5*8 THE POPULAR SCIENCE MONTHLY. 



ble, and is absorbed by the surrounding substances. Such, a pro- 

 cess takes place on a large scale in the chilled fields of the upper 

 air, the water vapor of the atmosphere being condensed into snow 

 and its latent heat lost to the surrounding frosty air. In a word, 

 snow is water which has lost its latent heat, or in a negative 

 sense has absorbed cold from the upper atmosphere. The fall- 

 ing snow conveys this chill to the earth, and thus acts as a great 

 refrigerating agent. To overcome the cold thus conveyed from 

 mid-air to the earth heat is necessary, and large supplies, which 

 might have been usefully employed in the service of man, are 

 lost in the conversion of vapor into snow, and thus indirectly 

 consumed in warming the upper air. 



It may be said here that the conversion of vapor into rain is 

 also exhaustive of latent heat. In the evaporation of the oceanic 

 waters a very considerable quantity of heat is absorbed, and con- 

 veyed to the upper air as the latent heat of water vapor. Of this 

 heat a part is lost in the formation of rain, and a larger part in the 

 formation of snow. But the rain reaches the earth in a condition 

 suitable for service. It does not, like the snow, need to be changed 

 in its physical state, at a great expense in heat, to render it serv- 

 iceable. In fact, the chilling influence of rain is inconsiderable, 

 the heat-consuming agency of snow great and important, and the 

 mode in which its work is performed calls for some considera- 

 tion. 



Snow has several curious methods of extending the sphere of 

 its hostile influence. The comparatively light snows which fall 

 in our latitude are of minor importance, since they readily yield 

 to the early spring sunbeams. They are in some degree beneficial 

 to the fertile surface and protective to its more tender annual 

 plants, while their only important adverse effect is the dangerous 

 flooding of the rivers, due to their rapid melting. But the deep 

 and persistent snows of northern regions are far more exhaustive 

 of solar heat, and reduce the agricultural season of those regions 

 to a dangerously short period. In their melting, also, the surface 

 air is chilled, and winds from the north convey this chilled air far 

 to the south, thus spreading widely over the warmer zones the 

 frost-inducing influence of the melting snows. 



We have seen how the tropic heats are carried toward the 

 poles by winds and waters. The frigid cold is carried toward the 

 tropics by the same agencies chilled winds and cold ocean cur- 

 rents. It is carried by another agent of great importance, the 

 direct creep of the snow itself toward the lower latitudes. This 

 agency has once perhaps many times produced an extraordi- 

 nary effect upon the surface of the earth, one far surpassing that 

 of volcanic explosions and lava outflows in its adverse influence. 

 At present this glacial action is greatly reduced, but is still of 



