Tidal Chart of England and Ireland
Cartographer: Brouscon, G[uillaume]
Medium: Ink and tempera on parchment
These maps of England and Ireland are from a small volume of coloured charts of the coast-lines of countries of Europe with a nautical calendar and tables. It dates from the 16th century and is the work of G[uillaume] Brouscon. The details of the charts reflect their use by sailors as only coastal detail is noted, places inland are not named. Names of important ports such as Portsmouth are written in red ink. London is noted at the top of the Thames. The map is relatively accurate; the indentation of the Bay of Cardigan, which only begins to appear on English maps after c.1550 is shown, although the Lleyn Peninsula is missing. Charts like these were invaluable to a sailor as 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 water at that port on a day of a new moon. This information allowed a sailor, once he had made allowances for the age of the moon and the known daily retardation, to estimate the optimum time at which to enter the port. Tidal diagrams date back to the medieval period but it was the author of this chart, the Breton man, G[uillaume] Brouscon who was the first to publish such charts.