MANOMETERS. 93 



AP above the mercury would be filled with an elastic vapor, 

 which would press down upon the column, so that its weight 

 would no longer be a measure of the atmospheric pressure, 

 but of the difference between this pressure and the elastic 

 force of the vapor given off. The second advantage is that, 

 on account of the great density of mercury, the height of 

 the column which measures the atmospheric pressure is so 

 small that barometers constructed with it are of a very con- 

 venient size. The pressure of the air may be measured by 

 using any kind of liquid. The density of mercury is about 

 13.595 times that of water,* and therefore, if water were 

 used, it would be necessary to have a tube of great length, 

 since the column of water in the water-barometer would be 

 about 33| feet. 



In order to measure easily and correctly the barometric 

 height, an accurately graduated scale is added, which can 

 be moved along the tube. 



REM. The instrument above described involves the essen- 

 tial parts of a barometer ; it is the province of Physics to 

 give a full description of different kinds of barometers, 

 to explain their use, etc. 



46. Manometers. Barometers are used not only to 

 measure the pressure of the external air, but also to deter- 

 mine the elastic force of gases or vapors which are enclosed 

 in vessels. When thus used, they are called manometers. 

 These instruments are filled with mercury, and are either 

 open or closed ; in the latter case, there may be air above the 

 column of mercury 6r there may be a vacuum. The manom- 

 eter with a vacuum above the column of mercury is like the 

 common barometer. In order to measure with it the elastic 

 force of the gas or vapor, it will be necessary to establish a 

 free connection between the cistern of the barometer and 

 the vessel containing the fluid. This is done by means of a 



* Enc. Brit., Vol. XVI., p. 38. 



