REPLY TO CRITICS. 49 



In short, there is not a place on the face of the 

 globe where the amount of heat received from the sun 

 is not far more than sufficient to melt all the snow 

 which falls upon it. If it were true, as the objection 

 assumes, that the amount of snow melted is propor- 

 tional to the amount of heat received by the snow, 

 then there could be no such thing as perpetual snow. 



The reason why the amount of snow and ice melted 

 is not necessarily proportional to the amount of heat 

 received is not far to seek. Before snow or ice will 

 melt, its temperature must be raised to the melting- 

 point. No amount of heat, however great, will induce 

 melting to begin unless the intensity of the heat be 

 sufficient to raise the temperature to the melting-point. 

 Keep the temperature of the snow below that point, 

 and, though the sun may shine upon it for countless 

 ages, it will still remain unmelted. It is easy to 

 understand how the snow on the lofty summits of the 

 Himalayas and the Andes never melts. According to 

 the observations made at Mount Whitney, to which 

 reference has already been made, the heat of even a 

 vertical sun would not be sufficient at these altitudes 

 to raise the temperature of the snow to near the melting- 

 point; and thus melting, under these conditions, is 

 impossible. The snow will evaporate, but it cannot 

 melt. But, owing to the frozen condition of the snow, 

 even evaporation will take place with extreme difficulty. 

 If the sun could manage to soften the snow-crystals 

 and bring them into a semi-fluid condition, evaporation 

 would, no doubt, go on rapidly ; but this the rays of the 

 sun are unable to do; consequently we have only the 

 evaporation of a solid, which, of course, is necessarily 

 small. 



It may here be observed that at low elevations, where 

 the snowfall is probably greater, and the amount of 



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