and the Mode of its Communication. log 
If one of the hot bodies was now brought nearer the ball to 
which it was presented, (the other hot body remaining in its 
place,) the bubble immediately began to move from the hot body 
which was advanced forward, towards the opposite ball, to which 
the other hot body was presented. 
If, instead of advancing one of the hot bodies nearer the ball 
to which it was presented, it was drawn backward to a greater 
distance from it, the action of its calorific rays on the ball was 
diminished by this increase of distance ; and, being overcome by 
the action of the rays from the hot body presented to the op- 
posite ball, (at a smaller distance,) the bubble was forced out 
of its place, and obliged to move towards the ball which had 
been drawn backward. 
When one of the hot bodies only was presented to one of the 
balls, the bubble was immediately put in motion ; and, by 
bringing the hot body nearer to the ball, it might be driven 
quite out of the tube, into the opposite ball; this, however, 
should never be done, because it totally deranges the instru- 
ment, as it is easy to perceive it must do. 
Having, by these trials, ascertained the sensibility and the 
accuracy of my instrument, I now proceeded to make the fol- 
lowing decisive experiment. 
Exper. No. 13. Having blackened the flat circular bottom 
of one of the cylindrical vessels, by holding it over the flame of 
a wax candle, I now filled both vessels again with water at the 
temperature of 180° F. and presented them, as before, to the two 
opposite balls of the instrument, at equal distances. 
The bubble was instantly driven out of its place, by the 
superior action of the blackened surface; and did not return 
to its former station, till after the vessel which was blackened 
