and in the other, the interior stratum much too 

 warm. The distribution of heat in a mountain eleva- 

 tion (an undulation of the earth's surface) is dependent, 

 as has been already remarked, on the form and mass of 

 the mountain, the conducting power of the rocks of 

 which it consists, the heat received from the sun's rays 

 on the one hand and lost on the other by radiation, both 

 in measure varying with the clear or clouded character 

 of the atmosphere, and on the contact and varying 

 play of ascending and descending currents of air: 

 granting these suppositions, we might reasonably expect 



