Cartographer: Marshall, John
Medium: Ink and tempera on parchment
This small volume is a collection of tide tables, dating from the 16th century. These folios show the coast line of the British Isles, with the names of the important ports, including London, written in red ink. For a sailor it was important to avoid a harbour at the times of the day when it was made inaccessible by low water levels. Here, the compass diagram doubles as a twenty four-hour clock, the lines that lead from it to the individual ports indicate the time of high tide at that port on a day of a new moon. This information allowed a sailor, once he had made allowances for the phases of the moon and the known daily retardation, to estimate the optimum time at which to enter the port. As such, tidal charts were invaluable. Tidal diagrams date back to the medieval period but it was the Breton man, G[uillaume] Brouscon who was the first to publish such charts. This example is probably a derivative of Brouscon’s charts and was made by John Marshall, a Scotsman, around c1555, during the reign of Queen Mary.