536 
Trials of Traction- Engines at Wolverhampton. 
rarely occur in actual practice, and certainly not in the case of 
traction-engines, where the cylinders are fitted with only one 
slide worked by that which is called a " link motion." 
Fig. 5 is a real diagram taken from the 10-horse engine, No. 
7001, of Aveling and Porter, when working at oO-horse-power 
upon the brake. 
Starting from the top point " P\" the steam remanls at an 
uniform pressure to about the point " R." Here the slide, which 
has been gradually closing, is so nearly closed as to "wiredraw" 
the steam, as it is called, and to cause it to fall in pressure 
following the curved line ; the closing being entirely effected, the 
expansion takes place as shown by the curved line " R, S." At 
" S," prior to the arrival of the piston at the end of the stroke, 
the steam is allowed to go into the atmosphere, and nearly sud- 
denly drops from " S " to " T ;" and it is at this moment that the 
waste-blast is heard in the chimney ; when the piston reverses, 
there remains a slight pressure in the cylinder, which becomes 
less and less until by the time the piston gets to " U," the waste 
steam, with the exception of about 1 lb. to the inch has gone out 
of the cylinder ; but this 1 lb. to the inch remains as a back 
pressure throughout the stroke, so that it keeps the line of the 
pencil 1 lb, above the line of no pressuie, or the atmospheric line 
O^, F^," until the closing of the connection with the blast-pipe 
by the movement of the slide, which occurs at the point " V," 
after which time the steam remaining in the cylinder is com- 
pressed, raising the indicator-pencil according to the curved 
lines " V, VV." 
In this instance, the effective work done by the engine is re- 
presented by the area contained within the irregular figure 
" P\ R, S, T, U, V, W ; " this is after allowing for the back 
pressure and the compression, which are contained between that 
fisrure and the lines " F^ P^" 
We have now described how a diagram is taken from one end 
of the cylinder. To obtain it from the other, all that has to be 
done is to connect a pipe, as shown by "X" in Fig. 1, to the 
left-hand end of the cylinder " A," and then the diagram, having 
been obtained from the right-hand end, and the cock " L " shut, and 
the cock " Z " in the pipe " X " opened, a diagram from the left- 
hand end may be got on the same piece of paper, and would, if 
the engine were perfectly equal in performance at the two ends, 
I)e represented by the dotted line upon Fig. 5. The sum of 
these two areas will represent pounds pressure through the length 
of the stroke of the piston in a whole revolution, which, multiplied 
by the area of the piston and the number of revolutions per minute, 
will, as before, give the foot-lbs., which, being divided by 33,000, 
will give the gross indicated horse-power of such an engine. 
