46 M. POUILLET ON SOLAR HEAT, 
be very uniform in its whole mass. The circle d, which receives 
the shadow of the vessel, serves to adjust the apparatus. The 
surface of the vessel which receives the solar action is carefully 
blackened with lamp-black. 
The experiment is made in the following manner :—The 
water in the vessel being nearly of the surrounding tempera- 
ture, the pyrheliometer is held in the shade, but very near the 
place where it is to receive the sun: it is placed so that it looks 
towards the same extent of sky, and there, for four minutes, 
its warming or its cooling is noted from minute to minute ; 
during the following minute it is placed behind a screen, and 
then adjusted so that on removing the screen at the end of 
the minute, which will be the fifth, the solar rays strike it per- 
pendicularly. Then, during five minutes, under the action of 
the sun, its warming, which becomes very rapid, is observed 
from minute to minute, and care is taken to keep the water in- 
cessantly agitated; at the end of the fifth minute the screen is 
replaced, the apparatus withdrawn into its first position, and 
for five minutes more its cooling is observed. 
Let R be the warming which it undergoes during the five 
minutes of the solar action, 7 and 7 the coolings which it under- 
went during the five minutes which preceded that action and 
during the five minutes which followed it, it is easy to see that 
the elevation of temperature ¢ produced by the heat of the sun 
s es soon 
2 
Let d be the diameter of the vessel expressed in centimetres, 
p the weight of the water which it contains expressed in grammes, 
_ p' the weight of the vessel itself and of the immersed portion of 
- the thermometer, this weight being reduced to what it would be 
for a specific heat equal to unity, we see that the elevation of 
temperature observed ¢, corresponds with a quantity of heat 
t(p + p'). “8 
This heat having fallen in five minutes on a surface — , each 
unity of surface has received 
4 (p + p') 
care during the five minutes, 
4 (p ‘) ‘ : 
and sag ¢ during one minute. 
