48 Inquiry concerning the Nature of Heat, 



to call his instrument), it is composed of two glass 

 balls, attached to the two ends of a bent glass tube ; but 

 the balls, instead of being near together, are placed at 

 a considerable distance from each other ; and the tube 

 which connects them, instead of being bent in its mid- 

 dle, and its two extremities turned upwards, is quite 

 straight in the middle, and its two extremities, to which 

 its two balls are attached, are turned perpendicularly 

 upwards, so as to form each a right angle with the mid- 

 dle part of the tube, which remains in a horizontal 

 position. 



At one of the elbows of this tube there is inserted a 

 short tube of nearly the same diameter, by means of 

 which a very small quantity of spirit of wine, tinged 

 of a red colour, is introduced into the instrument; and, 

 after this is done, the end of this short tube (which is 

 only about an inch long) is sealed hermetically ; and all 

 communication is cut off between the air in the balls 

 of the instrument and in its tube and the external air 

 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 f of an inch in length) remains station- 

 ary, 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 precisely 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. 



