The life of George Stephenson, railway engineer - Lighthouse ...

CHAP. VIII.] TREVETHICK'S EOAD LOCOMOTIVE. 77
of note, that the steam-cocks and the force-pump, as also the bel-
lows used for the purpose of quickening combustion in the fur-
nace, were worked off the same crank-axle.
This was the first successful high-pressure engine constructed
on the principle of moving a piston by the elasticity of steam
against the pressure only of the atmosphere. Such an engine
had been described by Leopold, though in his apparatus the
pressure acted only on one side of the piston. In Trevethick
and Vivian's engine, the piston was not only raised, but was also
depressed by the action of the steam, being in this respect an
entirely original invention, and of great merit. The steam was
admitted from the boiler under the piston moving in a cylinder,
impelling it upward. When the motion had reached its limit,
the communication between the piston and the under side was
shut off, and the steam allowed to escape into the atmosphere.
A passage was then opened between the boiler and the upper
side of the piston, which was pressed downwards, and the steam
again allowed to escape into the atmosphere. Thus the power
of the engine was equal to the difference between the pressure of
the atmosphere and the elasticity of the steam in the boiler.
This first steam-carriage adapted for actual use on common
roads, was, on the whole, tolerably successful. It excited con-
siderable interest in the remote district, near to the Land's End,
where it had been constructed. Being so far removed from the
great movements and enterprise of the commercial world, Treve-
thick and Vivian determined upon exhibiting their machine in
the metropolis, with a view, if possible, to its practical adoption
for the purpose intended. In furtherance of this object, they set
out with the locomotive to Plymouth, whence a sea-captain,
named Vivian, was to convey it in his vessel to town. Coleridge
relates, that whilst the vehicle was proceeding along the road
towards the port, at the top of its speed, and had just carried
away a portion of the rails of a gentleman's garden, Andrew
Vivian descried ahead of them a closed toll-gate, and called out
to Trevethick, who was behind, to slacken speed. He imme-
diately shut off the steam ; but the momentum was so great, that
the carriage proceeded some distance, coming dead up, however,
just on the right side of the gate, which was opened like lightning