

1905. | On the Thermal Emissivity of a Green Leaf: 133 
apparatus admitted of simplification, the use of a water-jacket around the 
cooling body being unnecessary in our case. 
In an aperture in one side of an air-tight box 3 feet cube was fixed an 
exhaust electric fan, 12 inches in diameter, and driven by an attached motor, 
which was connected with a speed-regulator under ready control. Into the 
side of the box opposite the fan opened a horizontal wooden shaft 5 feet in 
length. For the greater part of its length the cross section of this shaft was 
10 inches square, but just before entering the box it was reduced to 5 inches, 
and at this point there was inserted an aluminium fan-anemometer constructed 
by Richard Freres. By means of this simple apparatus a steady current 
of air could be maintained in the horizontal shaft at determinate velocities 
up to about 140 metres per minute (8°4 kilometres per hour). The thermo- 
metric coils carrying the leaves were inserted through a small door in the 
middle of the shaft. They were placed at a sufticient distance apart to prevent 
any mutual disturbance, and with their planes parallel to the current of air 
which passed over the leaf surfaces. 
The pairs of leaves covering the coils were arranged for differential 
transpiration in the manner already described, and a series of experiments was 
made for determining the thermal emissivity with air-currents of varying 
speeds on exactly the same lines as those for “ still air” conditions, 
The following results were obtained with leaves of Liriodendron tulipt- 

jera :— 
| 
| Thermal emissivity in calories per square 
centimetre of leaf surface per 1° C. 
Speed of air-current in metres temperature excess. 
per minute. 
Per minute. Per second. 
“CTCL, BENE ee een 0:0119 0-000198 
Be MICULES ee eek cece tens 0°0173 0:000288 
Fe ee ae 0:0238 0-000396 
108 3) Vl bee ee eee eee 0-0304 0-000506 
139 ES: ee er ae 0°0361 0-000601 

These results are plotted out in fig. 2, and clearly show that up to 
speeds of 140 metres per minute, the increased rate of cooling or heating 
of a leaf induced by a steadily moving current of air is directly proportional 
to the speed of the current passing over the surface of the lamina. This is 
