THE STEAM ENGINE. 



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 n 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 ex- 

 pressed 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 every 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. 



54. By means of the indicator, the actual mechanical effect pro- 

 duced 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 

 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 time- 

 piece indicate and record the number of vibrations of the pendulum 

 or balance-wheel. 



55. Such, then, is the machine, and such the principal expedients 

 by which it has been adapted as a moving power of unparalleled 

 importance and efficiency in all the industrial arts. In certain 

 applications of the engine some of these provisions are unnecessary 

 or inapplicable. In others supplementary expedients are required 

 and supplied. Our present purpose, however, will be attained, if 

 we have succeeded in rendering clearly intelligible the general 

 principle upon which the machine as described above acts, 

 and the special uses of the accessories that have been described. 

 These being well understood, no great difficulty will be encountered 

 in comprehending the mechanism and the action of any special 

 form of engine. 



