REPLY TO CRITICS. 41 



mantle removed a snow-shower in summer would be as 

 rare a phenomenon in those regions as it would be in 

 the south of England. 



Second. — ' The rays which fall on snow and ice are 

 to a great extent reflected back into space. But those 

 that are not reflected, but absorbed, do not raise the 

 temperature, for they disappear in the mechanical work 

 of melting the ice.' 



This reason is also regarded as absurd. The heat of 

 the sun during the perihelion summer would, he says, 

 suffice to melt the whole accumulation of winter snow 

 in three or four days. " The reader," he continues, "can 

 easily make a computation of the incredible reflecting 

 power of the snow and of the unexampled transparency 

 of the air required to keep the snow unmelted for three 

 or four months." Incredible as it may appear to Prof. 

 Newcomb, I shall shortly show that a less amount 

 of snow than the equivalent of the two feet of ice which 

 he assumes does actually, in some places, defy the melt- 

 ing-power of a tropical sun. But he misapprehends my 

 reasoning here also, by overlooking the more important 

 factor in the affair, namely, the keeping of the air in 

 the summer below the freezing-point. The direct effect 

 that this has in preventing the sun from melting the 

 snow and ice will be discussed shortly: but the point 

 to which I wish at present to direct special attention 

 is the fact that if the air is kept below, or even at the 

 freezing-point, snow will fall and not rain. Snow is a 

 good reflector of heat; consequently a large portion of 

 the sun's rays falling on the snow and icy surface is 

 reflected back into space. The aqueous vapour of the 

 air, on the other hand, as the vibrations of its molecules 

 agree in period with those of the snow and ice, cuts off a 

 large portion of the heat radicded by the snow surface; 

 but here in the case of reflection under considera- 



