At length Watt learnt the precise nature of the Hornblowers’ invention. “It is no less,” he wrote Boulton, “than our double-cylinder engine, worked upon our principle of expansion.” This was an old idea of Watt’s, which he had pursued while labouring upon his model at Kinneil. “It is fourteen years,” he said, “since I thought of the double-cylinder engine, and I think that I mentioned it to Mr. Smeaton, when I explained the expansion engine to him in your parlour, some years ago. Wm. Murdock and Mr. Henderson can testify to my having mentioned it to them; but this of the Horners seems to be a different thing, being hung on the same beam.”[224] As early as May, 1769, he had communicated to Dr. Small a clear and explicit description of his method of working steam expansively; and he adopted the principle in the Soho engine, in 1778, as well as in the Shadwell engine erected in the same year. He was, however, prevented carrying it out extensively in practice by the inexpertness of the workmen. “Though the effect of the steam,” he explained to a correspondent, “is thereby increased 50 per cent. (by theory 100 per cent.), it cannot be done without rendering the machine more complicated than we wish; and simplicity is a most essential point in mechanics. There are other contrivances known to us which would increase the effect in an inferior degree, say from one-fourth to one-sixth, but they are all attended with peculiar inconveniences which forbid their use until the illiterate and obstinate people who are intrusted with the care of the engines become more intelligent and better acquainted with the machine.”[225] [223] Watt to Boulton, 13th November, 1781. [224] Watt to Boulton, 19th November, 1781. [225] Watt to Samuel Ewer, jun., 9th July, 1781. Boulton MSS.