Evaporation in a Current of Air, 
423 
comparison, the difference of temperature between the water in the gauge 
6 inches from the fan and the dew-poinfc, and also the difference of 
vapour pressure at the temperatures of the water in the same gauge and 
the temperature of the dew-point. The loss evidently decreases as the 
relative humidity increases, and fairly regularly. Such variations as there 
are in the regularity must be attributed to occasional variations in the 
strength of the wind current during the 50 minutes of observation. A 
curious relation, for which I see no obvious physical reason, is shown 
by the sequences of evaporation and of temperature differences. Calling 
the final temperature of the water r, and the temperature of the dew- 
point d, the formula — 
E - A (r - e) 
is approximately true, E being the evaporation in grains, as before, and A 
a constant depending on the distance of the gauge from the fan. For 
a distance of 6 inches A is 8 "2 very nearly. 
One point must not be lost sight of in judging the results given in this 
paper, and that is that some time elapsed in each case before the water, 
falling from its initial temperature, attained its final stationary tempera- 
ture. It is not easy to measure the simultaneous rate of fall of the 
temperature of the water in a number of gauges in a strong draught ; but 
the following comparison between two of them will help to some extent : — 
Interval. 
15 inches from Fan. 
33 inches from Fan. 
0 minutes 
77-0° 
Eate 
77-0° 
Eate 
70-1 
6-9° 
71-0 
6-0° 
10 
66-1 
4-0 
68-0 
3-0 
15 
63-9 
2-2 
65-2 
2-8 
20 
62-0 
1-9 
64-0 
1-2 
25 
610 
10 
63-0 
10 
30 
60-2 
08 
62-0 
10 
35 
60-0 
0-2 
61-6 
0-4 
40 
59-8 
0-2 
61-0 
0-6 
45 
59-6 
0-2 
61-0 
0-0 
50 
59-1 
0-5 
60-9 
0-1 
55 
59-2 
-0-1 
60-9 
0-0 
60 
59-4 
-0-2 
61-0 
-0-1 
Air current at 13 inches from fan 1,004 feet per minute, and at 33 inches 
from fan 736 feet per minute. 
Dry bulb 
Dew-point 
Humidity, per cent. 
65-0' 
38-1 
37 
