{s2 Drs. H. T. Brown and W. E. Wilson. [Jan. 9, 
Moreover, and this is the point which is of particular interest for us, the 
cooling influence of the air is directly proportional to its speed up to velocities 
of about 300 metres per minute (18 kilometres per hour). Beyond this 
velocity of 300 metres the cooling influence in the case of the copper ball fell 
off somewhat. The explanation given, which is no doubt the correct one, is 
that when a body is cooling in air, the effect of radiation, which involves 
higher powers of the temperature excess (Stefan’s “ fourth-power law ”) is 
small compared with the effect produced by convection.* 
Crichton-Mitchell’s results are not expressed in absolute units, bat from 
the data given it is possible to make a fairly close approximation to these. 
With a temperature difference of 10° C. between the copper ball and the air 
(the lowest experimental difference recorded) we have calculated (approxi- 
mately) the calories lost per square centimetre of surface per minute for a 
1° temperature excess for each of the given velocities of the air-current from 
41 to 976 metres per minute. When the results were plotted the cooling 
effect of the air-current was seen to be practically a lnear function of the 
speed up to velocities of 300 metres per minute, and that the extra cooling 
effect induced by the air-current amounted to 0:000206 calorie per square 
centimetre per minute per 1° C. excess, for an increased speed of the current 
of 1 metre per minute. But here again we are prevented from applying 
these values with any degree of certainty to the rate of cooling of a leaf 
owing to the different nature and shape of the cooling bodies. In the paper 
just cited the author proposed to apply his method of investigation to the 
cooling in moving air of a strip of platinum foil heated by means of an 
electric current, and it is probable that owing to the closer similarity in the 
shape of a platinum strip to the leaf-lamina such an experiment might give 
results which would have a more direct bearing on the special case we are 
considering, although there would still be the uncertainty due to the very 
different nature of the two laminz.T 
Our experiments on the influence of currents of air on the thermal 
emissivity of foliage leaves were carried out on lines very similar to those of 
Crichton-Mitchell, but owing to the differential method employed the 
* How relatively small a part radiation plays in the cooling of a heated body in air is shown by 
J. T. Bottomley’s experiments (‘ Phil. Trans.,’ A, vol. 178, 1887, p. 429) on the emissivity of 
heated platinum wires. From a series of experiments made in a high vacuum and in air at 
varying pressures it was found that only about 5 per cent. of the emissivity in air at 740 mm. was 
due to radiation. 
+ Professor Crichton-Mitchell has recently laid before the Royal Society of Edinburgh a 
preliminary account of his experiments on the cooling of a strip of platinum under the above 
conditions, but at the time of writing these results are not available. 
