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LXVIII. On some Phenomena observed on Glaciers, and on 

 the internal temperature of large Masses of Ice or Snow, with 

 some Remarks on the natural Ice-caves which occur below 

 the limit of perpetual Snow. By Sir John Herschel, 

 Bart., F.G.S., $c* 

 IN a visit to the glacier of Chamouni in the summer of 1821, 1 was 

 ■*- struck with the very remarkable positions of several large blocks 

 of granite resting on the glacier in various parts. They were perched 

 on stools of ice of less diameter than the blocks themselves, which 

 overhang their supports on all sides, as a mushroom does its stalk. 

 The position of these large masses was rendered the more striking 

 when contrasted with that of small fragments of stone, equally (to 

 appearance) exposed to all the local heating and cooling influences, 

 but which were uniformly found to have sunk into the ice, and 

 that the deeper, (within certain limits) the less their size. On con- 

 sideration, the cause became apparent, and, as it affords a very 

 pretty illustration of the laws of the propagation of heat through 

 bad conductors, and the steps by which an average temperature is 

 attained in large masses from a varying source, I will here state it 

 as it occurred to me at the time. 



With regard to the sinking of small masses into the ice when 

 heated by the sun, it is the natural effect of the greater power of 

 absorbing heat which stone possesses beyond ice. Whenever the 

 sun shines, the stone will detain more of its heat than an equal sur- 

 face of ice would do ; and as it gives this out to the ice below nearly 

 as fast as it receives it, a greater depth of ice is melted in a given 

 time beneath the stone than in the parts around. On the other hand, 

 at night, ice radiates terrestrial heat nearly or quite as copiously as 

 stone, and thus they are on a par in frigorific power. 



The elevation of great masses above the general level, which at 

 first sight would appear to contradict this explanation, is however 

 equally a consequence of the laws of the propagation of heat. To 

 conceive this, let us imagine a very large block of stone, at the com- 

 mencement of the summer, to lie on a level surface of ice, in a si- 

 tuation exposed to the direct rays of the sun, where the mean tem- 

 perature of day and night is (even in summer) but little above the 

 freezing-point, but where, however, no fresh snow falls during the 

 whole summer. In the day time then, while receiving the sun's 

 rays, the upper surface of the stone will be strongly heated, and a 

 wave of heat will be propagated slowly downwards through the 

 stone towards the ice, diminishing in intensity rapidly, however, as 

 it travels, since each superior stratum only divides its excess of 

 temperature with that below. Long before this can reach the ice, 

 however, night comes on. The surface cools below the mean or 

 even below the actual temperature of the air by radiation, and a 

 wave of cold is propagated (or, which comes to the same thing, heat 

 is abstracted from stratum to stratum) by the same laws. This fol- 



* From the Proceedings of the Geological Society, vol. ii. part 2 ; having 

 been read March 9, 1842. 



