ON ITS EXTERIOR. THERMAL SPRINGS. 191 



portions which the elevations of the loftiest peaks bear 

 to those of the general crests of the mountains and of 

 the table-lands. 



If the semi-diameter of our planet were increased by 

 the height of the Himalayan summit of Kinchinjunga, 

 28-175 feet, this addition of only about -gj-g- of the earth's 

 radius would scarcely make any difference (according 

 to Fourier's analytic theory) in the temperature of the 

 surface as cooled by radiation; the thermal condition of 

 the upper portion of the crust of the earth would be 

 almost exactly the same as it now is. But where par- 

 ticular parts of the surface rise into the atmosphere, 

 whether in mountain chains, or in isolated summits, 

 there takes place in the interior of the upheaved strata a 

 decrease of warmth from below upwards, which decrease 

 is modified by contact with strata of air of unequal 

 temperatures; by the different capacity for heat and the 

 different heat-conducting power of different kinds of 

 rock ; by the exposure to the sun's rays of the bare or 

 forest-covered summits and declivities ; and by the 

 greater or less radiation from the mountain, according 

 to the particular form of its relief, w'hether forming a 

 wide mass, or one or more narrow cones or pyramids. 

 The particular height of the region of clouds; the 

 extent of icy or snowy covering (varying according to 

 the different heights of the snow limit); and the 

 frequency of the currents of colder air, which at par- 

 ticular parts of the twenty-four hours roll down the 

 steep declivities; all modify the effects of terrestrial 

 radiation. As the soaring jagged peaks become colder, 

 there arises in the interior a faint current of heat from 

 below upwards, tending towards, but never attaining, 



