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Cycling scientist Adam Hart-Davis visits London, where he re-creates a device invented by Hertha Ayrton to clear trenches of poison gas during the First World War. On the Thames he tests a 21ft megaphone, which Samuel Morland, the master mechanic to Charles I invented to conduct long-distance conversations. Hart-Davis views the work of Eleanor Coade, whose artificial stone can be seen all over the city, including the huge lions on Westminster Bridge. He also tests swimming umbrellas, designed to help people attain greater speeds in the water, and makes his own version of carbon paper, first developed by a London stationer. Last in the series.