348 REPORT—1846, 
which determines the rate of evaporation into the open air. Now by Dr. 
Apjohn’s researches f!—f" is exactly proportional to e x where d is the 
difference in,degrees between the temperature of the air and the tempera- 
ture of evaporation, and / the barometric pressure. Water undergoing eva- 
poration takes up for each unit of weight a certain measure of heat from the 
substances in contact ; its cooling effect on them is therefore proportioned to 
the quantity of water evaporated in a given time, which again is proportional 
piles ‘ hi enh 
to the force f'—f", that is, to a * 30° 
A convenient mode of experiment to obtain separately the cooling power 
of evaporation from that of air currents and radiation, is to note the times of 
cooling of one thermometer first dry and then wet, other circumstances being 
similar, as in the following experiment, made in the calm air of a large room 
(air 55°, evaporation 4:7°8, dew-point 38°6). 
Temp. of Time of cooling 5° 
‘Thermometer, in seconds, 
dry. wet 
105 0 0 
100 45 18 
95 52 21 
90 60 23 
85 70 29 
80 85 36 
75 102 42 
70 125 54 
65 168 70 
60 257 93 
DD) oy rith : nebees ve 137 
50 Gaunehier 253 
The reciprocals of these times (=) correspond to the cooling powers ex- 
erted on the two instruments and appear in the following table (column A 
and B). The difference between them obviously corresponds to the cooling 
power of evaporation exerted on the wet bulb (column C). Column D con- 
tains the numerical values of the forces of evaporation due to the successive 
temperatures of the bulb in the existing state of the air; and column E a 
series of numbers proportioned to these, and representing the successive 
cooling powers as they ought to be found experimentally if the theory 
already advanced be true. F shows the difference between theory and 
experiment. 
Mean Temp. A. B. Cc. D. E. F, 
wet. 
1025 | 293 | 555 | 333 | 1-758 | 352 | 419 
975 | 192 | 476 | 284 | 1-458 | 292 | +8 
925 | 166 | 435 | 269 | 1-208 | 242 | —28 
875 | 143 | 344 | 201 | 1014 | 203 | +2 
s25 | 117 | 279 | 162 | -a31 | 166 | +4 
775 98 | 237 | 139 | -673 | 135 | — 4 
70°5 80 | 185 | 105 | +535 | 107 | +2 
67°5 59 «| «(143 84 | -418 | ‘84 0 
62-5 38 | 107 69 | -316 | 63 | —6 
57°5 20 73 53 | 229 | 46 | —7 
50.5 eo ed: Aiwidy A SPN 155 | 31 
arp \eenieds Sen eee. 091 18 
LAD ERE BR! ASL Wet DN 000 0 
