Page:Popular Science Monthly Volume 38.djvu/534

518, and is absorbed by the surrounding substances. Such, a process 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 falling 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 conveyed 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 serviceable. 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 consideration.

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 currents. 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 extraordinary 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