Trials of Traction- En ffincs at Wolverhampton. 533 
cock by which the connection can be made when required between 
the end of the cn<rine-cyliiuler " A" and the bottom of the indi- 
cator-cylinder " D " ; assume also that the card " H, I, J, K " 
can be drawn backwards and forwards by the cord " M " being 
attached to some lever worked by the piston-rod, so that while 
the piston-rod moved through its whole stroke, say of 1 foot, the 
card "H, I, J, K" would move say 4 inches, and that as the piston- 
rod came back again through the foot, the card, by means of a 
spring " N," could be drawn the 4 inches back again. 
In the condition supposed, it will be seen that the traverse of the 
card is one-third that of the piston, and is made rateable with its 
motion. Assume the card to have thus travelled while a pencil 
" " is applied to its surface, it will draw upon the card a hori- 
zontal line 4 inches long from '• " to " " ; so far it will show 
nothing more tlian a proportion of the stroke of the engine. 
Next, suppose that the string "M" is disconnected from the 
engine and that the card " H," &c. remains stationary, and then 
suppose that the cock "L" is turned so as to connect the under- 
side of the cylinder " D " with the right-hand end of the engine 
cylinder " A." So soon as the slide-valve opens to admit steam to 
the right-hand end of the cylinder " A " to press the piston " B" 
towards the left-hand end, the same pressure of steam would 
clearly be exerted below the indicator piston " E." This piston 
being attached to the spring " G," would force that spring upwards 
rateably according to the pressure which came below it, and 
therefore the indicator piston-rod " F " and the pencil " F' " would 
rise as shown by the line " F^ "' to such a point upon the card 
as would represent the pressure per s(juare inch of the steam in 
the engine cylinder. 
Suppose, for example, that the indicator-spring were one which 
would collapse 1 inch for every 50 lbs. pressure per square inch 
applied to the piston, then if there Avere 100 lbs. steam applied, 
the pencil would rise 2 inches from " F^ " to " P^" 
Suppose the piston " B " to have completed its travel towards 
the left-hand end of the cylinder, and the slide-valve to have 
opened so as to allow the steam between the right-hand end 
and the piston to escape, then the indicator-piston "E" would 
return to its original position level with the line " F^ 0\" with 
the exception of any little pressure that might remain in the 
cylinder in consequence of the steam not having perfectly escaped 
into the atmosphere ; but leaving this out of the question, it would 
have returned to the point " F^ ". 
The card, therefore, used in the ways described would show 
two things : one, that a certain percentage of the whole stroke of 
the piston was indicated by the line " F^, " ; the other, that 
the maximum pressure of the steam within the cylinder was indi- 
