834 Transactions of the American Institute, 



working improperly, a mere glance at the diagram will reveal the 

 difficulty, and suggest the remedy. Large leaks in the valves or pis- 

 ton ma}' also be detected in this way. The indicated pressure at the 

 end of the stroke has very often been employed to determine the 

 (quantity of steam used by the engine.. Calculations founded on such 

 a basis, are entirely worthless, as will be explained when treating of 

 the cost of the power. It is customary, also, to calculate the friction 

 from the indicator friction diagrams ; but the system is often practi- 

 cally erroneous, as will be explained hereafter. 



The indicator is chiefly employed, hoM'ever, to determine the power 

 of an engine, it being supposed that the diagram shows correctly the 

 pressure at all parts of the stroke. Even this it fails to do under cer- 

 tain circumstances. The most perfect instrument must have some 

 friction in its moving parts, and the pressure of the pencil on the 

 paper adds considerable resistance. If, therefore, the pressure be 

 ascending, the indicatQi' will show less than it should ; and when the 

 pressure is descending, the instrument will show more than it ouglit. 

 In either case,, then, the length of the ordinates is increased during 

 any change of pressure, whence the mean pressure indicated, is greater 

 than actually exists in the cylinder. When the instrument is in 

 order^ a stiff" indicator sj^ring used, and the pencil adjusted to bear 

 lightly on the paper, their accuracies are too small to require serious 

 attention.. 



When the power is measured in the steam cylinder, by means of 

 .the indieator, it is difficult to ascertain accurately the vseful work 

 the engine is capable of performing. This can be done, approximately, 

 -by means of the indicator, as hereinafter explained, but a more satisfac- 

 tory result is obtained.by the use of the dynamometer. This instrument 

 is made in many different forms. The friction dynamometer consists 

 .substantially, of half, clamps, or boxes fitted to a revolving shaft, and 

 kept from turning therewith, by a lever held in position by weights 

 And a spring balance. When in use, the clamps are tightened until 

 they create sufficient friction to absorb the power ; the weights are 

 then adjusted till they nearly balance. The amount of weight, the 

 tension of the spring, and the speed of the shaft are then noted, when 

 the power transmitted through the shaft may easily be calculated ; for 

 the force of the weight and spring is multiplied by the lever in pro- 

 portion to its length, divided by the radius of the shaft, and this mul- 

 tiplied by tlie velocity of the bearing surface in feet, per minute, gives 

 the footrpoimds. This form of dynamometer is little used, because it 



