is the position of the pencil at the moment the piston commences its descent, 

 B is its position at the middle of the stroke, and C at the termination of the 

 stroke. On closing the upper steam-valve and closing the exhausting-valve, 

 the indicator-piston being gradually relieved from the pressure of the steam 

 the pencil descends, and at the same time the paper moving from left to right, 

 the pencil traces the curve C D E, the gradual descent of this curve showing 

 the progressive increase of the vacuum. As the atmospheric pressure con- 

 stantly acts above the piston of the indicator, its position will be determined 

 by the difference between the atmospheric pressure and the pressure of the 

 steam below it ; and therefore the difference between the heights of the pencil 

 at corresponding points in the ascending and descending stroke, will express 

 the difference between the pressure of the steam impelling the piston in the 

 ascent and resisting it in the descent at these points. Thus at the middle of 

 the stroke, the line B D will express the extent to which the spring governing 

 the indicator-piston would be stretched by the difference between the force of 

 steam impelling the piston at the middle of the descending stroke, and the 

 force of steam resisting it at the middle of the ascending stroke. The force 

 therefore measured by the line B D will be the effective force on the piston 

 at that point ; and the same may be said of every part of the diagram produced 

 by the indicator. 



The whole mechanical effect produced by the stroke of the piston being 

 composed of the aggregate of all its varying effects throughout the stroke, 

 the determination of its amount is a matter of easy calculation by the measure- 

 ment of the diagram supplied by the indicator. Let the horizontal play of the 

 pencil from A to C be divided into any proposed number of equal parts, say 

 ten : at the middle of the stroke, B D expresses the effective force on the 

 piston, and if this be considered to be uniform through the tenth part of the 

 stroke, as from /"to g, then the number of pounds expressed by B D multiplied 

 by the tenth part of the stroke expressed in parts of a loot, will be the mechani- 

 cal effect through that part of the stroke expressed in pounds' weight raised 

 one foot. In like manner m n will express the effective force on the piston 

 after three fourths of the stroke have been performed, and if this be multiplied 

 by a tenth part of the stroke as before, the mechanical effect similarly express- 

 ed will be obtained ; and the same process being applied to any successive 

 tenth part of the stroke, and the numerical results thus obtained being added 

 together, the whole effect of the stroke will be obtained, expressed in pounds' 

 weight raised one foot. 



By means of the indicator, the actual mechanical effect produced by each 

 stroke of the engine can be obtained, arid if the actual number of strokes made 

 in any given time be known, the whole effect of the moving power would be 

 determined. An instrument called a counter was also contrived by Watt, to be 

 attached either to the working beam or to any other reciprocating part of ihe 

 engine. This instrument consisted of a train of wheel-work with governing 

 hands or indices moved upon divided dials, like the hands of a clock. A record 

 of the strokes was preserved by means precisely similar to those by which 

 the hands of a clock or timepiece indicated and recorded the number of vibra- 

 tions of the pendulum or balance-wheel. 



To secure the boiler from accidents arising from the steam contained in it 

 acquiring an undue pressure, a safety-valve is used, similar in principle to 

 those adopted in the early engines. This valve is represented in fig. 52, at 

 N. It is a conical valve, kept down by a weight sliding on a roJ upon it. 

 When the pressure of the steam overcomes the force of this weight, it raises 

 the valve and escapes, being carried off through the tube. 



With a view to the economy of heat, this waste-steam tube is sometimes 



