THE STEAM-ENGINB. 



507 



Fig. 61. 



the mercury up in the tube A B, until the column which is suspended in it is 

 equal to the difference between the atmospheric pressure and the pressure of 

 the uncondensed steam. The difference between the column of mercury sus- 

 tained in this instrument and in the common barometer, will determine the strength 

 of the uncondensed steam, allowing a force proportional to one pound per 

 square inch for every two inches of mercury in the difference of the two 

 columns. In a well constructed engine which is in good order, there is very 

 little difference between the altitude in the barometer-gauge and the common 

 barometer. 



To compute the force with which the piston descends, thus becomes a very 

 simple arithmetical process. First, ascertain the difference of the levels of the 

 | mercury in the steam-gauge ; this gives the excess of the steam pressure above 

 the atmospheric pressure. Then find the height of the mercury in the barome- 

 ter-gauge ; this gives the excess of the atmospheric pressure above the uncon- 

 densed steam. Hence, if these two heights be added together, we shall obtain 

 the excess of the impelling force of the steam from the boiler, on the one side 

 of the piston, above the resistance of the uncondensed steam on the other side : 

 this will give the effective impelling force. Now, if one pound be allowed for 

 every two inches of mercury in the two columns just mentioned, we shall have 

 the number of pounds of impelling pressure on every square inch of the piston. 

 Then, if the number of square inches in the section of the piston be found, and 

 multiplied by the number of pounds on each square inch, the force with which 

 it moves will be obtained. 



From what we have stated it appears that, in order to estimate the force 

 with which the piston is urged, it is necessary to refer to both the barometer 

 and the steam-gauge. This double computation may be obviated by making 

 one gauge serve both purposes. If the end C of the steam-gauge, fig. 60, in- 

 stead of communicating with the atmosphere were continued to the condenser, 

 we should have the pressure of the steam acting upon the mercury in the 

 tube B A, and the pressure of the uncondensed vapor which resists the piston 

 acting on the mercury in the tube B C. Hence the difference of the levels 

 of the mercury in the tubes would at once indicate the difference between the 

 force of the steam and that of the uncondensed vapor, which is the effective 

 force with which the piston is urged. 



But these methods of determining the effective force by which the piston is 

 urged, can only be regarded as approximations, and not very perfect ones. If 

 the condensation of steam on one side of the piston were instantaneously 

 effected, or the uncondensed vapor were of the same tension during the whole 

 stroke ; and if, besides this, the pressure of steam on the piston were of uni- \ 

 form intensity from the beginning to the end of ihe stroke, then the steam and ] 

 barometer gauges taken together would become an accurate index of the effec- j 



