42 Irrigation and Drainage 



of-doors, with the plants surrounded by the field crop and under 

 the out-of-door meteorological conditions, than they did in the 

 house. 



This difference, however, shows larger than it really is, for it 

 has been shown that the use of water is usually more economical 

 in those cases in which the yields are largest, and in these cases 

 there has been a larger yield of dry matter per unit area in the 

 plant-house cylinders than were secured from the cylinders in the 

 field. The total mean yield per acre for the oats, maize and 

 clover in the field cylinders was 6.312 tons and in the plant-house 

 7.397 tons of dry matter per acre, making the latter yields on the 

 average 17.19 per cent larger; and to this difference in yield must 

 certainly be ascribed a part of the difference in the amount of 

 water given off from the plants and from the soil during the 

 periods of growth. It is quite plain, for example, that the loss 

 of water from the soil surface would tend to be relatively larger, 

 and probably, also, absolutely larger from the cylinders bearing 

 the smallest crop of a given kind. The absolute loss would cer- 

 tainly be largest from the cylinders where the 'crop had the thin- 

 nest stand on the ground, and some of the cases of larger yield 

 per unit area in the plant-house are due to the fact that more 

 plants occupied the same area. 



While, therefore, from the general principles governing the 

 rate of evaporation, we are led to expect that more moisture must 

 be lost from vegetation growing in a dry atmosphere than under 

 more humid conditions, we are not able to point to our data as 

 bearing out such a view in any emphatic manner. The rate of 

 air movement in the plant -house has certainly been less than it 

 was in the field, but the higher temperature in the plant-house 

 has probably left the air relatively dryer during both day and 

 night than in the field. 



The conditions which did exist, both in the plant -house and 

 in a field of maize, were noted on July 27, 28 and 29. The rela- 

 tive humidity of the air was measured with a wet -and -dry bulb 

 thermometer, and the rate of evaporation was also measured under 

 the two conditions with a form of Piche evaporometer. Two of 

 these instruments were hung among the corn plants in the plant- 



