PARALLEL MOTION BAROMETER GAUGE. 



The force of steam which moves the piston is indicated by the 

 steam gauge already described, and which is shown attached to 

 the exposed end, K, of the boiler in fig. 7. The reaction of the 

 uncondensed steam and gases is indicated by a gauge called the 

 barometer gauge, inasmuch as it would be in fact a barometer if 

 an absolute vacuum were produced before the piston. This gauge 

 consists of a glass tube, A B (fig. 29), more than thirty inches long, 

 and open at both ends, 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 atmospheric pressure on the surface 

 of the mercury in the cistern c will force the mercury 

 up in the tube A B, until the column which is sus- 

 pended in it is equal to the difference between the 

 atmospheric pressure and the pressure of the uncon- 

 densed/ steam. The difference between the column of 

 mercury sustained 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. 



49. 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 barometer 

 gauge ; this gives the excess of the atmospheric pressure above 

 the uncondensed 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 



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