Proceedings of the Polytechnic Association. 833 



tloiible the stroke of the engine (when the latter is double acting). 

 "When the tension of a belt or series of springs is to be used in calcu- 

 lating the power, the movement of each must also be found, and 

 must be calculated from the sj)eed of the engine. It will thus be 

 seen that two elements of the 'power are easily ascertained, viz. : The 

 time and the distance through which the force is exerted. The mean 

 driving force is more difficult to obtain. There are two instruments- 

 in use for measuring this, viz. : The indicator and dynamometer.. 

 These two names are used in this paper in a restricted sense. The- 

 lirst is applied only to the well known steam engine indicator, and 

 the latter to that form of dynamometer which is used to measure the 

 force transmitted by revolving wheels or shafts. 



It would be impossible, in the limits of this paper, X(y give a 

 detailed description of the indicator. We therefore wnll mention 

 only such features as are necessary to explain its mode of operation.. 

 The indicator is so constructed and attached that steam fi'om the- 

 main cylinder presses upon one side of a small piston in the instru- 

 ment, the atmosphere pressure being upon the other side. To the 

 indicator piston is attached a spring and a pencil, the latter arranged 

 to mark on paper. The predominating pressure on the indicator 

 piston, whether of the steam or the atmosphere, extends or com- 

 presses the spring in proportion to the intensity of the pressure, and! 

 moves the pencil up and dowm on the paper. The paper is an-anged 

 on a drum, which is so connected that it has a side motion corres- 

 ponding to that of the engine piston. Consequently, as the engine 

 piston mo\'es, the faper is moved sideways., aiiid as the pressure 

 changes, th-e jK-iicil is correspondingly moved tip dnd down,, sO' 

 that the figure or diagram, traced on the paper, is a combination of 

 the two movements, and should show the pressure at each and all 

 points of the stroke. The mean of a number of ordinates on the' 

 diagram, represents the mean pressure per square inch of piston,, 

 which, multiplied by the area of the piston, gives the total force whicli 

 produces the piston movement, from which the power may be calcu- 

 lated, as has been bef.)re explained. The indicator is a beautiful 

 instrnment, of such great value to the steam engine, that it may be 

 said to deserve the numerous words that have been spoken in its 

 praise. Still in many cases, where it has hitherto been considered 

 practically perfect, its indications are of the most deceitful and unre- 

 liable character. It shows very perfectly whether the valves are 

 adjusted properly; and often, when applied to an engine which is 

 [Inst.] 53 



