Description:Plan and side view of the 'Double Expansive Engine', Soho Manufactory, by James Watt.
In 1781, James Watt paid considerable attention to utilising the expansive power of steam in his engines. He may have been spurred on to this by the appearance of Jonathan Hornblower’s compound engine (i.e. an engine with two cylinders). Watt devoted much of his patent of 12th March 1782 to expansive working. He also included in it a curious form of compound engine which is illustrated here. One cylinder had a beam over it as in an ordinary engine, while the other had an 'equalizing beam' or wheels over it. One of these engines was built at Soho, and was later used to power a hammer. It could be worked as a compound engine, or the two cylinders could be worked separately. However, the design proved inefficient and unwieldy, and was not used again.
This drawing is part of the Archives of Soho collection, which is held by Birmingham City Archives [Ref. MS3147/Portfolio/5/1341]