404 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK iv. 



leg b is open ; only it is not the pressure of the atmosphere that raises 

 the column of mercury, but that of the steam ; the short leg has 

 direct communication at a, Fig. 278, with the steam in the boiler. The 

 difference of the heights of the mercury in the two legs increased by 

 the atmospheric pressure expresses the pressure of the steam. 



The compressed air yauges (Figs. 279, 280) are nothing else 

 than Mariotte's tubes. In one of the branches the steam freely 

 exerts its pressure, which in the other branch is kept in equilibrium 

 by the compressed air and the difference of level 'of the mercury. 

 The instrument is regulated in such a way that the two columns of 

 mercury are at the same height, mm, when the pressure of the steam 

 is equal to one atmosphere. When the pressure gradually becomes 



FIG. 278. An open 

 pressure gauge, 



FIG. 279. A compressed air 

 pressure gauge. 



FIG. 280. Pressure gauge with 

 conical tube. 



greater, the level rises in A, but with lessening increments for equal 

 additions of pressure, according to Mariotte's law. The instrument 

 is therefore less and less sensible for the greatest pressures. This 

 disadvantage is overcome by giving the gauge the form shown in 

 Fig. 280. The conical form of the branch which contains the air 

 gives to the divisions corresponding to successive atmospheres lengths 

 which are nearly equal, so that it is easier to read off high pressures 

 than in the first system. 



The handiness and cheapness of metallic pressure-gauges (Fig. 281) 

 have caused them to be adopted for a great number of boilers. But 



