1080 Transactions or the American Institute. 



is, in most cases, also very difficult of determination by means of the 

 diagram, while it is not difficult to decide upon the point of exhaust, 

 ■with sufficient accuracy for our present purpose ; consequently a 

 nearer approximation to the true amount of steam used, may be 

 obtained by measuring the steam exhausted than by the induction. 



But it should be remarked here, that the indicator diagram does 

 not show the full quantity of steam used under any circumstances ; 

 as there is more or less water evaporated from the interior surfaces 

 of the cylinder during the return stroke of the piston, while the 

 exhaust valve is open, the steam from which evaporation does not 

 show on the diagram, but nevertheless carries off a quantity of heat 

 that has to be replaced by the condensation of more live steam, while 

 the induction valve is open, on the return stroke, and this steam also 

 is unrepresented upon the diagram. Again, there may be, and often 

 is a leak from the cylinder either past the piston or exhaust valve, 

 during the forward stroke, which may be detected by, but is not 

 measurable upon, the diagram, although a leak into the cylinder is 

 shown by a higher terminal pressure. There is, consequently, a 

 quantity of steam used which it is impossible to estimate by means of 

 the indicator alone, the effect of which upon this investigation will 

 be considered hereafter. 



Having assumed, therefore, that the terminal pressure represents 

 a sufficiently near approximation to the quantity of steam used in 

 developing the power represented by the diagram, we will proceed to 

 construct a diagram of a theoretical engine of the same capacity, 

 including clearance, as the engine under consideration, and having 

 the same terminal pressure. But in order to do this we must first 

 know the pressure of steam in the boiler^ and the clearance in the 

 cylinder and passages. The former is generally given on the 

 diagram, and, where practicable, should be corrected by testing the 

 gauge with the indicator spring. The clearance is, however, rarely 

 given, and varies in different engines from one and a half to ten per 

 cent per of the space swept through by the piston in one stroke. If 

 we have the drawings of the engine we can calculate it ; if we know 

 the style of engine we can approximate it ; and if there is any com- 

 pression, we can estimate it from the diagram, by assuming any two 

 convenient points in the curve, measuring the pressure from absolute 

 vacuum at those points, and their distance from a line vertical to 

 the vacuum line, and touching the end of the card ; thus in Fig. 1, 

 let p == pressure and d = distance of the first point, or the one far- 



