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 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 utilisation could be worked out. If one studies Stephenson's "Rocket" in detail, one begins to understand as one realises its profound complexity, 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