514 POPULAR SCIENCE MONTHLY. 



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 inti- 

 mate knowledge of the laws governing the movements in our atmos- 

 phere 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 circum- 

 stances, 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 Pro- 

 fessor 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 thor- 

 ough examination of the internal laws of motion, quite apart from any 

 external forces which may modify the results. 



