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 [Pg278] is a matter of easy calculation by the measurement 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 _f_ to _g_, then the number of pounds expressed by B D multiplied by the tenth part of the stroke expressed in parts of a foot, will be the mechanical 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 expressed 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. (162.) By means of the indicator, the actual mechanical effect produced by each stroke of the engine can be obtained, and 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 the engine. This instrument consisted of a train of wheel-work with governing hands or indices moved upon divided dials, like the hand of a clock. A record of the strokes was preserved by means precisely similar to those by which the hands of a clock or time-piece indicated and recorded the number of vibrations of the pendulum or balance-wheel. (163.) 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._ 71. at N. It is a conical valve, kept down by a weight sliding on a rod 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. [Pg279] With a view to the economy of heat, this waste steam tube is sometimes conducted into the feeding cistern, where the steam carried off by it is condensed, and heats the feeding water.