Not till the invention of Watt's second and so called double acting steam engine, was a prime mover found, that begot its own force by the consumption of coal and water, whose power was entirely under man's control, that was mobile and a means of locomotion, that was urban and not, like the waterwheel, rural, that permitted production to be concentrated in towns instead of, like the water wheels, being scattered up and down the country, that was of universal technical application, and, relatively speaking, little affected in its choice of residence by local circumstances.