Marina: Folkestone

Turner painted this around 1845, when he was about 70 and near the end of his life. He had gone back to Folkestone on the Kent coast, and what he made of it is almost pure weather and water. The sea and sky dissolve into swirling greens, and you have to hunt for the two boats pushing out through them. Look closely and one of them has a funnel. The age of steam was arriving at sea, and Turner, who had already painted steam trains and steamboats, set an old-fashioned sailing boat beside a new steamer without any comment. That quiet pairing is the whole modern world creeping into a picture that otherwise feels ancient. Work this loose and experimental had been mocked when he put it on show, so this one he held back, and it was never exhibited in his lifetime.