All About Engines

James Watt: The Man and his Work 5* all the expansion he needed in a single cylinder. But as Jonathan Hornblower took out a patent in 1781 for a double-cylinder engine in which the steam passed first through one and then worked against a vacuum in a second, he laid greater stress on the principle of working expansively in the patent which he took out in 1782. So far the engines constructed had only been suitable for pumping—they gave a to and fro motion and not a rotary one, suitable for driving machinery. Watt, always on the look out for new applications of, as well as improvements in, his engine, saw what was required, and puzzled long and earnestly how to secure it. It is curious to us, who see the crank so frequently, that this device was not thought of sooner, but to the engine builders of those days it was not so clear. A model of the crank was, among other contrivances, made in the Soho works, and a workman gave the secret away, so that when Watt was ready to put a rotary engine on the market he found that a Mr. Pickard, of Birmingham, had patented the crank ; and as he never adopted other men’s inventions on his engines, he was driven to seek for some other device. Finally he settled on the " sun and planet ” motion shown in Fig. 19, Plate 2. In this arrangement one toothed wheel was fixed rigidly, so that it could not rotate, to the arm hanging from the “ pump ” end of the beam. The other toothed wheel was fixed to the shaft carrying the flywheel. As the arm rose and fell the first wheel ran round the second, with the