FOE DETEEMINING THE EADIATING POWEES OF SUEFACES IN AIE. 
387 
outer one radiating into the atmosphere, when the temperature of the lower surface 
is much higher than that of the atmosphere 
6. We may now proceed to describe the manner of conducting each experiment, the 
radiating surface having attained its required stationary temperature, and the calorimeter 
being filled with distilled water. A vessel containing water was placed near at hand, 
the temperature of the water being kept a little lower than that of the surrounding 
atmosphere of the room. The calorimeter was then immersed in this water, and allowed 
to remain there till the temperature of the distilled water and the tin vessel (A) con- 
taining it became stationary, as indicated by the sensitive L-shaped thermometer already 
described. It was then taken out of the water, and all the moisture adhering to it 
externally very carefully removed. The temperature would then remain sensibly sta- 
tionary, and a little lower than that of the surrounding air. The thermometer was then 
carefully read, and the calorimeter transferred to its position above described, imme- 
diately over the radiating surface. After remaining there a certain observed time 
(during which the distilled water in it was thoroughly stirred as above described), it 
was removed, and the thermometer again read. The difference of the readings afforded 
a measure of the quantity of heat which had emanated from the heated surface and been 
absorbed by the vessel A of the calorimeter through its bottom, the observed result 
being subject to certain corrections to be hereafter considered. 
The time during which the calorimeter was exposed to the influence of the radiating 
surface was determined with great acciu’acy. It seldom exceeded five or six minutes, 
generally not more than three or four. An assistant placed close to his ear a watch 
which ticJced very audibly 150 times in a minute. It was easy to indicate to him the 
exact instant when the calorimeter was placed over the radiating surface, and he then 
began to count aloud the beats of the watch to a number varying according to the circum- 
stances of each experiment, from 300 to 1000. The instrument was rapidly removed 
at the instant the counting of the number fixed upon was completed. The error to which 
the experiments were liable in this estimation of the time was very small. 
I was also able to make the readings of the thermometer very accurately. I have 
stated that the first reading was made while the mercury was stationary. Such was the 
case also with the second reading ; the rise of the thermometer of the calorimeter during 
an experiment seldom exceeded three or four degrees, and therefore the temperature of 
the distilled water and its containing vessel, when just removed from the influence of the 
radiating surface, differed from that of the surrounding atmosphere only by a small 
amount. Consequently the thermometer remained sensibly stationary for a longer time 
than was required to read it with care. I was thus able to obtain the reading with ease 
within the hundredth part of a degree of Faheenheit. 
* It might seem that aflat bulb would have answered the purpose, and been more simple in construction ; 
but it was pointed out to me by Mr. Casella that thermometers of this construction are not to be relied 
upon. They may yield to a small external pressure sufficiently to affect the indication of temperature to 
the amount of several degrees. 
3 F 2 
