of Edinhurgli, Session 1883-84. 
069 
simple conditions for observation. Suppose the sky overcast, with 
a uniform and thick covering of clouds, that a fresh breeze is blowing, 
and that the temperature has been constant for some time. Under 
these conditions, the surfaces of all bodies, large and small, in the 
open air, have nearly the same temperature. But now suppose the 
the clouds to clear away, and the sun to shine out brightly. The 
temperature equilibrium is soon destroyed, and the surfaces of the 
different bodies quickly take up different temperatures, all becoming 
more or less warmer than the air. The degree to which the different 
bodies are heated will depend on — 1st, their exposure to the sun ; 
2nd, their exposure to the wind : 3rd, the absorbing power of their 
surfaces for heat ; 4th, their conducting power ; and 5th, their 
capacity for heat. But, in addition to these, I find that the size of 
the body, or ratjier the extent of its surface, has a most important 
influence. These remarks apply to bodies placed in shade as well 
as to those exposed to sunshine. 
Before describing the experiments which show the effect of size 
on the temperature of bodies, it will be necessary for me to describe 
the nature of the objects surrounding the spot where these ex- 
periments were made, as the results in experiments of this kind are 
determined very much by the surroundings, every position giving 
a different result according to the objects to which it is exposed. 
The view from the position of these experiments is bounded on the 
south and west by trees at a considerable distance, on the east by 
shrubs close at hand, and backed by a wall which extends south- 
wards, and closes in the view to the south-east. One half of the 
north is bounded by a wall at a distance of 6 feet, and the other 
half by trees at a distance, while the ground is covered with grass 
in nearly every direction. This site was selected on account of its 
being sheltered from the wind, and otherwise suited for the experi- 
ments. 
When studying the action of the draught tube thermometer 
screen, the experiments were begun by erecting, in the position 
above described, a horizontal sunshade made of wood, 3 feet long by 
2 feet broad. This shade was erected at a height of about 5 feet from 
the ground. In the sunshade was made a round opening, into 
which fitted a tall metal tube. This tube projected 1 foot down- 
wards into the shade and 2 feet upwards into sunshine. Expori- 
