INDICATOR. 



will show the pressure of the uncondensed steam resisting the 

 piston in its ascent, and a comparison of the two will exhibit the 

 effective force on the piston. Fig 30 represents such a diagram 

 as would be produced by this instrument. A B c is the curve 

 traced by the pencil during the descent of the piston, and c i> E 



Fig. 30. 



that during its ascent. A is the position of the pencil at the 

 moment the piston commences its descent, B is its position at the 

 middle of the stroke, and c at the termination of the stroke. 

 On closing the upper steam valve and closing the exhausting 

 valve, the indicator piston being gradually relieved from the 

 pressure of the steam, the pencil descends, and at the same time 

 the paper moving from left to right, the pencil traces the curve 

 c D E, the gradual descent of this curve showing the progressive 

 increase of the vacuum. As the atmospheric pressure constantly 

 acts above the piston of the indicator, its position will be deter- 

 mined by the difference between the atmospheric pressure and the 

 pressure of the steam below it ; and therefore the difference be- 

 tween the heights of the pencil at corresponding points in the 

 ascending and descending stroke will express the difference 

 between the pressure of the steam impelling the piston in the 

 ascent and resisting it in the descent at these points. Thus, at 

 the middle of the stroke, the line B D will express the extent to 

 which the spring governing the indicator piston would be stretched 

 by the difference between the force of steam impelling the piston 

 at the middle of the descending stroke, and the force of steam 

 resisting it at the middle of the ascending stroke. The force, 

 therefore, measured by the line B D will be the effective force on 

 the piston at that point, and the same may be said of every part of 

 the diagram produced by the indicator. 



The whole mechanical effect produced by the stroke of the 

 piston being composed of the aggregate of all its varying effects 

 throughout the stroke, the determination of its amount is a matter 

 of easy calculation by the measurement of the diagram supplied 



47 



