live force of steam on the piston : but such is not the case. When the steam- 

 is first admitted through the steam-valve it acts on the piston with a pressure 1 

 which is first slightly diminished, and afterward a little increased, until it 

 arrives at that part of the stroke at which the stearn-valve is closed, after which 

 the pressure is diminished. The pressure, therefore, urging the piston is- 

 subject to variation ; but the pressure of the un condensed vapor on the other 

 side of the piston is subject to still greater change. At the moment the ex- 

 hausting-valve is opened, the piston is relieved from the pressure upon it by 

 the commencement of the condensation ; but this process during the descent 

 of the piston is gradual, and the vacuum is rendered more and more perfect, 

 until the piston has nearly attained the limit of its play. These variations, 

 both as well of the force urging the piston as of the force resisting it, are such 

 as not to be capable of being accurately measured by a mercurial column, since 

 they would produce oscillations in such a column, which would render any 

 observations of its mean height impracticable. 



To measore the mean efficient force of the piston, taking into account these 

 circumstances, Mr. Watt invented an instrument, which, like all his mechanical 

 inventions, has answered its purpose perfectly, and is still in general use. This 

 instrument, called an indicator^ consists of a cylinder of about 1 J inch in diame- 

 ter, 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 friction in any part of its play. At the bot- 

 tom of the cylinder is a pipe governed by a stop-cock and turned 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 extension and compression, and which by its 

 elasticity is capable of measuring the force which extends or compresses it in 

 the same manner as a soring 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 upward, 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 

 downward, 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 indi- 

 cator-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 cyl- 

 inder of a steam-engine, and the stop-cock opened so as to leave a free com- 

 munication between the cylinder of the indicator below its piston and the cyl- 

 inder 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 upward : 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. 



If it were possible to observe and record the varying position of the index 

 on the piston-rod of the indicator, and to refer each of these varying positions 



