SECT, viii.] MANAGEMENT OF THE STEAM ENGINE. 2G7 



the condenser, the mercury rises as much in the long leg as it falls in the short 

 one, these divisions will be equivalent to inches on the common barometer. 



The condenser gauge should indicate the state of the vapour in the condenser, 

 to be capable of sustaining from two to three inches of mercury. While it does 

 not exceed three inches, the condensation may be esteemed very good ; and about 

 two inches is the best I have seen obtained in practice. 



The difference between the elastic force of the vapour in the condenser, and the 

 elastic force of the steam in the boiler, as shown by the gauge, added to the height 

 of the barometer at the time, gives the relative force of the steam to move the 

 engine, but many deductions have to take place before we have the real moving- 

 force ; nevertheless they show the state of two very important parts of the engine. 

 (See Sect. v. and vi.) 



560. THE INDICATOR. The force of the steam and the state of exhaustion 

 in the cylinder, at the different periods of the stroke of the engine, cannot be 

 ascertained by the condenser gauge, and for that purpose it was necessary to form 

 an instrument less subject to vibration ; the instrument in use is called the IN- 

 DICATOR, and is found to answer the end tolerably well. It consists of a cylinder 

 about one inch and three quarters in diameter, and eight inches long, exceedingly 

 truly bored, with a solid piston accurately fitted to it, so as to slide easy by the 

 help of some oil ; the stem of the piston is guided in the direction of the axis of 

 the cylinder, so that it may not be subject to jam or cause friction in any part of 

 its motion. The bottom of this cylinder has a cock and small pipe joined to it ; a 

 flat pillar D, Plate xvi. Figs. 1 and 2. is screwed to the cylinder of the indicator 

 C, and supporting the frame E E, which is twelve inches by seven inches, with 

 the upper and under rail grooved to retain the sliding-board K. 



The piston rod G is about five-eighths of an inch in diameter, and sixteen inches 

 long ; and H is the guide for it screwed to the pillar D, at about six inches above 

 the top of the cylinder C. 



A spiral spring I is attached to the piston at F, and to the guide at H. It 

 should be about seven inches long when at rest, and of such a strength as to allow 

 the piston to descend to within about an inch of the bottom of the cylinder C, 

 when it is loaded with 15 Ibs. upon every square inch of its area ; and the 

 spring should admit of being compressed one inch and a half. 



The board or panel K slides in the grooves of the frame E E, and should be 

 seven inches square ; and a small brass slider L should be set at any height on the 

 piston rod G, by means of a screw. A short pencil is inserted in the other end, 

 with a weak spring to push it against the surface of the board K, which is caused 

 to slide by a weight N, attached to a line passing over a pulley ; the opposite line 



