Mariners have it very easy nowadays. They use satellites to find their exact location on the globe and can pinpoint their position anywhere on the Earth's surface to within a yard or two.
Three hundred years ago, however, it was a different story; on the open sea it was almost impossible for a sailor to know exactly where he was.
The distance north of the equator was easily established. Latitude could be determined with surprising accuracy by finding the elevation of the pole star or the sun at local noon.
But the measurement of longitude - the distance east or west - presented a problem that was far more fundamental. In principle it was simple enough. Since the sun appears to complete one circle of the earth in 24 hours, it must take four minutes to traverse each of the 360 of longitude.
A sailor who was equipped with a clock set to Greenwich time could easily calculate his distance east or west of that meridian by noting the time registered by this clock at his own local noon - the time when the sun attained its highest point; he allowed one degree of longitude for each four minutes that the Greenwich timepiece was either slow or fast of 12 o'clock.
But until the middle of the 18th century, the clocks available were notoriously unreliable.
The problem was solved in a novel way in the reign of Charles II. A man called Kenelm Digby claimed to have discovered a remarkable panacea, a powder that could heal a wound at any distance.
All one had to do was to apply the powder to some artefact that had come into contact with the ailment. The only disadvantage was the cure could hurt: patients allegedly would jump in pain when some distant item recently in contact with their sore was sprinkled with a dash of Digby's powder. A way was found to put this strange telepathy to navigational use.
Before a voyage was embarked on, a wounded dog would be put aboard the ship, care being taken to leave in the custody of some trusted person ashore a bandage that had touched the canine sore. The assistant's job was to apply Digby's powder to the band age at noon precisely every day in Greenwich.
Naturally, the unfortunate mid-ocean animal would yelp in response to this distant painful stimulus, thereby providing the navigator with an accurate time-signal. By comparing this to the ship's clock, which was reset regularly to local time, the longitude of the vessel could be quickly calculated - or so, at the time, it was apparently believed.