and the Mode of its Communication. 49 



If now, the bubble being at rest in its proper place, 

 one of the balls of the instrument be exposed to the 

 calorific rays which proceed in all directions from a hot 

 body, while the other ball is defended from those rays 

 by a screen, the air in the ball so exposed to the action 

 of these rays will be heated ; and, its elasticity being 

 increased by this additional heat, its pressure will no 

 longer be counterbalanced Dy the elasticity of the colder 

 air in the other ball, and the bubble will be forced to 

 move out of its place and to take its station nearer to 

 the colder ball. 



By presenting two hot bodies at the same time to 

 the two balls of the instrument, taking care that each 

 ball shall be defended from the action of the hot body 

 presented to the opposite ball, the distances of these 

 hot bodies from their respective balls may be so regu- 

 lated that their actions on those balls may be equal, 

 however the temperatures of those hot bodies may 

 differ, 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 immovable 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 mstrument. 



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. 



VOL. II. 4 



