Constant- Volume Air-Thermometer, 151 



instrument, must be put in position with other pieces of ex- 

 perimental apparatus, of such a form as to be easily handled. 



For all these objects I find it most convenient to construct 

 separately the manometric columns, and the air-reservoir with 

 its volume-indicator ; connecting these two parts of the in- 

 strument only by flexible tubing. This arrangement neces- 

 sitates an apparatus for regulating the pressure under which 

 the air in the thermometer is maintained. 



The complete instrument is shown in fig. 1. A is the air- 

 reservoir and volume-indicator, B is the manometric gauge, 

 and C is the pressure-apparatus. 



Ficr. 1. 



The air-reservoir and volume-indicator I shall call, for 

 brevity, the volume-gauge. It is made in two forms (figs. 2 

 and 3) — the form shown in fig. 2 for the lower, and the other 

 for the higher temperatures. The bulb, a, which is generally 

 either globular or cylindrical, is connected by a very fine ca- 

 pillary tube, c, with a somewhat wider tube, d. At b, b' there 

 are two cylindrical bulbs of the same size. The tubes dd and 

 d! dl are of precisely the same diameter, being cut from the 

 same length of uniform glass tubing. The diameter of this 

 tube is about 1 millim. It is snch tubing as is used for the 



