6 
378 On the Heat in the Sun’s Rays. 
portion or nearly so to the difference between its temperature 
and that of the medium in which it is placed. 
I regard it therefore as well established that the differential 
thermometer affords a correct measurement of the differences be- 
tween the heat of the canister and that of the surrounding air. 
These differences may evidently be varied in two ways: by 
changing either— 
ist. The heat of the canister ; or— 
2dly..The temperature of the air. 
An increase or diminution in the heat of the canister would 
directly increase or diminish the differences; whilst an increase 
in the temperature of the air would diminish the difference until 
an equality between the two was obtained. If the temperature 
of the air were uniform and the changes were those of the canis- 
ter alone, the instrument measuring the differences would cor- 
rectly indicate those changes. But if the heat of the canister 
were uniform and that of the air were varied, then would the in- 
strument equally indicate those changes, but in a contrary direc- 
tion. case the heat of both the canister and the air was vari 
ment to obtain*the desired measurement. 
It is upon these principles that I have applied the differential 
thermometer to measure the comparative heat in the sun’s rays. 
One of its bulbs received their direct action in the same way 
that it received the rays proceeding from the canister. ‘The tem- 
perature of the air was at the same time obtained by a common 
thermometer. An increase was added to, and a diminution sub- 
tracted from, the indications of the instrument to obtain the real 
changes in the heat of the rays proceeding from the sun. 
y first experiment was of the simplest kind. It was a win- 
ter’s day. The differential thermometer was placed on the out- 
side of a window where the temperature was below the freezin 
point. The effect measured by the scale (which merely divide 
the stem into equal parts) was 53°.~ It was then placed on 
the inside of the window where the temperature was about 
70°, and to my surprise the effect rose to 115°. The experiment 
was many times repeated with similar results, although varying 
