94 Steam-Engitie Indicator. 



D A flat pillar — screwed to the side of the cylinder C, and 

 supporting the frame EE. 



F The piston — fitted so as to work easily up and down, and 

 to be, at the same time, air-tight. 



EE A frame — 12 inches, by 7 inside, the under and upper 

 rail grooved to retain the sliding-board K. 



G The piston rod — about 5-8ths diameter, and 16 inches long. 



H A guide — screwed to the pillar D, at 6 inches above the 

 top of the small cylinder, and through which the piston-rod 

 passes. 



I A spiral spring — attached to the piston at F, and the guide 

 at H. It should be about 7 inches long when at rest, and of 

 such a strength as to allow the piston to descend nearly to the 

 bottom of the cylinder, when it is loaded with a weight equal to 

 14 lbs. upon every square inch of its area. It should also admit 

 of being compressed about la inches. 



K A small board — about 7 inches square, sliding in grooves 

 in the upper and under rails of the frame EE. 



L A small brass socket — which may be fixed at any height 

 upon the piston-rod, by the tightening screw M. It carries in 

 the other end a short pencil, with a weak spring to push it 

 forward against the surface of the sliding-board. 



N A weight— attsiched by a cord to the sliding-board K. 



O — Any convenient part of the parallel motion, traversing 

 a space of about 4| inches during each half stroke of the 



From this description the principle on which the instrument 

 acts will be evident. 



By opening the stop-cock B, a direct communication is made 

 between the interior of the large and small cylinders, and the 

 density of the steam in the indicator becomes the same as in 

 the steam-engine cylinder above the piston. When this den- 

 sity is less than that under the atmospheric pressure, the indi- 

 cator piston will sink, when it is greater the piston will rise; but 

 the spiral spring, which, if carefully made, stretches through 

 equal distances with equal weights, restrains the motion of the 



