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

96 LIFE OF GEOKGE STEPHENSON. [chap. ix.
vinced him that the complication arising out of the action of the
two cylinders being combined by spur wheels would prevent its
coming into practical use. He accordingly directed his attention
to an entire change in the construction and mechanical arrange-
ments of the machine ; and in the following year, conjointly with
Mr. Dodds, who provided the necessary funds, he took out a
patent, dated the 28th of February, 1815,* for an engine which
combined in a remarkable degree the essential requisites of an
economical locomotive ; that is to say, few parts, simplicity in
their action, and directness in the mode by which the power was
communicated to the wheels supporting the engine.
This locomotive, like the first, had two vertical cylinders, which
communicated directly with each pair of the ' four
wheels that
supported the engine, by means of a cross head and a pair of
connecting-rods. But in attempting to establish a direct com-
munication between the cylinders and the wheels that rolled
upon the rails, considerable difficulties presented themselves.
The ordinary joints could not be employed to unite the parts of
the engine, which was a rigid mass, with the wheels rolling upon
the irregular surface of the rails ; for it was evident that the two
rails of the line of way—more especially in those early days of
imperfect construction of the permanent road—could not always
be maintained at the same level,—that the wheel at one end of
the axle might be depressed into one part of the line which had
subsided, whilst the other wheel would be comparatively ele-
vated ;
and, in such a position of the axle and wheels, it "was ob-
vious that a rigid communication between the cross head and the
wheels was impracticable. Hence it became necessary to form
a joint at the top of the piston-rod where it united with the cross
head, so as to permit the cross head to preserve complete parallelism
with the axle of the wheels with which it was in commu-
nication.
In order to obtain that degree of flexibility combined with
direct action, which was essential for ensuring power and avoid-
* A grant to Ralph Dodds and George Stephenson, both of Killiugworth, engineers,
for their various Improvements in the Construction of Locomotive En-
gines. Patent Office, No. 8887.