( 17 ) 



Oil some Phenomena observed on Glaciers, and on the internal 

 Temperature of large masses of Ice or Snow, with some Be- 

 viarks on the natural Ice-caves which occur belorc 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, 

 I was struck with the vei'y 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) ex- 

 posed 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 consi- 

 deration, 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 tempera- 

 ture is attained in large masses from a varying som"ce, I will 

 here state it as it occurred to me at the time. 



With regai'd to the sinking of small masses into the ice when 

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

 of absoi'bing heat which stone possesses beyond ice. "WTien- 

 ever the sun shines, the stone will detain more of its heat than 

 an equal surface 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 

 liowever equally a consequence of the laws of the propagation 



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

 been read March 'J. 1842. 



VOL. XXXIV. NO. LXVII. JANUARY 1843. B 



