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Several sketches in this sketchbook show variants of this arched structure, especially folio 18 verso (D05515) but also folios 20 verso and 21 verso (D05519, D05521). In the context of a classical seaport, these seem to anticipate buildings in Dido Building Carthage, exhibited in 1815 (National Gallery, London).1
Martin Butlin and Evelyn Joll, The Paintings of J.M.W. Turner, revised ed., New Haven and London 1984, pp.94–6 no.131 (pl.133).