Page:The evolution of worlds - Lowell.djvu/237

Rh it at its minimum, which it should do to produce permanent glaciation, it so far falls short of its fellow that during the last opposition at which it could be well observed, it disappeared entirely. The short, hot summer, then, far exceeded in melting capacity that of the longer but colder one.

Let us now suppose the precipitation to be increased, the winters and summers remaining both in length and temperature what they were before. The amount of snow which a summer of given length and warmth can dispose of is, roughly speaking, a definite quantity. For it depends to a great extent only on its amount of heat. The summer precipitation may be taken as offsetting itself in the two hemispheres alike. If, then, the snow-fall in the winter be for any reason increased daily in both, a time will come when the deposition due the longer winter of the one will exceed what its summer can melt relatively to the other, and a permanent glaciation result in the hemisphere so circumstanced. Increased precipitation, then, not eccentricity of orbit, is the real cause of an Ice Age. And this astronomic deduction we owe not to theoretic conclusions, for which we lack the necessary quantitative data, but wholly to study of our neighbor in space. Had any one informed our geologic colleagues that they must look to the sky for definite information about the cause of an Ice Age, they would probably have been surprised.