WATER SUPPLY. 69 
is part of an air current moving from west to east. The basin having no 
outlet, the precipitation of rain and snow within its limits must be counter- 
balanced by the evaporation. The air current must on the average absorb 
the same quantity of moisture that it discharges. Part of the absorption is 
from land surfaces and part from water, the latter being the more rapid. 
If, now, the equilibrium be disturbed by an augmented humidity of 
the inflowing air, two results ensue. On the one hand the precipitation is 
increased, and on the other, the absorbent power of the air being less, the 
rate of evaporation is diminished. In so dry a climate the precipitation is 
increased in greater ratio than the humidity, and the rate of evaporation is 
diminished in less ratio; while of the increased precipitation an increased 
percentage gathers in streams and finds its way to the lake. That reservoir, 
having its inflow augmented and its rate of evaporation decreased, gains in 
volume and grows in breadth until the evaporation from the added expanse 
is sufficient to restore the equilibrium. Giving attention to the fact that the 
lake receives a greater percentage of the total downfall than before, and 
to the fact that its rate of evaporation is at the same time diminished it is 
evident that the resultant augmentation of the lake surface is more than 
proportional to the augmentation of the precipitation. 
We are therefore warranted in assuming that an increase of humidity 
sufficient to account for the observed increase of 17 per cent. in the size of 
the lake would modify the rainfall by less than 17 per cent. The actual 
change of rainfall cannot be estimated with any degree of precision, but 
from a review of such data as are at my command I am led to the opinion 
that an allowance of 10 per cent. would be as likely to exceed as to fall 
short, while an allowance of 7 per cent. would be at the verge of possibility. 
The rainfall of some other portions of the continent has been recorded 
with such a degree of thoroughness and for such a period that a term of 
comparison is afforded. In his discussion of the precipitation of the United 
States, Mr. Schott has grouped the stations by climatic districts, and 
deduced the annual means for the several districts. Making use of his 
table on page 154 (Smithsonian Contributions, No. 222), and restricting 
my attention to the results derived from five or more stations, I select the 
following extreme cases of variation between the mean annual rainfalls of 
