102 Count Rumford’s Enquiry concerning the Nature of Heat , 
this short tube (which is only about an inch long) is sealed her- 
metically ; and all communication is cut off, between the air in 
the balls of the instrument and in its tube, and the external arr 
of the atmosphere. 
A small bubble of the spirit of wine (if I may be allowed to 
use that expression) is now made to pass out of the short tube, 
into the long connecting tube ; and the operation is so managed, 
that this bubble (which is about ± of an inch in length) remains 
stationary, at or near the middle of the horizontal part of the 
tube, when the temperature ( and consequently the elasticity ) of 
the air in the two balls, at the two extremities of the tube , is pre- 
cisely the same. 
By means of a scale of equal parts, attached to the horizontal 
part of the connecting tube, the position of the bubble can be 
ascertained, and its movements observed. 
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 ex- 
posed to the action of these rays, will be heated ; and, its elas- 
ticity being increased by this additional heat, its pressure will 
no longer be counterbalanced by 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 de- 
fended 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 regulated, that their actions on those balls may be 
equal, however the temperatures of those hot bodies may differ, 
