On this day in 1696, James Lyddell, compass maker, sent his bill for compasses for the St George Galley to Captain Thomas Bowrey.
In the days of GPS, it is easy to forget how reliant a seventeenth-century mariner was on the simple compass. At the time, there was no reliable way to calculate a ship’s position at sea. The modern sexant, used to determine latitude, was not invented until 1731 although the mariner’s quadrant could be used to measure the altitude of the sun or the North Star. It was to be many more years before the ability to accurate determine longitude was developed. In Bowrey’s time a technique called dead reckoning had be used.
This involved calculating the distance and direction travelled from the last known position. To measure distance, a log or wooden quadrant attached to a line marked out with knots, was thrown overboard. The distance travelled in an hour, measured using an hour-glass, could calculated from the number of knots that had run out.
Having estimate the distance travelled, a compass was essential to measure the direction of travel. Today’s document confirms their importance. The St George Galley carried a number: a large wooden one, three large brass ones, one for the galley’s boat, and a Coronett compass. In addition, there were spare cards and glasses. Lyddell also supplied the hour-glasses – 12 half-hours glasses and 6 half-minute and quarter-minute glasses.