640 
Proceedings of the Royal Society 
from the air passing through its meshes, the air on these occasions 
being always hotter than the exposed surfaces of bodies. The plant 
therefore makes its heat-exchanges with a surface but little cooled 
by radiation instead of with the cold of space, as tempered by our 
atmosphere. We see from this that the protection afforded by 
these cloths on frosty nights is efficient only on what are called 
radiation frosts ; the cloth has no power to heat the air. Further, we 
see that the efficiency of the cloth will be a good, deal affected by the 
amount of wind ; the better the air circulation the warmer the cloth 
will be — that is, the less it will be cooled by radiation. It may also 
be observed that an open cloth may be under certain conditions a 
better protection than a close covering. From the experiments with 
muslin in the draught tube it was seen that one thickness of this 
material reduced the radiation effect to less than half. From experi- 
ments made at night with one radiation thermometer, protected by 
a piece of muslin stretched horizontally at a little distance above 
it, while another was freely exposed to the sky, I found that one 
thickness of the same muslin reduced the cooling effect of radiation 
to almost exactly one half, the less circulation of the air at the time 
accounting for the difference. 
It is now long since Dr Joule first laid down certain imaginary 
conditions under which it might be possible to get the true tem- 
perature of the air. The method proposed by him was to place the 
thermometer in a long vertical tube, open at both ends, but with a 
cap by which the lower end could be closed. This tube was to be 
surrounded with a jacket by means of which it could be heated to any 
desired temperature. The temperature of this tube was to be care- 
fully adjusted, so that the air inside of it tended neither to ascend 
nor to descend when the cap was taken off. When this condition 
of matters was arrived at, the air inside the tube would evidently 
have the same temperature as the air outside of it, and the thermo- 
meter would be at the true temperature of the air, as the radiating 
surfaces all round it would have the same temperature as the air. 
It is needless to say these conditions could not be carried into 
practice, for taking observations except where the temperature was 
nearly constant, and the air free from currents ; in the open air, 
winds and the constant changes of temperature there taking place 
would make it quite impracticable. 
