534 Causes which occasion the Variation 



from the effects of frost, which, it is well known, cannot 

 penetrate the earth to a greater depth than 3 or 4 ft. 



Many instances might be given of the strong effect of this 

 interior heat. The glaciers of the Alps, for instance, fre- 

 quently cover an extent of three or four square leagues, with 

 a mass of ice 400, 500, or even 600 ft. deep ; thus entirely 

 preventing the access of exterior heat to the soil ; yet the 

 radiation of heat from the ground itself is so powerful as to 

 dissolve the ice very rapidly, and to occasion streams of no 

 inconsiderable size beneath the ice, whose temperature, in 

 summer, is, I believe, as far as can be ascertained, not many 

 degrees below that of streams exposed to the air; and the 

 radiation of heat from the water of these streams forms vaults 

 under the ice, which are frequently 40 ft. or 50 ft. above the 

 water ; and which are formed, as a glance will show, not by 

 the force of the stream, which would only tear itself a broken 

 cave sufficient for its passage, but by the heat which radiates 

 from it, and gives the arch its immense height, and beautifully 

 regular form. 



These streams continue to flow in winter as well as in 

 summer, although in less quantity; and it is this process 

 which chiefly prevents the glacier from increasing in size; 

 for the melting at the surface is, in comparison, very inconsi- 

 derable, even in summer, the wind being cold, the sun 

 having little power, and slight frosts being frequent during 

 the night. It is also this melting beneath the ice (subglacial, 

 suppose we call it) which loosens the ice from the ground, 

 and occasions, or rather permits, the perpetual downward 

 movement, with which 



" The glacier's cold and restless mass 

 Moves onward day by day." 



But more forcible and striking evidence is afforded by ex- 

 periments made in mines of great depth. Between 60 ft. and 

 80 ft. down, the temperature of the earth is, I believe, the 

 fcame at all times and in all places ; and below this depth it 

 gradually increases. Near Bex, in the Valais, there is a 

 perpendicular shaft 677 ft. deep, or about 73 6 2ft. English, 

 with water at the bottom, the temperature of which was ascer- 

 tained by Saussure. He does not tell us whether he used 

 Reaumur's or the centesimal thermometer; but the result of 

 his experiments was this : — In a lateral gallery, connected with 

 the main shaft, but deserted, and, therefore, unaffected by 

 breath or the heat of lamps, at 321 ft. 10 in. below the sur- 

 face, the temperature of the water and the air was exactly 



