1887.] Mr John Aitken on Thermometer Screens. 77 
with a low than with a high snn. If this is the correct explanation, 
then it confirms our conclusion that the fluctuations observed in this 
thermometer in summer were due to imperfectly mixed hot and 
cold air, and not to variations in the radiation. 
We now come to the consideration of the second point, namely, 
the power of the passing air to check the effects of radiation. It is 
very evident that this will depend on the rate at which the air 
passes over the radiation-heated surfaces. Now, in summer, we 
have— ^-apart altogether from what we call wind — an unstable con- 
dition of the atmosphere which acts quite locally. Owing to the 
heating of the ground, the air over it is rarefied and tends to rise ; its 
stability is thus constantly disturbed by slight movements. But in 
winter we have a totally different condition of matters. Radiation 
from the earth being now in excess of that towards it, the surface 
of the ground gets cooled, and the air on it tends to become denser, 
and so keep closer to the ground, and if the country is flat, there is 
no tendency as in summer to local movements, but on the contrary, 
the tendency is towards stability. We thus see that the solar radia- 
tion effect in summer is to cause — in addition to winds proper — slight 
local airs, which prevent the surface of the louvres in the screen from 
being highly heated ; whereas in winter the tendency owing to the 
lowness of the sun is the other way, and we have degrees of calmness 
in winter, quite unknown and impossible in summer. With these 
two things, namely, the high heating power of even a low winter sun, 
and the great calmness of air possible and frequent at this season, 
we seem to have the explanation of the somewhat unexpectedly 
great error of the Stevenson screen observed during the winter season. 
The error at this season is due principally to the heating of 
the louvres, and but little to heat radiated in through the open 
bottom. 
In connection with the more perfect calms which take place in 
winter than in summer, and as showing their effects on the com- 
parative cooling produced by them at the different seasons, we may 
here refer to the readings given by the two forms of radiation ther- 
mometers used in this investigation, namely, the ordinary black- 
bulb thermometer in vacuo, and the flat black surface thermometer 
having an area of 14 inches square. The ordinary black bulb gives 
a very complicated result, indeed it is extremely difficult to interpret 
