506 THE STEAM-ENGINE. 



the steam-pressure on the mercury in A B balances the atmospheric pressure 

 on the mercury in B C. If, however, the level of the mercury in B C be above 

 the level of the mercury in B A, the pressure of the steam will exceed that of 

 the atmosphere. The excess of its pressure above that of the atmosphere may 

 be found by observing the difference of the level of the mercury in the tubes 

 B C and B A, allowing a pressure of one pound on each square inch for every 

 two inches in the difference of the levels. 



If, on the contrary, the level of the mercury in B C should fall below its 

 level in A B, the atmospheric pressure will exceed that of the steam, and the 

 quantity of the excess may be ascertained exactly in the same way. 



If the tube be glass, the difference of levels of the mercury would be visible ; 

 but it is most commonly made of iron ; and in order to ascertain the level, a 

 thin wooden rod with a float is inserted in the open end of B C, so that the 

 portion of the stick within the tube indicates the distance of the level of the 

 mercury from its month. A bulb or cistern of mercury might be substituted 

 for the leg A B, as in the common barometer. This instrument is called the 

 steam-gauge. 



If the steam-gauge be used as a measure of the strength of the steam which 

 presses oil the piston, it ought to be on the same side of the throttle-valve 

 (which is regulated by the governor) as the cylinder ; for if it were on the 

 same side of the throttle-valve with the boiler, it would not be affected by the 

 changes which the steam may undergo in passing through the throttle-valve, 

 when partially closed by the agency of the governor. 



For boilers in which steam of very high pressure is used, as in those of 

 locomotive engines, a steam-gauge, constructed on the above principle, would 

 have inconvenient or impracticable length. In such boilers the pressure of the 

 steam is equal to four or five times that of the atmosphere, to indicate which 

 the column of mercury in the steam-gauge would be four or five feet in height. 

 In such cases a thermometer-gauge may be used with advantage. The prin- 

 ciple of this gauge is founded on the fact, that between the pressure and tem- 

 perature of steam produced in contact with water there is a fixed relation, the I 

 same temperature always corresponding to the same pressure. If, therefore, j 

 a thermometer be immersed in the boiler which shall show the temperature of i 

 the steam, a scale may be attached to it, on which shall be engraved the I 

 corresponding pressures. Such gauges are now very generally used on locomo- 

 tive engines. 



The force with which the piston is pressed depends on two things : 1st, the 

 actual strength of the steam which presses on it ; and, 2dly, on the actual 

 strength of the vapor which resists it. For although the vacuum produced by 

 the method of separate condensation be much more perfect than what had been 

 produced in the atmospheric engines, yet still some vapor of a small degree 

 of elasticity is found to be raised from the hot water in the bottom of the con- 

 denser before it can be extracted by the air-pump. One of these pressures is 

 indicated by the steam-gauge already described ; but still, before we can es- 

 timate the force with which the piston descends, it is necessary to ascertain 

 the force of the vapor which remains uncondensed, and resists the motion of 

 the piston. Another gauge, called the barometer-gauge, is provided for this 

 purpose. A glass tube A B, fig. 61, more than thirty inches long and open at 

 both ends, is placed in an upright or vertical position, having the lower end B 

 immersed in a cistern of mercury C. To the upper end is attached a metal 

 tube, which communicates with the condenser, in which a constant vacuum, 

 or rather high degree of rarefaction, is sustained. The same vacuum must 

 therefore exist in the tube A B, above the level of the mercury, and the at- 

 mospheric pressure on the surface of the mercury in the cistern C will force 



