LOCOMOTIVE ENGINE. 
439 
On reversing the slide valve to admit the steam behind the piston, and produce a 
fore stroke, the piston has no power to move the crank, as it is pulling directly in a 
line with the centre of the axle, and only tending to break the axle; and the crank has 
therefore to be moved on, so as to rise a little above the centre line, when the piston is 
able to move it, and pulls it round to the opposite position, or the side next the 
cylinder, and the piston arrives at the end of its stroke. On the commencement of 
a back stroke the piston has also no power to move the crank, it being again on the 
centre, and requiring to be assisted on a little, to enable the piston to act upon it and 
push it round to the opposite side again. The crank requires assisting over these 
two centres, or dead points, in order that continued rotation may be produced by the 
reciprocation of the piston; and for this purpose the two cranks upon the axle, 
which are worked by the two pistons, are placed at right angles to each other : for 
when one of the cranks is on the centre, and the piston connected with it has no 
power to make it revolve, as is the case with the left hand crank in Plates XC. and 
XCI., the other is at half stroke, and its piston has the greatest power upon it, so 
that it moves the axle round and makes the other crank pass the centre. In a 
similar manner, when they have made a quarter revolution, the right hand crank 
comes to the centre, but the piston of the left crank is then in full action, and this 
continues throughout the revolution, one crank being always in full action, when 
the other comes to the centre and ceases to propel the axle. 
The power of the piston to propel the crank is greatest at half stroke, when the con 
necting rod is at right angles to the crank, and acts with the leverage of the full length 
of the crank ; but as the piston advances to the end of the stroke, the angle that the 
connecting rod forms with the crank is continually diminishing, and with it the leverage 
and power to propel the crank, until at the end of the stroke the crank comes to 
the centre, and the connecting rod ceases to have any power to move it; the mean 
leverage throughout the stroke being only about two thirds of the length of the crank. 
This would cause therefore the motion to be very irregular if there were only one 
crank ; but with two cranks placed at right angles to each other, the irregularity 
is very nearly corrected; as at half stroke the power of one piston only is effective, 
and at quarter stroke the power of both pistons together, though acting with equal 
advantage, very little exceeds the full power of the one alone. In stationary engines 
which have only one cylinder, the irregularity of the action of the crank is com 
pensated for by the use of a large and heavy fly-wheel; which when once set 
moving, has sufficient momentum to bring the crank over the centre, and to render 
the velocity of the motion very uniform, as there is not time to accelerate its ve 
locity perceptibly in the middle of the stroke, or to diminish it at the ends. Marine