STEAM ENGINES 



383 



There are mechanical difficulties which must be taken into con- 

 sideration; hence the diagram as usually obtained from a steam 

 engine cylinder is not like Fig. 264, but is like Fig. 265. Here the 

 corners are rounded off, due to wire drawing and slow-acting valves. 

 The line BC drops, due to the resistance of steam moving through 

 the boilers. The point C is not a sharp one, since the valve cannot 

 move quickly enough to cut off steam instantaneously, but commences 

 to cut off at C', and complete cut-off takes place at C. This fall in 

 pressure after the valve commences to cut off and before it com- 

 pletely tuts off is known as wire drawing. Often the exhaust valve 

 does not open soon enough for the pressure to fall to the back 

 pressure line before the piston starts in the return stroke ; hence the 

 line DE of Fig. 264 is more like the line DE of Fig. 265. 



530. Attaching indicator to engine. Where indicator diagrams 

 are to be taken from engines of 100 H.P. or more it is better 

 to have two indicators, one for each end of the cylinder; but for 

 engines of a capacity such as are used on the farm or in cream- 

 eries one indicator connected to both ends of the cylinder by means 

 of a three-way cock is fully as accurate as two. If there are no 

 holes for attaching the indicator when the engine comes from the 

 factory, drill into each clearance space A A (Fig. 266) of the cylin- 



FIG. 266 ATTACHING AN INDICATOR TO AN ENGINE 



der a hole of sufficient size to thread for 2^-inch or ^-inch pipe, and 

 by means of pipe fittings connect up to the three-way cock B. The 

 connection on the indicator will screw into the cock at (7. Since 



