All these issues were compounded by the real problem – that in the early 18th Century, there was no accurate way of determining a ship’s exact longitude (its east–west position) at sea. Sailors used a process called “dead reckoning”, measuring speed, direction and distance to estimate their location. But it was an educated guess at best. Shovell and his officers knew they were aligned with the English Channel but could never have known which side of the Scillies they were.
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Losing the Admiral of the Fleet and so many men alongside him, “stirred public opinion [and] was quoted as an illustration of the urgent need of a means to find longitude at sea,” wrote curator Lieutenant-Commander David Waters in the catalogue for 4 Steps to Longitude, an exhibition at the National Maritime Museum in 1962. Larn goes a step further, believing that parliament introduced the Longitude Act of 1714 as a direct result of the disaster. The act offered a reward – the Longitude Prize – of £20,000 to whoever could produce a solution that was “practicable and useful at sea”. Sir Isaac Newton and Edmond Halley (of comet fame) set their minds to the task, but the problem was eventually solved by a carpenter-turned-clockmaker from Yorkshire.
It took John Harrison 25 years and four attempts, but in 1759 he invented a marine chronometer that allowed a ship to calculate its longitude by comparing the difference in local time at sea with the time in Greenwich. His prize-winning pocket watch, known as H4, overcame the challenging conditions on board – the issues of motion and variation in temperature – and offered the stability required.
“H4 works in principle just like any other mechanical watch,” explained Emily Akkermans, curator of time at the Royal Observatory in Greenwich, London. “But the difference is the level of precision [Harrison] achieved by using a ‘high energy’ balance that would beat faster than the smaller and lighter ones found in traditional watches.”
