Hand-coloured ink and watercolour drawing showing elevations, cross-sections and plan views of several key components of an atmospheric condensing beam pumping engine designed by James Watt and supplied by the firm Bolton & Watt, of Birmingham, to the Margaret Mine, at Wanlockhead, Scotland, in 1785-86. Scale: 1/4 inch to 1 inch. Undated, but likely 1785.
The drawing is part of a set of technical drawings supplied by the manufacturers showing details of 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.7, it shows (at upper left) details of the crop bar for the air pump with details for attachment of linkages and top of the valve plug, (at lower left) details of the gland on the king post of the engine beam, (at upper right) details of Martingales for the piston rod and air pump and for the plug rod and condenser pump rods on the engine beam, and (at lower right) details of the great gland on the main gudgeon on the engine beam. Martingales are iron stays that form distinctive diagonal braces between the curved sectors and main beam as shown in sheet No.1. 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 and other notes to the erectors.