STEAM ENGINES 179 



a smaller wheel c, on which is wound the cord leading to the 

 indicator b. Hence, as the wheel a turns, the drum of the 

 indicator is given a rotary motion that is proportional to 

 the motion of the wheel c, and hence proportional to the 

 crosshead movement also. But since the wheel c is so much 

 smaller than wheel a, the movement of a point on the drum 

 surface is much less than the movement of the crosshead. 

 On the forward stroke, the wheel a is rotated against the 

 resistance of a spring at d; but on the return stroke, this 

 spring rotates the wheel in the opposite direction. Both 

 wheels a and c are made as light as possible, in order that their 

 inertia may not affect the accuracy of the reduction. The 



FIG. 9 



cord leading from the wheel a to the upright on the crosshead 

 must be parallel to the axis of the cylinder, but the cord from 

 the wheel c to the indicator may incline upwards or downwards. 

 Reducing wheels, employing gears, are often made of alumi- 

 num for the sake of lightness. Such a wheel is shown in Fig. 9. 

 It really consists of two wheels; on the larger one, shown at a, 

 is wound the string that is attached to the arm on the cross - 

 head, and from the smaller one b runs the cord to the indicator. 

 A spring in the horizontal case c takes up the slack in the string. 

 Frequently, the reducing wheel is attached directly to the 

 body of the indicator, as shown in Fig. 2, thus avoiding the 

 necessity of fastening it to the engine frame, as in Fig. 8. 



