STEAM ENGINES 181 



bears a constant ratio to the distance from the pivot to the 

 point where the lever is connected to the crosshead. This 

 ratio must be the same at all points in the stroke, or at every 

 position of the crosshead; if it is not, the reducing motion is 

 not exact. In the reducing motion shown in Fig. 4, for instance, 

 the distance from the pivot b to the center of the pin d is 

 variable, depending on the position of the crosshead, whereas 

 the distance from b to g, where the cord is attached, is always 

 the same; in other words, the length of the long arm of the 

 lever changes while the length of the short arm remains con- 

 stant. As a result, the motions of the crosshead and the 

 cord differ at different parts of the stroke, and the indicator 

 diagram is correspondingly distorted. The reducing motions 

 shown in Figs. 5 and 6, and the reducing wheels shown in Figs. 

 8 and 9, are accurate, inasmuch as the motion of the cord is at 

 all times proportional to the motion of the crosshead. 



Reduction of Errors. For ordinary work with the indicator, 

 the amount of error caused by the - inexactness of reducing: 

 motions like those in Figs. 3, 4, and 7 is not serious and may 

 be ignored. To secure a minimum of distortion of the diagram, 

 the long lever should always be pivoted in such a position that 

 it will be perpendicular to the line of movement of the cross- 

 head when the latter is at the middle of its stroke. The 

 accuracy of the motion will, in general, be increased by increas- 

 ing the lengths of the long lever. For most purposes it is 

 sufficient to use a lever whose length is twice the stroke of the 

 engine. 



Taking the Diagram. After the instrument is properly 

 attached, a blank card is slipped over the drum so as to fit 

 smoothly, as in Fig. 1. The hook on the indicator cord is 

 then engaged with the loop on the cord from the reducing 

 motion, and the drum is allowed to rotate back and forth several 

 times, to see that it works properly and that the cord is adjusted 

 correctly. The cock is then opened and the indicator is allowed 

 to work freely while the engine makes several revolutions. 

 This warms up the parts to the working temperature. The 

 pencil is then pressed lightly against the card during a single 

 revolution. Next, the cock is closed and the pencil is again 

 pressed against the card, recording the atmospheric line. 



