Hand-coloured ink and watercolour drawing showing a general arrangement and cross-sections for an atmospheric condensing beam pumping engine and associated steam boiler designed by James Watt and supplied by the firm Bolton & Watt, of Birmingham, to the Margaret Mine, at Wanlockhead, Scotland, in 1785-86. Scale: 1/3 inch to 1 foot.
The drawing is part of a set of technical drawings supplied by the manufacturers showing general arrangements, details of foundations and key components for a 55-inch (1397 mm) bore x 8 foot (2438 mm) stroke single-acting single-cylinder steam pumping engine ordered by Gilbert Meason, general manager of the Wanlockhead Mining Company for the Margaret Mine in June 1785. Labelled as sheet No.4, it shows (at left) a transverse cross-sectional elevation through the three-storey engine house with inside front view of the access stairways, steam cylinder, condenser and air pump in position within the building, and (at right) transverse and longitudinal cross-sections through a 'hay stack' style steam boiler in its masonry setting, including details of the internal flues, fire grate, ash pit, air damper, water gauge glasses, safety valve, chimney and steam pipe connecting the boiler to the steam chest on the engine.
The drawing is believed to have been executed by James Watt, or an assistant working under his direction, and includes a number of annotations written in blue ink in what appears to be Watt’s hand, giving selected dimensions of the general assembly and other notes to the erectors.