Page:Popular Science Monthly Volume 66.djvu/518

514 surface, and the moisture thus drawn up being carried away by the movements of the atmosphere, may be deposited elsewhere in the form of snow or rain. It is almost impossible without a much more intimate knowledge of the laws governing the movements in our atmosphere to say in advance what the effect of a slow and periodic change in the sun's heat will be. Although the change in any one year may be small, its effect on the earth in the course of a number of years may be cumulative and thus become very evident; or other circumstances, for example, a corresponding periodic change in the radiation of heat from the earth, may so counterbalance the change in the heat received from the sun that the effect of the latter is scarcely perceptible. The conclusion of the whole matter has been well expressed by Professor Cleveland Abbe: that the key to the weather problem is not to be found in the sun or indeed in any external influence, but that the solution is to be worked out by the conditions which hold in the atmosphere itself—conditions which can only be discovered by a thorough examination of the internal laws of motion, quite apart from any external forces which may modify the results.