that spread outward from that centre played their part.
The men whose names are cardinal in the history of this development
invented, for the most part, in a quite empirical way, and Trevithick's
engine was running along its rails and Evan's boat was walloping up the
Hudson a quarter of a century before Carnot expounded his general
proposition. There were no such deductions from principles to
application as occur in the story of electricity to justify our
attribution of the steam engine to the scientific impulse. Nor does this
particular invention seem to have been directly due to the new
possibilities of reducing, shaping, and casting iron, afforded by the
substitution of coal for wood in iron works; through the greater
temperature afforded by a coal fire. In China coal has been used in the
reduction of iron for many centuries. No doubt these new facilities did
greatly help the steam engine in its invasion of the field of common
life, but quite certainly they were not sufficient to set it going. It
was, indeed, not one cause, but a very complex and unprecedented series
of causes, that set the steam locomotive going. It was indirectly, and
in another way, that the introduction of coal became the decisive
factor. One peculiar condition of its production in England seems to
have supplied just one ingredient that had been missing for two thousand
years in the group of conditions that were necessary before the steam
locomotive could appear.

This missing ingredient was a demand for some comparatively simple,
profitable machine, upon which the elementary principles of steam
utilization could be worked out. If one studies Stephenson's "Rocket" in
detail, as one realizes its profound complexity, one begins to
understand how impossible it would have been for that structure to have
come into existence _de novo_, however urgently the world had need of
it. But it happened that the coal needed to replace the dwindling
forests of this small and exceptionally rain-saturated country occurs in
low hollow basins overlying clay, and not, as in China and the
Alleghanies for example, on high-lying outcrops, that can be worked as
chalk is worked in England. From this fact it followed that some quite
unprecedented pumping appliances became necessary, and the thoughts of
practical men were turned thereby to the long-neglected possibilities of
steam. Wind was extremely inconvenient for the purpose of pumping,
because in these latitudes it is i