168 APPENDIX. 



may seem minute and trifling, but serious errors may occur from the neglect of them, when 

 from a piston of 1^ inches area and 3 inches stroke, we judge of another of 3 feet in diameter 

 and 8 feet stroke. 



Those manufacturers who use the indicator have the lower part of the stop-cock formed into 

 a plug, which can be unscrewed at pleasure, and made to fit in the place of the plug or barrel 

 of the grease cock on the cylinder cover. It is to be presumed that in most engines of the 

 best makers, the action on both sides of the piston is the same ; but if this be matter of doubt, 

 it is easy by means of a pipe bent at right angles, and screwed into the cylinder at the bottom, 

 to apply the indicator there also. The apparatus being fixed, and the stop-cock closed, the 

 point of the tracing pencil should be carefully adjusted on the line of rest. The sliding table 

 carrying the ruled paper may then be put in motion by attaching the cord to it, which has 

 already been fastened to some convenient moving point of the engine ; then, observing that 

 the pencil makes its mark on the line of rest, open the stop-cock, and so set the piston of the 

 indicator in motion. The pistons of the instrument and the steam engine now move simul- 

 taneously, but in opposite directions, the force of the steam driving them asunder, and the 

 vacuum bringing them, as it were, together, at the same time that the horizontal motion of 

 the sliding table represents the vertical motion of the larger piston ; consequently, on the 

 indicator, the horizontal motion shows the stroke of the steam engine piston, and the vertical 

 motion shows the pressure upon it. The preceding explanation is requisite to enable the 

 general reader to understand the annexed diagram of a trial which we made on a steam 

 engine applied to drive a rolling mill for the manufacture of iron. This engine had a cylinder 

 of 36 inches in diameter, and a stroke of 7 feet, making 16 strokes per minute (that is to say, 

 16 revolutions of the crank shaft) ; the fly wheel, as in most rolling mill engines, was on the 

 second motion making 64'6 revolutions per minute ; it was 20 feet in diameter, and weighed 

 about 25 tons, serving to accumulate the engine power, as well as to regulate its motion, and 

 thereby overcome any sudden resistance, as the passing a mass of iron between the rolls of 

 the mill. Such irregular application of power made this engine peculiarly eligible for exhibiting 

 the use of the indicator in delineating its varying action. The instrument being fixed as before 

 stated, and the steam engine in its usual order having been at work for some hours before 

 the experiment was made, it was stopped, and all the machinery detached; the condenser 

 was cleared of air and water by blowing the steam through it, and the engine started in the 

 usual manner, when the pencil of the indicator described the curvilinear figure marked A 

 (see the annexed figure), showing the force required to put the engine into motion from a state 

 of rest. This figure gradually diminished until the engine acquired a uniform speed, which 

 was maintained and regulated by the governor at the proper rate of going, namely, 16 revolu- 

 tions per minute, for a considerable time, and the pencil of the indicator continued to trace 

 the curve marked B, showing the force expended in overcoming the constant friction of the 

 engine, in changing the motion of its parts, and in working the air-pump, &c. The principal 

 machinery of the rolling mill was now attached to the engine, and set in motion ; it consisted 

 of a pair of rollers for the puddled iron, a pair of boiler plate rollers, a pair of rolls for making 

 small bars, a pair of shears for cutting them to the proper length, and a large lathe used for 

 turning the rollers. The above machinery in motion, but without performing any work, pro- 

 duced the figure C on the tablet of the indicator. When this had been continued for some 

 time, the small rolls were kept at work making bars (small squares), and the figure D was 



