

1905. | On the Thermal Emissivity of a Green Leaf. 131 
found that the heat emitted per second, per 1° difference of temperature, per 
square centimetre of surface, was 0°000252 calorie. 
The Thermal Enussivity of a Leaf in Moving Arr. 
In the experiments so far recorded, we have confined our attention to the 
“rate of cooling” of a leaf under “still air” conditions, that is to say, under 
conditions in which the leaf was shielded from direct draughts, and, as regards 
mass-movement of the air, was only subjected to the slight convective 
currents induced by the temperature differences between the leaves and their 
environment. 
We have still to consider the influence of comparatively rapid air-currents 
of determinate velocity on the rate of transference of energy to and from the 
leaf, a matter of considerable importance in any study of the energetics of 
the leat,-especially as regards the dissipation of the excess of incident solar 
energy under open air conditions. 
It has already been pointed out elsewhere* that, owing to the structure of 
the leaf, the rapidity of the transpiratory process, under fixed conditions of 
temperature and air-humidity, will not be much influenced by merely 
increasing the velocity of the air passing over the surface of the leaf when 
once a very moderate degree of velocity is exceeded. If, therefore, we have 
two pairs of leaves placed in the thermometric apparatus already described, 
and we arrange them so as to produce differential transpiration, the general 
tendency of a steady current of air passing over the leaves will be to diminish 
the temperature difference which they exhibit in still air conditions. Hence 
by extending our observations to leaves placed in air-currents of known 
velocity, we can determine the increase in the thermal emissivity due to 
this cause. : 
Before proceeding to describe our experiments in this direction we must 
refer to a previous paper which has a direct bearing on the subject. 
So far as we know the only experiments which have been made on the 
influence of air-currents of definite velocity on the rate of cooling of a heated 
body are those of Crichton-Mitchell.t The author employed a blackened 
copper sphere which had a diameter of 2 inches, and the rate of cooling was 
determined in steady air-currents of determinate velocity. It was found that 
within limits of 200° C. temperature excess and a speed of air-current of 
1000 metres per minute Newton’s, “ Law of Cooling” is accurate, provided the 
speed of the air-current passing the surface of the cooling body be sufficient, 
* Brown and Escombe, ‘ Roy. Soc. Proe.,’ this vol., p. 79. 
+ ‘ Roy. Soc. Edin. Trans.,’ vol. 40, 1900, p. 39. 
