160 Proceedings of the Uoyal Physical Society. 



been shaped entirely by heat radiated from the stone. The 

 stone remains on the pedestal until the melting of the ice 

 below its southern end causes it to topple off, when the 

 whole process is begun anew. 



Films of water lying upon ice heat up in the sunshine, 

 and help materially to raise the temperature of the surface 

 of the ice beneath and melt it, even though the thermal 

 conductivity of water is only '002. That of ice is '0057 (or 

 very nearly the general average of rocks). The specific heat 

 of air is '2669, and therefore the films of air whose presence 

 gives rise to the white bands of glacier ice, cause these white 

 bands to melt faster than the pure ice of the " blue bands," 

 which are consequently left on the surface of a glacier as 

 ridges, while the " white bands " sink into furrows, and are 

 further deepened by receiving dust and fine particles of rock, 

 as well as films of water. 



The effect of radiated heat upon glacier ice is well shown 

 by the marginal zone of depression, which occurs where the 

 glacier meets the walls of its enclosing valley. Much of the 

 convexity in the cross section of the surface of the glacier is 

 due to this cause. 



A little reflection over these generally well-known facts 

 will lead us to see that something of much importance in the 

 present connection can be made out of them. Let us take 

 the case of a stone which, instead of remaining at the surface, 

 has fallen into a crevasse that has subsequently been closed 

 up by the onward movement of the ice past the place 

 where the crevasse originated. The Sun's rays pass down 

 through the overlying ice, impinge upon the surface of the 

 stone, and straightway begin to warm it. Heat is now 

 radiated from the stone, which, if it is a large one, will only 

 radiate from the surface next the Sun, i.e., from its upper 

 surface. If the stone is dark coloured, it will probably both 

 absorb, and radiate to the overlying ice, a large amount of 

 heat, of that quality which is competent both to warm the 

 ice itself and to melt it. Hydrostatic pressure squeezes the 

 stone upward to take the place of the water, the melting 

 proceeds, and, in course of time, the stone is back again at 



