We are grateful to Dr Jeroen Giltaij of Museum Boymans van Beuningen, Rotterdam and Dr Jan Kelch formerly of the Staatliche Museen, Berlin for confirming the attribution on the basis of a photograph (written communication, 4 and 19 March 2008, respectively). Dr Kelch points out that this is probably a late work by Porcellis dating from the late 1620s. He compares it to Shipping in an estuary by stormy weather in Museum Boymans van Beuningen, Rotterdam (inv. no. 1675).