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 RAILWAYS. 149
bar are joined two connecting rods, the lower ends of which
work two cranks fixed on an axis extending across and
beneath the boiler, and immediately under the centre of the
cylinder. This axis is sustained in bearings formed in the
legs which support the boiler, and upon its extremity is
fixed the fly wheel as represented at B. A large-toothed
wheel is placed on this axis, which, being turned with the
cranked axle, communicates motion to other wheels; and,
through them, to any machinery which the engine may be
applied to move.
As the four-way cock is represented in the figure, the
steam passes from the boiler through the curved passage G
above the piston, while the steam below the piston is carried
off through a tube which does not appear in the figure, by
which it is conducted to the tube H, and thence to the chimney. The steam, therefore, which passes above the piston
presses it downward; while the pressure upward does not
exceed that of the atmosphere. The piston will therefore
descend with a force depending on the excess of the pressure
of the steam produced in the boiler above the atmospheric
pressure. When the piston has arrived at the bottom of
the cylinder, the cock is made to assume the position represented in the figure 52. This effect is produced by the
motion of the piston-rod. The steam now passes from
above the piston, through the tube H, into the chimney,
while the steam from the boiler is conducted through another
tube below the piston. The pressure above the piston, in
this case, does not exceed that of the atmosphere; while the
pressure below it will be that of the steam in the boiler.
The piston will therefore ascend with the difference of these
pressures. On the arrival of the piston at the top of the
cylinder, the four-way cock is again turned to the position
represented in fig. 51, and the piston again descends; and
in the same manner the process is continued. A safety
valve is placed on the boiler at v, loaded with a weight w,
2