M. POUILLET ON SOLAR HEAT. 45 
labours of those who have preceded me, particularly those of 
M. de Laplace, M. Fourier and M. Poisson. 
1. I have attempted to determine the quantity of solar heat 
by three different processes :— 
(1.) By means of the apparatus which is described in the 
two first editions of my Elémens de Physique et de Météoro- 
logie. 
(2.) By means of a direct pyrheliometer. 
(3.) By means of a pyrheliometer with a lens. 
The direct pyrheliometer is represented in the annexed figure. 
V 
men; WES 
The vessel V is very thin, of silver or plated metal: one de- 
cimetre in diameter, and 14 or 15 millimetres high; it con- 
tains about 100 grammes of water. The stopper which fixes 
the thermometer to the vessel is fitted to a metal tube which is 
supported at its extremities by two collets c, c',in which it plays 
freely ; so that on turning the button 6, the whole apparatus 
turns round the axis of the thermometer, and the water of the 
vessel is incessantly agitated in order that the temperature may 
