17. Valve Motion, admitting steam alternately to each side of the
pistons.
18. Cranks, operating on the axle: at the ends of the axle are crotches
(No. 21,) which, as the axle turns round, catch projecting pieces of
iron on the boxes of the wheels, and give them the rotatory motion. The
hind wheels only are thus operated upon.
19. Propellers, which, as the carriage ascends a hill, are set in
motion, and move like the hind legs of a horse, catching the ground, and
then forcing the machine forward, increasing the rapidity of its motion,
and assisting the steam power.
20. The Drag, which is applied to increase the friction on the wheel in
going down a hill. This is also assisted by diminishing the pressure of
the steam--or, if necessary, inverting the motion of the wheels.