1890.] NEW YORK ACADEMY OF SCIENCES. 211 



gineer of the Geological Survey, to whom I am indebted for the 

 following facts, Colorado has to-day 3,550 square miles of her 

 land under irrigation ; Wyoming, 780 square miles ; Utah, 1,250 

 square miles; and Kansas and Nebraska less than 100 square 

 miles each, making a total of about 6,730 square miles. This 

 spreading of water over the principal portion of the central plains 

 has induced an increased rainfall, but to what extent or amount 

 is not easily determined. Comparative tables show the increase 

 in some months to be most remarkable ; then in a year or two 

 following there is a considerable deficiency. But it is safe to say 

 that all indications point to an increase, — if not in a percep- 

 tible fall of rain or snow according to measurement, then in the 

 depositing of moisture in the form of dew, thus nourishing and 

 sustaining wheat fields and farms, even outside of the reach of 

 irrigation; which bears evidence of the fact that the moisture in 

 that part of the country is increasing, whether it comes from 

 actual rainfall or deposits of dew. The large amount of water 

 thinly spread over such an immense area must either be absorbed 

 by the ground or evaporated ; at any rate, a certain percentage 

 is evaporated, for evaporation goes on at all times and under all 

 atmospheric conditions. During the warmer part of the day the 

 atmosphere is capable of containing the most moisture, and in 

 that country where few clouds exist, and the sky at night is per- 

 fectly clear and transparent, the radiation is rapid and the con- 

 densation of moisture takes place almost as soon as the sun sets. 

 By light winds the moisture maybe carried for miles from where 

 it has been gathered during the day, and deposited outside the 

 fields of irrigation, thus accounting for the fertile and pliable 

 soil where nature alone supplies the moisture. The planting 

 of trees and general vegetation on what was a barren waste 

 has a further tendency to increase the rainfall, and the larger 

 the area covered the greater will be the influence to prevent a 

 relapse. The evidence of a steady increase in rainfall is not as 

 conclusive as might be desired, but the evidence of increased 

 moisture is plainly shown. 



Now, we find that during the past ten years immense tracts 

 of land in the States of Colorado, Utah, Wyoming, Kansas, and 

 Nebraska have greatly advanced in the system of irrigation. 

 Colorado has by far the largest irrigable area, and furnishes water 

 sufficient to cover over 2,000,000 acres. 



To sum it up, we have shown, first, that the change that has 

 taken place in our climate is due to a change in the course of 

 storms; and, second, that that change in the course of storms is 

 induced by the irrigated lands of the West. 



Now the question arises, What will be the effect upon our 

 climate of further irrigation ? 



