nsists of
a cylindrical boiler filled with brass tubes, through which the hot air
passes on its progress from the furnace to the chimney, and attached to the
boiler are two horizontal cylinders fitted with pistons, valves, connecting
rods, and other necessary apparatus to enable the power exerted by the
pistons to turn round the cranked axle to which the driving wheels are
attached. There are, therefore, two independent engines entering into the
composition of a locomotive, the cranks of which are set at right angles
with one another, so that when one crank is at its dead point, the other
crank is in a position to act with its maximum efficacy. The driving
wheels, which are fixed on the crank shaft and turn round with it, propel
the locomotive forward on the rails by the mere adhesion of friction, and
this is found sufficient not merely to move the locomotive, but to draw a
long train of carriages behind it.

130. _Q._--Are locomotive engines condensing or high pressure engines.

_A._--They are invariably high pressure engines, and it would be impossible
or at least highly inconvenient, to carry the water necessary for the
purpose of condensation. The steam, therefore, after it has urged the
piston to the end of the stroke, escapes into the atmosphere. In locomotive
engines the waste steam is always discharged into the chimney through a
vertical pipe, and by its rapid passage it greatly increases the intensity
of the draught in the chimney, whereby a smaller fire grate suffices for
the combustion of the fuel, and the evaporative power of the boiler is much
increased.

131. _Q._--Can you give an example of a good locomotive engine of the usual
form?

_A._--To do this I will take the example of one of Hawthorn's locomotive
engines with six wheels represented in fig. 29; not one of the most modern
construction now in use, nor yet one of the most antiquated. M is the
cylinder, R the connecting rod, C C the eccentrics by which the slide valve
is moved; J J is the steam pipe by which the steam is conducted from the
steam dome of the boiler to the cylinder. Near the smoke stack end of this
pipe is a valve K or regulator moved by a handle _p_ at the front of the
boiler, and of which the purpose is to regulate the admission of the steam
to the cylinder; _f_ is a safety valve kept closed by springs; N is the
eduction pipe, or, as it is commonly termed in locomotives, the _blast
pipe_, by which the steam, escaping from the cyli