1887.] Mr John Aitken on Thermometer Screens. 
75 
and the other was one of my radiation thermometers having a plate 
14 inches square. The following are the maximum readings re- 
corded : — 
Date. 
Vacuum Thermometer. 
14" Black Plate. 
November 17 
81°'5 
96°-5 
„ 18 
91° 
113° '5 
These radiation temperatures seem high for so low an elevation 
as the sun attained at that date, for we must remember that the 
temperature of the air was low at the time. Suppose it had been 
a warm summer day, and the temperature of the air 75°, then the 
same solar radiation as that of the 18th would have raised the 
temperature of the vacuum thermometer to 121°, and the other one 
to 143°'5. Still stronger radiation effects were observed on the 2nd 
December; the air on this day was 32°, vac. 81°, and black surface 
111°. This seems to point to a wonderful diathermancy of the 
air in winter compared to summer. I write without sufficient 
observations, but I imagine an equally low sun in summer would 
not have anything like this heating effect.* Then again, the louvres 
of the Stevenson screen are exposed almost perpendicularly to the 
rays of a low sun ; they therefore receive more heat than from an 
equally hot but higher sun in summer. 
As stated, owing to shadows passing over the screen, it was not 
found possible to make a comparison during the winter months 
between the ordinary Stevenson and the modified form. This, 
however, does not seem to be a matter of much importance, as we 
can scarcely expect to find much difference in their readings at this 
season. The daily maximum readings up to the 16th November 
do indicate an advantage in favour of the closed form ; but this 
* The greater diathermancy of the air in winter than in summer has been 
observed by M. Soret at Geneva, and Professor Langley has carefully measured 
it. He finds the greater transparency in winter to be chiefly for rays of short 
wave lengths, and he finds a close relation between the diathermancy and the 
amount of vapour in the air. I much regret I have no low-sun summer observa- 
tions with which to compare my winter ones, but the difference in this climate 
seems to be very much greater than that observed by M. Soret or Professor 
Langley. This difference will probably be due to the position of my observa- 
tions being much further north than Geneva or Mount Whitney, and subject 
to greater extremes of dryness and moisture. 
