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In this study for a composition of Caernarvon Castle the building is farther from the viewer than in that on folio 40 verso (Tate D01856; Turner Bequest XLIII 39a); compare the series of composition studies in the contemporary Dolbadarn sketchbook, including Tate D02069 and D02071 (Turner Bequest XLVI 49a, 50a).
With the page turned horizontally, the general layout is closer to the composition of the Picturesque Views in England and Wales watercolour of about 1832 (British Museum, London),1 engraved in 1835 (Tate impressions: T04603, T06112). There the atmospheric effect is of a warm summer evening just after sunset, and a crescent moon rises beyond the castle. In this summary study, Turner may have intended to represent moonlight; that seems to be the case in the very similar studies on folios 43 verso, 44 verso and 45 verso (D01862, D01864, D01866; Turner Bequest XLIII 42a, 43a, 44a).
Andrew Wilton, J.M.W. Turner: His Life and Work, Fribourg 1979, p.400 no.857, pl.196.