THE DYNAMOMETER-CHRONOGRAI'H. 77 



the pulley. This departure, (hen, corresponds to the effect of a definite and 

 constant weight so applied, so long as we use the same spring under the same 

 adjustment. When widely different ranges of power are to be measured, the 

 additional range of tension required is obtained with the same spring by insert- 

 ing a set-screw in successive holes, numbered to 15, around the end of the 

 cylinder D, so as virtually to shorten or lengthen the clock-spring. A separate 

 calibration is, of course, required for each setting. 



(2) MEASUREMENT OF THE END-THRUST. 



I have thus far spoken of the shaft or axis as if it were in one piece between 

 the clock-spring and the pulley, but for the purpose of measuring the end-thrust 

 the shaft is also cut in two within the cylinder F. The two pieces are maintained 

 in line by suitable guides, and forced to rotate together by a fork within F, but 

 the propeller end of the shaft is given freedom of longitudinal motion. Any end- 

 thrust on the axis, whether received from the propeller or otherwise, causes, then, 

 this portion carrying the pencil P to slide up within the other toward the pulley, 

 telescoping the part of the shaft next the propeller within that next the clock- 

 spring, and causing the longitudinal compression of the spiral spring in cylinder 

 F, as shown in the drawing. All the parts of the axis, then, between the 

 clock-spring and the propeller must rotate together when the latter is revolved, 

 but the end of the axis nearest the propeller, and this end only, has the 

 capacity not only of rotatory but of a longitudinal motion, which latter is per- 

 mitted by this portion of the axis telescoping into the other, as above described. 

 The force of the end-thrust is recorded by the "departure" of the pencil P, which 

 bears a definite relation to its own spring, determined by independent calibration. 

 The record made by P on the recording cylinder is a curve Avhose abscissae are 

 proportional to time and whose ordinates are proportional to end-thrust. This 

 curve cannot by itself properly be called an indicator-diagram, since, taken 

 alone, it records a static pressure only, but when the experiments are adjusted 

 in a manner later described in this chapter the record of the speed of the turn- 

 table (on which it will be remembered this apparatus is being carried forward) 

 supplies the requisite additional data that an indicator-diagram demands. Hence, 

 while the pencil P" actually traces an indicator-diagram giving the expenditure 

 of power at every moment, the pencil P traces in part a second indicator-diagram 

 giving synchronously the useful result attained. 



A third pencil, P', records the seconds of a mean time-clock through the 

 action of an electro-magnet, M, and obviously gives the means of determining 

 with all needful precision the time corresponding to each element of angular 

 rotation of the cylinder, even should this vary. This time record, then, serves 

 two purposes: (1) it gives the speed of rotation of the cylinder, and (2) permits 



