The steam engine familiarly explained and illustrated; with an historical sketch of its invention and progressive improvement; its applications to navigation and railways; with plain maxims for railway speculators. By the Rev. Dionysius Lardner ... With additions and notes by James Renwick ...

LOCOMOTIVE ENGINES ON TURNPIKE ROADS. 227
through the tube T to the lowest vessel D, to be again circu
lated through the boiler.
The pistons of the engine work on the axles of the hind
wheels of the carriage which bears the engine, by cranks, as
in the locomotives on the Manchester railway, so that the
axle is kept in a constant state of rotation while the engine
is at work. The wheels placed on this axle are not permanently fixed or keyed upon it, as in the Manchester locomotives; but they are capable of turning upon it in the
same manner as ordinary carriage wheels. Immediately
within these wheels there are fixed upon the axles two projecting spokes or levers, which revolve with the axle, and
which take the position of two opposite spokes of the wheel.
These may be occasionally attached to the wheel or detachedfrom it; so that they are capable of compelling the wheels:
to turn with the axle, or leaving the axle free to turn independent of the wheel, or the wheel independent of the axle,
at the pleasure of the conductor. It is by these levers that
the engine is made to propel either or both of the wheels.
If both pairs of spokes are thrown into connexion with the
wheels, the crank shaft or axle will cause both wheels to
turn with it, and in that case the operation of the carriage is
precisely the same as those of the locomotives already described upon the Liverpool and Manchester line; but this is
rarely found to be necessary, since the adhesion of one wheel
with the road is generally sufficient to propel the carriage,
and consequently only one pair of these fixed levers are generally used, and the carriage propelled by only one of the
two hind wheels. The fore wheels of the carriage turn
upon a pivot similar to those of a four-wheeled coach. The
position of these wheels is changed at pleasure by a simple
pinion and circular rack, which is moved by the conductor,
and in this manner the carriage is guided with precision and
facility.
The force of traction necessary to propel a carriage upon
common roads must vary with the variable quality of the