This is John Thornton’s immediate reaction to the separate publication of Edmond Halley’s FUNDAMENTAL THEMATIC MAP of the English Channel. It is difficult to overestimate the importance of this chart. We hear a lot about the importance to navigation of understanding longitude but the tides produced just as much of a headache. Their causes and their variations around the globe were not understood. They caused heavy losses of life in maritime vessels particularly in approaches to harbour. Halley’s last voyage in the ‘Paramore’ during the summer of 1701 produced a map of the English Channel which for the first time enabled a prediction of the tides. It was separately published no earlier than 20 May 1702. He used a series of roman numerals across the map to indicate the hour of the high tide. By a computation described in the Advertisement the precise hour of the day could be computed for each region. As was Halley’s great strength he again managed to bring something that was so complicated down to a level that was understandable.
Quickly John Thornton published this derivative by an unknown engraver with all of the features of the original. The magnetic variation of the compass is recorded. With insets of the Isles of Scilly, Plymouth, the Solent, Thames Estuary and St. Malo. Published in the ‘English Pilot Third Part’, 1703. Howse & Sanderson p. 80; Shirley BL M.Thor 1c no. 7; Thrower, ‘The Compleat Plattmaker’ pp. 220-5; Thrower ‘The Three Voyages of Edmond Halley in the Paramore 1698-1701’, fig. 9.