INDICATOR. 



before this ; and produced an instrument of most felicitous per- 

 fection, called an Indicator, by which the object was perfectly and 

 simply attained. 



This contrivance consists of a cylinder of about 1| inch in diameter, 

 and 8 inches in length. It is bored with great accuracy, and fitted 

 with a solid piston moving steam-tight in it with very little friction. 

 The rod of this piston is guided in the direction of the axis of the 

 cylinder through a collar in the top, so as not to be subject to fric- 

 tion in any part of its play. At the bottom of the cylinder is a pipe 

 governed by a stop-cock and terminated in a screw, by which the 

 instrument may be screwed on the top of the steam cylinder of the% 

 engine. In this position, if the stop-cock of the indicator be 

 opened, a free communication will be made between the cylinder 

 of the indicator and that of the engine. The piston-rod of the 

 indicator is attached to a spiral spring, which is capable of exten- 

 sion and compression, and which by its elasticity is capable of 

 measuring the force which extends or compresses it in the same 

 manner as a spring steel-yard or balance. If a scale be attached 

 to the instrument at any point on the piston-rod to which an index 

 might be attached, then the position of that index upon the 

 scale would be governed by the position of the indicator piston in 

 its cylinder. If any force pressed the indicator piston upwards, 

 so as to compress the spring, the index would rise upon the scale ; 

 and if, on the other hand, a force pressed the indicator piston 

 downwards, then the spiral spring would be extended, and the 

 index on the piston-rod descend upon the scale. In each case 

 the force of the spring, whether compressed or extended, would 

 be equal to the force urging the indicator piston, and the scale 

 might be so divided as to show the amount of this force. 



Now let the instrument be supposed to be screwed upon the top 

 of the cylinder of a steam engine, and the stop-cock opened so as to 

 leave a free communication between the cylinder of the indicator 

 below its piston and the cylinder of the steam engine above the 

 steam piston. At the moment the upper steam valve is opened, 

 the steam rushing in upon the steam piston will also pass into the 

 indicator, and press the indicator piston upwards : the index upon 

 its piston-rod will point upon the scale to the amount of pressure 

 thus exerted. As the steam piston descends, the indicator piston 

 will vary its position with the varying pressure of the steam in the 

 cylinder, and the index on the piston-rod will play upon the 

 scale, so as to show the pressure of the steam at each point during 

 the descent of the piston. 



52. If it were possible to observe and record the varying positions 

 of the index on the piston-rod of the indicator, and to refer each 

 of these varying positions to the corresponding point of the 



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