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 inconstant: it was costly, too, because at any time the labourers might be obliged to sit at the pit's mouth for weeks together, whistling for a gale or waiting for the water to be got under again. But steam had already been used for pumping upon one or two estates in England--rather as a toy than in earnest--before the middle of the seventeenth century, and the attempt to employ it was so obvious as to be practically unavoidable.[3] The water trickling into the coal measures[4] acted, therefore, like water trickling upon chemicals that have long been mixed together dry and inert. Immediately the latent reactions were set going. Savery, Newcomen, a host of other workers, culminating in Watt, working always by steps that were at least so nearly obvious as to give rise again and again to simultaneous discoveries, changed this toy of steam into a real, a commercial thing, developed a trade in pumping engines, created foundries and a new art of engineering, and almost unconscious of what they were doing, made the steam locomotive a well-nigh unavoidable consequence. At last, after a century of improvement on pumping engines, there remained nothing but the very obvious stage of getting the engine that had been developed on wheels and out upon the ways of the world.