STEAM EXGIXES 



175 



springs of varying degrees of stiffness, which are distinguished 

 by the numbers 20, 30, 40, etc. These numbers indicate the 

 pressure, in pounds per square inch, required to raise the 

 pencil 1 in. Thus, if a 40 spring is used, a pressure of 40 Ib. per 

 square inch raises the pencil 1 in., and therefore, the vertical 

 scale of the diagram is 40 Ib. per in. That is, the vertical 

 distance, in inches, of any point on the diagram from the 

 atmospheric line, multiplied by 40, gives the gauge pressure 

 per square inch at that point. The scale of the spring chosen 

 should not be less than half the boiler pressure, because it 

 is not desirable to have the indicator card more than 2 in. in 

 height. 



Attachment of Indicator. To attach the indicator to the 

 engine, a hole is drilled in the clearance space of the cylinder 



FIG. 3 



and tapped for a $-in. nipple, which should be as short as 

 possible. The nipple has an elbow, into which is screwed a 

 cock. The indicator may then be attached directly to the 

 cock by the nut n. Fig. 1, the conical projection i of the indi- 

 cator wedging tightly into the cock to prevent the leakage 

 of steam. It is preferable to have an indicator at each end 

 of the cylinder, but if that is not convenient, one indicator 

 may be connected with both ends of the cylinder by means 

 of a three-way cock, as shown at k, Fig. 3. This construction 

 is undesirable, however, if the engine has a long stroke, as the 



