Sleam-Eiigiiie Indicator. 95 



piston; and, by the distance to which it allows it to move from 

 its state of rest, indicates the pressure it is undergoing. 

 During each stroke of the engine, therefore, the indicator 

 piston will rise at the instant the upper steam valve opens, and 

 during the descent of the large piston will maintain a height 

 proportioned to the density of the steam in the cylinder. 

 When the eduction valve opens it will sink, and by the rapidity 

 of its descent, and the distance to which it falls, denote the 

 quality of the vacuum. If, during this perpendicularly alter- 

 nating motion of the small piston, the sliding-board be made 

 to perform its reciprocating and horizontal course, the pencil, 

 in the socket L, will trace upon the board, or upon a piece of 

 paper applied to its surface, a figure, something like P Q R S ; 

 of which figure, the part P Q is drawn during the descent of 

 the large piston. At Q, the condensation taking place, the 

 atmospheric pressure acts upon the piston indicator, and makes 

 it descend until the tension of the spring counteracts the force 

 of the pressure. Meanwhile the engine begins to perform 

 the up-stroke, and, as the board traverses, produces the line 

 R S. When the engine piston arrives at the top, the admis- 

 sion of the steam destroys the vacuum that existed below the 

 indicator piston, and allows the spring to raise the latter until 

 the equilibrium is restored. It consequently follows that the dis- 

 tance between the line P Q, and the line R S, will be greater in 

 proportion to the difference between the pressure in the cylinder 

 during the existence of the vacuum and the pressure of the steam, 

 and the curve Q R be more acute in proportion to the rapidity 

 with which the vacuum is formed. If this distance be mea- 

 sured in eight or ten places, and an average taken, then a 

 simple proportion gives the pressure upon each square inch of 

 the piston. Let a = area of indicator piston, — b, any weight 

 applied experimentally to that piston, — d, the distance to 

 which it falls with that weight, — and let e be the average dis- 

 tance taken from a diagram, and / the average pressure in 

 pounds upon the steam-engine piston during the formation of 



that diagram. Then, as d to — so is e to/, or — - =: / And 

 "a da 



