Solar Energy in Relation to Ice. 145 



the Sun do not warm dry and clear air on their way through 

 it, but have to reach the earth first and warm something 

 there, which something in its turn warms the air, which is 

 thus heated by the Sun only in a roundabout way. And this 

 fact is quite independent of what the temperature in the sun- 

 shine may be. When the Sun's rays fall upon a stone, for 

 example, the surface of this is heated, and from that there 

 is radiated heat undulations of a longer and slower kind than 

 those which came direct from the Sun, and it is these which 

 are endowed with the property of heating the air in contact 

 with the stone. There is a difference in the original quality 

 of the heat direct from the Sun, as compared with what we 

 may term the " second-hand " heat given back by the stone. 

 Air, therefore, is warmed by radiation from a land-surface, 

 if that surface is not covered by snow or ice, and is not 

 warmed over a surface which is so covered. It follows from 

 this that the temperature of the air over a snow-covered 

 surface cannot rise above the freezing point, be the tempera- 

 ture in the sunshine what it may. The result we are 

 concerned with is that solar radiation produces no effect 

 upon the climate in such a case ; and, furthermore, that the 

 air, even beyond the margin of the snow, is also chilled ; so 

 that the area where the precipitation takes the form of snow, 

 instead of that of rain, tends to enlarge its boundaries. Not- 

 withstanding the fact that the Sun's rays during the Glacial 

 Period may well have been at least as hot as they are now, 

 and on the mountain summits must have been even hotter, 

 on account of their greater elevation, yet, once started, these 

 icy conditions tended to affect an increasingly larger area, 

 spreading outwards from the mountain tops to their lower 

 slopes, and eventually from them to the lowlands. The warmth 

 arising from the condensation was, therefore, counteracted 

 by the chill from the snow, as well as by frequent down- 

 draughts of icy cold air from the upper regions of the 

 atmosphere. So the glaciers increased in size, and, presently, 

 coalesced with their neighbours. Then the conjoined 

 streams extended far from their originating centres, and 

 covered an ever-increasing area of the lowlands ; until, 

 in the end, they enlarged to such an extent that all Britain, 



