THE STORAGE OF COLD. 523 



the long-continued inhospitable condition thus produced, and it 

 is quite possible that, but for the glacial period, the civilization 

 of mankind would have been much further advanced than at 

 present, and most of the awkward questions which are troubling 

 us now would have been settled ages ago. They might, however, 

 have been succeeded by other questions quite as awkward, for 

 the solving of perplexing problems of social relations seems part 

 of the destiny of man. It has been suggested that the glacial age 

 may have aided human advancement, by forcing primitive man 

 to adopt new methods of shelter, clothing, and food-getting, in 

 self-defense against the cold. Thus, instead of hindering it may 

 have helped to break the reign of savagery. 



Here it may be well to advert to another probable refrigerat- 

 ing agency of snow to which no attention has hitherto been paid. 

 Aerial snow snow that forms in the upper strata and is melted 

 at lower levels of the air may have always been an important 

 agent in the cooling of the earth, aiding essentially in the upward 

 transport of heat during the ages when the surface was at a high 

 temperature. In those ages the great quantity of water vapor in 

 the air hindered the free radiation of heat, whose conveyance 

 upward was mainly accomplished by warm ascending currents. 

 This may have been greatly aided by the conversion of the vapor 

 of these vertical winds into snow in the upper air, the descent of 

 this snow, and the exhaustion of much of the lower heat in melt- 

 ing it. 



Such a state of affairs may have extended much further back 

 in time than would at first thought be deemed possible ; perhaps 

 to that period when the earth was still too hot to permit the 

 existence of liquid water, and the substance of the present oceans 

 was held in the air as water vapor. Even then the rarer regions 

 of the atmosphere were probably chilled below the temperature of 

 congelation, and a snow limit existed, though very much higher 

 than at present. The range of vapor must also have extended 

 much higher than at present, possibly far within the region of 

 congelation. Therefore, at the period when the surface heat pre- 

 vented the existence of liquid water, there may have been a con- 

 tinuous formation and fall of snow in the upper strata of the 

 atmosphere. The melting of this snow at lower levels, and the 

 vaporizing, at still lower levels, of the rain which it yielded, must 

 have been highly important agents in the upward transit of the 

 surface heat. 



There is thus much reason to believe that the snow-fall, which 

 within the recent period has played so prominent a part in terres- 

 trial affairs, has been from a very early era an active agent in the 

 cooling of the earth, the snow limit of the atmosphere gradually 

 descending through the ages until, in the glacial era, it nearly 



