112 TBANSACTIONS OF THE ILLINOIS 



comes so filled with water by springtime. During the summer, how 

 ever, vast amounts are returned to the atmosphere in the shape of 

 vapor. At Rothamsted, England, the average rainfall is 28.30 inches. 

 Of this, 43.1 per cent, is lost by drainage — tile sixty inches deep — 

 and 56.9 per cent, by evaporation. The records made at the New 

 York experimental station during 1888-'4-'5-'6, show an average 

 rainfall of 25.0 inches, and a loss by drainage of 7.34 inches, by evap- 

 oration of 17.66 inches. Of the drainage loss at Rothamsted, some- 

 what over seventy per cent, escaped from September to February in- 

 clusive, leaving only thirty per cent, from March to August. Our 

 summers are much warmer, and the air is drier, hence our evapora- 

 tion would be still greater. The water of summer rains is largely 

 absorbed by the earth for a time, but gradually passes back to the 

 air as vapor. So far as our crops are concerned, this summer rainfall 

 is of much more importance, while the precipitation of the winter 

 fills our wells and streams. 



We will endeavor now to compare the proportional amounts 

 given off to the air as vapor from the surface of the ground, and 

 from the foliage of plants. The average of a large number of ex- 

 periments gives us about one and one-quarter ounces of water to 

 every square foot of leaf -surf ace, as the amount normally thrown 

 into the atmosphere during one clear summer day. Since a pint of 

 water weighs sixteen ounces, it therefore requires about thirteen 

 square feet of leaf-surface to exhale a pint during a day of clear 

 weather. (Much less or none is transpired in cloudy weather.) Thus 

 stated, this amount does not seem large; but when we consider the 

 enormous aggregate measure of leaf-surface, even on one forest tree, 

 we may gain some idea of the total quantity of water so returned 

 to the air. A large tree is estimated to have 200,000 square feet of 

 foliage. With a little mathematical computation, we reach the as- 

 tounding result that such a tree throws into the air every sunshiny 

 day forty barrels of forty gallons each! Ten such trees may stand 

 upon an acre (fifty feet apart); we therefore have four hundred 

 barrels of water as the amount daily pumped from the soil into the 

 air by trees. Looked at in this way, the amount becomes enormous, 

 if not incredible. It will be at once asked : Where can any such 

 amount come from, for any continuous length of time? But this 

 great amount would, after all, form a layer over the acre only one- 

 twentieth of an inch thick. This again is small — less than would 

 be evaporated from the same ground area of soil or water. 



From the records given above, there falls in Illinois, from May 

 to September inclusive (the growing season), an average of 18.61 

 inches of water. During this time there is reported seventy-nine 

 days '^ clear," to which we may add enough of '^fair" weather to 

 make one hundred days of evaporation. Multiplying one-twentieth 

 of an inch by this, we have five inches as a possible and probable 

 amount transpired during a season from the foliage of plants. Some 



