134 Drs. H. T. Brown and W. E. Wilson. [Jan. 9, 
quite in accordance with the general results obtained by Crichton-Mitchell, 
from which we are justified in conciuding that the same law would hold good 
for even higher speeds, z.e., up to at least 300 metres per minute (18 kilo- 
metres per hour). 
The increase in thermal emissivity for a velocity of 1 metre per minute is 
seen to be 0°000174 calorie per square centimetre of leaf surface per minute 
per 1° C. difference of temperature between the leaf and the air. 
Another similar experiment with the leaves of Helianthus multifiorus, 
having a thermal emissivity under “still air” conditions of 0-0150 calorie per 
per minute for 
1°Temp. excess. x 100 

Emissivity in calorles per $q. c.m. 
0 -8 90 100 no 
120 130 140 I50 
Air velocities in metres per minute. 
Pres .2; 
square centimetre per minute per 1° C. excess temperature, showed an 
emissivity of 0°0338 calorie in an air-current of a speed of 109°8 metres per 
minute. This result is also plotted in the accompanying figure, and it will 
be seen that the line is sensibly parallel to that given by the experiments on 
Liriodendron, the rise of emissivity for an increased speed of 1 metre per 
minute being 0:000171 calorie, against 0:000174 calorie in the case of the 
Liriodendion. This renders it probable that although there are small initial 
differences in the emissivity of leaves under “still air” conditions, the 
influence of moving air-currents on all glabrous and moderately thin leaves 
in effecting transference of heat from and to the leaf is practically identical. 
From the magnitude of the thermal effects produced by moving air it will 
