Watt had from the first regarded the employment of the steam-engine in producing continuous rotary motion as one of its most useful applications, and with this object he invented his original wheel-engine. No steps were taken to introduce the invention to practical use; but it occurred to Watt that the same object might be better effected by employing the ordinary engine for the purpose, with certain modifications.[202] The subject had partially occupied his attention during his first visit to Cornwall; for we find him writing Boulton from Chacewater, in 1779, “As to the circular motion, I will apply it as soon as I can, but foresee that I shall be very busy shortly, and much out of doors.” On his subsequent return to Birmingham, after frequent conferences with his partner on the subject, he proceeded to prepare a model, in which he made use of a crank connected with the working beam of the engine to produce the rotary motion. There was no originality in the employment of the crank, which was an expedient that Watt had long before made use of.[203] The crank was, indeed, one of the most common of mechanical appliances. It was in daily use in every spinning-wheel, in every grindstone turned by hand, in every turner’s and knife-grinder’s foot-lathe, and in every potter’s wheel. It was one of the commonest, as it must have been one of the oldest, of mechanical expedients. “The true inventor of the crank rotative motion,” said Watt, “was the man who first contrived the common foot-lathe: applying it to the engine was like taking a knife to cut cheese which had been made to cut bread.” [202] In June, 1780, we find Boulton describing to Colonel Watson the progress of the Soho business, as follows:--“Since I had the honour of seeing you in England we have erected upwards of 40 of our new steam-engines, and have (from so much experience) obviated every difficulty, and made it a most practicable and perfect machine. The steam wheel we have not meddled with since you were at Soho, as we have been fully employed upon large beam-engines; besides, we have applied the beam engine to rotative motions so successfully that the wheel engine seems almost unnecessary.” [203] Watt had made use of the crank at a very early period. Thus we find him writing to Dr. Small on the 20th September, 1769,--“As to the condenser, I laid aside the spiral wheels because of the noise and thumping, and substituted a crank: in other respects it performed well enough.” [Illustration: THE CRANK AS APPLIED IN THE FOOT-LATHE.] [Illustration: INTERIOR OF THE “WAGGON AND HORSES.” [By Percival Skelton.]]