76 
Proceedings of Royal Society of Edinburgh, [jan. 17 , 
may possibly be the result of accident, and due to the open screen 
being more frequently in sunshine than the other, at the time the 
maximum temperature was attained. The sun at this season being 
very low, its rays strike so nearly horizontally they scarcely heat the 
ground, all their heat being received by trees and other vertical 
surfaces. There will therefore be but little heat radiated into the 
screen from the ground, and there will not be much reflected. While 
the observations were being made on the 17th and 18th November, 
no thermometer was placed on the grass, as its indications would 
have been valueless, owing to its showing a great difference in 
temperature according to the situation of the bulb. It would have 
read high and above the temperature of the air if it happened to be 
in sunshine, but if shaded by the grass it would have read much 
below the temperature of the air, as it was observed that the small 
hollows in the grass which had got frozen during the night remained 
frozen all day. It is evident from this that a thermometer placed on 
the grass would not enable us to say whether an excess or deficiency 
of heat was radiated in through the open bottom of the screen ; but 
of course some heat would be reflected inwards, however cold the grass. 
We may here refer to a point of some interest observed when 
making these trials in November. In the previous parts of this 
communication frequent mention has been made of the quick 
fluctuations in the temperature of the passing air, as revealed 
by the constant pulsations of the fine-bulbed thermometer. It was 
observed in these last trials that these quick changes were almost 
entirely absent. The mercury in the fine-bulbed thermometer rose 
and fell nearly as steadily as the one in the C screen, seldom 
varying from it more than 0°T. The apparent dulness in the 
movements of this instrument would seem to be caused by the 
entire change in the manner in which the air is heated by solar 
radiation in winter compared to summer. When the sun is low the 
ground is but little heated by its rays, and there is no layer of hot 
air near its surface swept along by the wind, and imperfectly mixed 
with the colder air above, to give rise to a succession of warm 
and cold parts ; and the heating received by vertical surfaces is 
distributed through a much greater depth of atmosphere, and is more 
easily and perfectly mixed with the passing air. The air passing 
the thermometer has therefore a much more uniform temperature 
