SECTION OF IRRIGATION AND METEOROLOGY. 
29 
■water remains on the surface. In heating, therefore, above 39° the 
whole mass is heated, and the increase in temperature is slow, while 
in cooling the surface cools without the mass of water cooling materi- 
-ally. Hence for temperatures above 39° the surface averages of 
higher temperature than the body of water, and for temperatures 
■between 32° and 39° it averages lower. In the one case it tends to 
make the surface warmer than the air, in the other cooler. 
In our evaporation tank it is noticeable that the temperature of 
the surface shows an excess of from 6° to 7° above the air tempera- 
tures. 
At higher elevations similar differences prevail, and in all proba- 
bility the differences are nearly the same, though exact observation 
is lacking. But as the prevailing temperatures are lower, the water 
temperature is less than the critical temperature of 39° for a greater 
part of the year than at lower elevations. It seems probable, there- 
fore, that the excess of the water temperature above the air tem- 
perature is less than at the lower elevations. 
§ 41. But even if more, the evaporation, so far as this factor is 
•concerned, may be less, for as the evaporation seems to vary directly 
with the difference of vapor pressure corresponding to the tempera- 
tures of the water surface and of the air, and as the vapor pressures 
decrease much faster than the temperatures, the same difference in 
•degrees means a greater difference in vapor pressure, or a greater 
capacity for moisture at a high temperature than at a low. Thus 
the table shows the pressure of the water vapor corresponding to the 
•ordinary air temperatures. 
Temperature. Corresponding Vapor Pressure. 
100 degrees 1.91 inches. 
90 
80 
70 
60 
50 
40 
30 
20 
1.40 
1.02 
.73 
.52 
.36 
.25 
.17 
.11 
From this table a difference of 10° would correspond to a dif- 
ference of vapor pressure or capacity for moisture : 
For 10 Degrees Difference. Difference of Vapor Pressure. 
Between 80 and 70 degrees .29 inches. 
“ 70 and 60 “ .22 
“ 60 and 50 “ .16 “ 
“ 50 and 40 “ .11 “ 
“ 40 and 30 “ .08 
Since the evaporation varies directly as this difference of vapor 
pressure, or, so far as this factor is involved, when the temperature of 
