This is a preparatory study for Flatford Mill ('Scene on a Navigable River') (1816-17, Tate Gallery N01273), which was largely painted outdoors in the artist's native Suffolk. The drawing is a pencil tracing of an image made with a brush on a sheet of glass held on an easel in front of the subject itself. The tracing was made by placing a piece of paper over the image on the glass. Because it was still wet, it produced an accidental offprint on the back of the paper. It was squared for transfer to the canvas. The drawing includes the two barges seen in the finished painting, but none of the figures. Arthur Parsey describes Constable using such a method in his book The Science of Vision; or, Natural Perspective (1840). A number of such tracings are known, and Constable made this one to establish the perspective in his large canvas.
Leslie Parris and Ian Fleming-Williams, Constable, exhibition catalogue, Tate Gallery, London 1991, pp.179-180, reproduced fig.60