Irrigation tends in part to correct these difficulties, since it furnishes the soil with the needed water at times when the lack of rain would cause an entire failure in future growth. Irrigation, of course, cannot prevent rainfall, and it may be that after a copious soaking of the ground with the irrigating water a heavy rain may follow, resulting in an excess of moisture as bad for the ground as none at all. The possibility of irrigation cannot prevent excessive rains at the time of harvest, but the advantages of being able to control the soil moisture during the period of growth are more than enough to counterbalance any possible disadvantages. During the summer months evaporation is very high, the dryness of the air and the high temperature combining to draw moisture from the soil in considerable quantity. Then, too, the plants themselves, while absorbing moisture from their roots, evaporate moisture through their leaves, and agricultural stations have made extensive studies on the amount of this evaporation from different plants. The teaching of it all is that the amount of water which can be utilized by the soil, not merely for the sake of the growth of the plants themselves, but to make up for the demands of evaporation, is very high. Mr. Newell, of the United States Geological Survey, points out that while the amount of water required for raising crops varies according to soil and other conditions, yet a large quantity is required to maintain the soil in such a degree of saturation as to best promote the vitality of the plant life. He shows that for each ton of hay raised upon an acre, from three hundred to five hundred tons of water must be furnished either by rainfall or by artificial means. In other words, since water covering an acre to a depth of one inch weighs about one hundred and thirteen tons, it would be necessary to cover an acre to a depth of from three to five inches if that acre produced one ton of hay. From actual conditions, he shows that it has been necessary, in order to produce five tons of barley hay per acre, to provide an amount of water which would cover the acre to a depth of twenty inches. Although his figures have special reference to the semi-arid regions of the West they furnish a guide for the amount of water which may profitably be used in addition to the rainfall, which, in the summer months, may be practically nothing even in the East. From three to six inches in depth each month is his estimate of the needed water for successful crop growing, the difference depending upon the character of the soil, more being required in sandy soils and less where the texture is finer.