534 Trials of Traction- Engines at Wolverhampton. 
cated by the line " F\ " ; but if this were all the indicator 
showed it would be but of little use, and it is not thus that the 
indicator is worked. The string " M " is kept in communication 
with the lever worked by the piston-rod at the same time that the 
cock " L " is open, so as to keep the indicator cylinder in commu- 
nication with the engine cylinder. The effect of this is, that 
assuming the pressure to exert itself with great rapidity, the pencil 
" F^" (see Fig. 2) would trace a vertical line " F\ P'"on the card 
" H, 1, J, K;" and supposing the pressure to be maintained uni- 
formly throughout the whole of the stroke, then as the point " F^ " 
would be maintained at the level "pi" throughout the stroke, 
while the card travelled from right to left its 4 inches, the pencil 
would describe the top line " P^, ;" then the slide-valve 
being opened to allow the steam to escape, the pencil would make 
a sudden drop from " " to " O^," and on the return journey of 
the card would trace the line " " " F^ " ; thus it would have 
described a parallelogram of which the horizontal line would 
represent a proportion of the stroke of the piston, and the vertical 
line would represent the pressure upon the piston ; the area of that 
parallelogram would therefore represent pounds pressure into feet 
moved through by the piston in its stroke or half revolution of the 
engine. Now, as has been before explained in the Oxford Report, 
a horse-power is supposed to be 33,000 lbs. moved through 1 foot 
in a minute of time, or equal thereto, viz., 11,000 moved through 
3 feet, or 1 lb. moved through 33,000 feet, &c. 
Figs. 2-5. — Indicator Diagrams, 
FIG. a- FIG..1. 
0' 
p-1 
100 Ills 
qi p"'P" 
P' 
J}. 
w 
c) 
I 
0 
pi 
0' F' 
PIG. 4:. TIG. 5. 
"N? 7001 
0» U V F' 
Supposing now, to take an example, that the lines " F^, O^' 
(see Fig. 2) represent 1 foot stroke of the piston, that that piston 
has an area, say of 100 square inches, that the line " F^, P^ " re- 
presents 100 lbs. to the inch, then we shall have 100 lbs. x 1 foot 
X 100 inches area = 10,000 as the work performed by the piston 
