order to obtain that degree of flexibility combined with direct
action, which was essential for ensuring power and avoiding needless
friction and jars from irregularities in the road, Stephenson made use of
the "ball and socket" joint for effecting a union between the ends of the
cross heads where they united with the connecting rods, and between the
ends of the connecting rods where they were united with the crank-pins
attached to each driving-wheel. By this arrangement the parallelism
between the cross head and the axle was at all times maintained and
preserved, without producing any serious jar or friction on any part of
the machine. Another important point was, to combine each pair of wheels
by means of some simple mechanism instead of by the cogwheels which had
formerly been used. And, with this object, Stephenson made cranks in
each axle at right angles to each other, with rods communicating
horizontally between them.

A locomotive was constructed upon this plan in 1815, and was found to
answer extremely well. But at that period the mechanical skill of the
country was not equal to forging cranked axles of the soundness and
strength necessary to stand the jars incident to locomotive work.
Stephenson was accordingly compelled to fall back upon a substitute,
which, although less simple and efficient, was within the mechanical
capabilities of the workmen of that day, in respect of construction as
well as repair. He adopted a chain which rolled over indented wheels
placed on the centre of each axle, and was so arranged that the two pairs
of wheels were effectually coupled and made to keep pace with each other.
The chain, however, after a few years' use, became stretched; and then
the engines were liable to irregularity in their working, especially in
changing from working back to working forward again. Eventually the
chain was laid aside, and the front and hind wheels were united by rods
on the outside, instead of by rods and crank axles inside, as specified
in the original patent. This expedient completely answered the purpose
required, without involving any expensive or difficult workmanship.

Thus, in 1815, by dint of patient and persevering labour,--by careful
observation of the works of others, and never neglecting to avail himself
of their suggestions,--Stephenson succeeded in manufacturing an engine
which included the following important improvements on all previous
attempts in the same direction:--viz.,