and the Mode of its Communication . 103 
or however different may be the quantities, or intensities, of the 
calorific rays which they emit. 
The instrument will show, with the greatest certainty, when 
the actions of these hot bodies on their respective balls are equal ; 
for, until they become unequal, the bubble will remain im- 
moveable in its place. 
And, when the actions of two hot bodies on the instrument 
are equal, the relative intensities of the rays they emit may be 
ascertained, by the distances of the bodies from the balls of the 
instrument. 
If their distances from their respective balls are equal, the 
intensities of the rays they emit must of course be equal. 
If those distances are unequal, the intensities will probably 
be as the squares of the distances, inversely. 
A distinct and satisfactory idea may be formed, of the instru- 
ment I have been describing, from Figure 2. 
AB is a board, 27 inches long, 9 inches wide, and 1 inch 
thick, which serves as a support for the bent tube CDE, at the 
two extremities of which the two balls are fixed. The two pro- 
jecting ends of the tube, C and E, which are in a vertical 
position, are each 10 inches long ; and the horizontal part D of 
the tube, which is fastened down on the board, is 17 inches in 
length. 
The balls are each 1.625 inches in diameter. The diameter of 
the tube is such, that 1 inch of it in length would contain 15 
grains Troy of mercury. 
The pillar F, which, by means of a horizontal arm projecting 
from it, serves for supporting the circular vertical screen repre- 
sented in the figure, is firmly fixed in the board AB. 
This circular screen (which is made of pasteboard, covered 
