The subject of this painting is the same as Constable’s Diploma picture for the Royal Academy,  A boat passing a lock 1826 .  Both compositions are horizontal and their general structure is identical. Like the Diploma painting there is a rainstorm in the sky. However this work differs in that Constable painted the background in a looser fashion – and he did not include the dog that appears in the right foreground  of the Diploma painting. Moreover the lock keeper wears a hat (as opposed  to a cap)  and has raised arms as in Constable’s original representation of the figure. A pentimento suggests that one of the posts at the entrance to the lock was originally higher than it now appears (W.G. Constable, ‘“The Lock” as a theme in the work of John Constable’, in F. Philipp and J. Stewart (eds), Essays in Honour of Daryl Lindsay, Melbourne: Oxford University Press, 1964, p. 139).

Scholars have put forward a number of suggestions regarding the relationship of this work to Constable’s Diploma picture and his other versions of the subject. At first it was generally accepted to be a preliminary oil study for the Diploma picture. Then in 1956 W.G. Constable claimed that this painting might be an independent work, as ‘there seems to be nothing lacking that is necessary to a complete and finished picture’ (ibid., p. 138). Following this, in 1976, the fully finished foreground and carefully painted sky of this work led Leslie Parris and Ian Fleming-Williams  to suggest that it was an unfinished replica of the Diploma picture  (Tate 1976, p. 155). Robert Hoozee agreed. Reynolds, however, in his catalogue raisonné of Constable’s work, argued that it was indeed  a sketch for the Diploma picture. He pointed out that it was in keeping with Constable’s practice ‘to make a sketch when he was undertaking a major change in an existing composition’. He suggested that in transforming  his vertical Lock images into a horizontal format, with an extension of the view to the right, Constable may well have made Study of ‘A boat passing a lock’ as a preparatory sketch. (Constable made at least three versions that had a vertical format: Carmen Thyssen-Bornemisza Collection on loan to the Museo Thyssen-Bornemisza, Madrid; Philadelphia Museum and Art Gallery; and private collection). In 1991 Parris and Fleming-Williams revised their earlier opinion to suggest that this painting might be an abandoned work, that Constable may have painted it before his Diploma picture, and even before the upright Lock which he exhibited in 1824 (Tate 1991, p. 289). In 1996 Sarah Cove, in a detailed technical report of the Diploma picture, concluded that ‘this version is the sketch for, or certainly the precursor to, the Royal Academy picture’  (Sarah Cove to Anne Lyles, 12 September 2005, NGA file 04/0501–0).

While it seems most likely that the painting in the National Gallery  of Victoria’s collection was the sketch for the Diploma picture, it would appear unlikely that it was a precursor to any of the upright Locks, as in this painting Constable introduced a number of features that he also included in the Diploma picture, but which do not appear in the upright versions – the boat with a sail, willows (rather than oaks),  and the rainstorm – and he positioned the vessel as it would be travelling up river, as opposed to down river as in the upright Lock paintings.  Although it is possible that Constable chopped and changed in subject and in format, it is more likely that he painted the three upright versions and then the two horizontal versions. The Fitzwilliam drawing of c.1826 acts as a intermediary in the process of working out the compositional ideas with neither the barge in the upper lock chamber (as in the upright versions) or the boat with a sail below (as in the horizontal versions).

Whatever the purpose of this work, in painting it Constable used more finished brushwork and greater definition and coherence than  he frequently employed in his full-scale preliminary sketches. He carefully modulated the light in the sky to create a sense of wind and weather, and he depicted the plants on the riverbank and the lock’s wooden structure with considerable attention to detail.