232 APPENDIX A. 



enough for the air enclosed in it to take almost 

 instantly the temperature of the glass. We shall 

 try to ascertain the time necessary for the estab- 

 lishment of this equilibrium of temperature by 

 placing the tube under the receiver of the air- 

 pump, making a partial vacuum, and admitting 

 the air. We shall see whether, some seconds after 

 the introduction, the index perceptibly moves. 

 The index must be of very light weight to avoid 

 oscillation as much as possible. 



For the same reason, the capillary tube should 

 be also as narrow as possible. If the straight part 

 of the tube is equal to the bent part and the index 

 be placed at the beginning of the bent part, for a 

 pressure equal to atmospheric pressure, it would 

 not be necessary to subject the instrument to a 

 less pressure than -J atmosphere. It is between 

 these two limits that it would serve as a measure. 



It might end in an open enlargement to prevent 

 the projection of the mercury outside the tube. 

 Disposed in this way, it could be used as a general 

 measure for pressures between p and _/?; p being 

 anything whatever. The apparatus will be fast- 

 ened to a board bearing a graduated scale placed 

 against the straight tube. The scale will be, for 

 instance, numbered by fives or tens. A correspond- 

 ing table denoting pressures would be required. 



