378 



THE POPULAR SCIENCE MONTHLY. 



Here the horizontal lines represent the number of feet below the 

 plane of 1838. 



It requires but a glance at the diagram to show that some relation- 

 ship exists between the lake and rainfall periods. The first impression 

 conveyed is that the curves are in opposition — that the high in one 

 corresponds with the low in the other, and the reverse. But I think 



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the true relation will be made to appear when we notice the important 

 fact (which I endeavor to render more apparent by dotted lines), viz., 

 that the water extremes lag behind the rainfall extremes — following 

 them at intervals of from two to four years. Thus the seeming want 



