Watt was exceedingly busy about this time in superintending the erection of new engines. No fewer than twelve were in progress in different parts of the county. As he travelled about from one mine to another on horseback, and spent a good deal of his time in the open air, his mind was diverted from preying upon itself according to his ordinary habit, and his health and spirits improved accordingly. Boulton was equally busy at Soho, where he was erecting a powerful engine for blowing the furnaces at Walker’s ironworks at Rotherham, and another for Wilkinson’s forges at Bradley, in which he proposed to employ a double cylinder, with a double crank[220] and a pair of fly-wheels. At intervals he went into Yorkshire, Lancashire, and Shropshire, to look after various other engines in progress; writing Watt cheerful letters as to the improving prospects of the firm. He found the steam-engine everywhere gaining in public estimation. “The more it is known,” he wrote, “the more it will be in demand. As to the scheme of the Hornblowers, they shall sooner press me down into the earth than they shall press down a piston with steam.” And again, “Give yourself no uneasiness about the Horners’ engine. Our title to the invention is as clear as can be; and it is as well secured as an Act of Parliament can make it-- “Doubt that the sun is fire, Doubt all the powers of sight, Doubt truth to be a lyer, But never doubt our right.” [220] Watt suggested caution as to making use of the cranks. “In relation to Wilkinson’s forges, I wish you would execute them without the double crank. We shall soon have a bad enough lawsuit on our hands without it.”--Watt to Boulton, 19th July, 1781. Watt’s first surmise, that the Hornblowers intended to work their engine by heated air or gas, had set Boulton upon a series of inquiries and experiments on the subject, in which he was assisted by Dr. Priestley, who had shortly before settled in Birmingham, and was a willing co-operator in all investigations of this nature. Their object was to ascertain whether it was practicable to produce mechanical power by the absorption and condensation of gas on the one hand, and by its disengagement and expansion on the other.