Sir J. Herschel on some Phenomena on Glaciers, fyc. 363 



lows close on the wave of heat below and travels with equal velo- 

 city. In consequence, the heated stratum parts with its heat, now, 

 both upwards and downwards, and thus the intensity of the wave of 

 heat diminishes with much greater rapidity as it proceeds down- 

 wards. It is manifest, that were the thickness of the stone infinite, 

 the wave of heat being always followed close up by the wave of 

 cold, and a perpetual tendency to an equilibrium of temperature 

 going on between them, they would ultimately reduce each other to 

 their mean quantity and (not to take the extreme case of infinity) 

 at some very moderate depth, the fluctuations above and below the 

 mean temperature of the air, as the successive nocturnal and diur- 

 nal waves pass through a particle of the stone there situated, will be 

 rendered very trifling, and may for our present purpose be regarded 

 as evanescent. Beyond this depth, whatever mass of stone may 

 exist, may be regarded as a slow conducting mass, interposed be- 

 tween a surface of ice constantly maintained at 32°, and a surface 

 of stone constantly maintained at the mean temperature of the air, 

 which by hypothesis is very little above it. Through this then the 

 heat will percolate uniformly but feebly, and the ice below will be 

 very slowly melted, and the more so in proportion to the thickness 

 of the interposed stratum. Let us now consider what happens to 

 the ice on the parts undefended by the stone. In the day time these 

 experience the direct radiation of the sun, and therefore melt and 

 run off in water. At night, it is true, the remaining surface cools 

 by radiation ; but this cold is propagated downwards, and on the 

 return of day the superficial lamina is necessarily put in equilibrium 

 with the air and melted by the sun, and however cold the interior of 

 the mass may be, the surface will still be kept all day in a state of 

 fusion. Thus the degradation of the general surface of the ice will 

 be in proportion to the direct intensity of the sun's rays and the time 

 they shine, while that of the surface beneath the stone will only be 

 in proportion to the excess of the mean temperature of day and night 

 above 32°, diminished by the effect of the thickness of the stone. 

 This of course will produce a difference of level, and a relative ele- 

 vation of the stone sunk as really observed. One curious and, at 

 at first sight, paradoxical consequence seems to follow from this 

 reasoning, viz. that the ice of a glacier, or other great accumulation 

 of the kind, may, at some depth beneath the surface, have a per- 

 manent temperature very much below freezing, though in a situa- 

 tion whose mean annual temperature is sensibly above that point. 

 In fact (continuing to use the metaphorical expression already em- 

 ployed), there is no reason why waves of cold, of any intensity be- 

 low 32°, may not be propagated downwards into the interior of the 

 ice ; but waves of heat above that point, of course, never can. Thus, 

 the cold of winter and the frost produced by radiation in the clear 

 nights of summer, will enter the mass and lower its internal tempe- 

 rature, while the heat of the summer air and that imparted by solar 

 radiation will mainly be employed in melting the surface, and will 

 run off with the water produced. 



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