168 Mr. E. Foote on the Heat in the Sun's Rays. 



the differential thermometer receives heat from the canister, it at 

 the same time radiates it to surrounding bodies, and that in pro- 

 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 

 between the heat of the canister and that of the surrounding air. 

 These differences may evidently be varied in two ways : by 

 changing either — 



1st. The heat of the canister ; or — 



2ndly. 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 

 canister alone, the instrument measuring the differences would 

 correctly indicate those changes ; but if the heat of the canister 

 were uniform and that of the air were varied, then would the 

 instrument equally indicate those changes, but in a contrary 

 direction. In case the heat of both the canister and the air was 

 varied at the same time, if we knew the change in one and its 

 effects upon the instrument, we could easiltj deduce the changes 

 in the other. Suppose, for example, an increase of ten degrees 

 on the scale of the instrument, and an elevation of five degrees in 

 the temperature of the air ; the effect of the latter having been 

 to depress the thermometer five degrees, and the canister having 

 not only ovei'come that effect but increased the indications ten 

 degrees, the sum of the two or fifteen degrees would be the real 

 change which had taken place in the heat of the canister. Had 

 there been a depression in the temperature of the air, it obviously 

 should be subtracted from the indications of the instrument 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 tempe- 

 rature 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. 



My first experiment was of the simplest kind. It was a win- 

 tei-'s day. The differential thermometer was placed on the out- 

 side of a window where the temperature w'as below the freezing- 

 point. The effect measured by the scale (which merely divided 

 the stem into equal parts) was 53°. It was then placed on the 

 inside of the window, where the temperature waa about 70", and 



