532 Trials of Tradion-Eiujines at Wolverhampton. 
it is a reconl which, to the uninstructed eye, is unintelligible, 
but by engineers is looked upon as t!ie most reliable statement 
they can have of the duty done by an engine, inasmuch as it tells 
at each and every part of the stroke of the piston what are the 
effective pressures tending to produce motion, and what are the 
back pressures tending to detract from those effective pressures. 
We fear we must introduce a sketch here of an elementary 
indicator, as without it, probably, a verbal description would not 
be intelligible. 
Fig. l.—Illusirating the Sfrudure and mode of using the Steam-engine 
Indicator. 
c 
Supposing " A," Fig. 1, to represent the cylinder of a horizontal 
steam-engine from which it is desired to obtain indicator dia- 
grams, and supposing "B" to represent the piston of that engine 
now at the right-hand end of its stroke, " C " to be the clearance 
between the piston and the right-hand cover. Suppose further 
that " D " is the cylinder of the indicator, of, say, half an inch 
area ; and suppose that " E" is its piston attached to the rod "F,' 
having on it a spiral spring " G," and that in its normal con- 
dition" the spring "G" holds the indicator-piston "E_" at the 
place shown on the drawing, and that the top of the piston-rod 
" F " of the indicator carries a pencil at " F^ " which presses 
a<rainst a card or piece of paper " H, 1, J, K," and let " L " be a 
