The British Longitude Act Reconsidered
Was it responsible governance or was it subterfuge?
Travel by sea was slow and dangerous during the age of sail. Many ships intending to carry cargo or passengers to distant ports, to wage war or to discover new lands, met disaster at sea and were never heard from again. It was not at all uncommon for coastal vessels simply transporting goods from port to port along the rugged British coastlines to get caught up in fast-moving storms and be driven ashore, at the cost of cargo, ships and lives. But it was extraordinary when, in 1707, a fleet of British naval vessels under the command of Admiral Sir Cloudesley Shovell sailed headlong into the Isles of Scilly off the coast of Cornwall. Having the bodies of hundreds of British seamen, including members of some of the nation’s leading families, wash up on homeland beaches was simply unacceptable. People demanded that an effort be made to prevent such a tragedy from occurring again.
Shovell and his fleet had sailed from Gibraltar bound for England on or about October 10, 1707. They suffered stormy weather nearly continuously for the next 10 days. They were unable to make reliable astronomic observations and were forced to navigate by dead reckoning, a process that combined compass readings and speed estimates. On October 21, soundings found the water depth to be some 90 fathoms, suggesting that the fleet was nearing land, and the skies cleared enough to permit some astronomic observations. The results were inconsistent but seemed to indicate that the fleet was just offshore of Ushant, France. If that was the case, sailing northeast would carry the fleet safely into the English Channel and Admiral Shovell issued orders to do just that.
Tragically, the fleet was not off Ushant. It was nearly a degree and half to the north and a degree to the west—in the immediate vicinity of the Scilly Islands. The only reasonable explanation for such a large navigational error appears to be that the fleet made more rapid headway than reckoned, either because the current they faced was weaker than normal or because gusty winds resulted in faster sailing speeds than were estimated. Admiral Shovell’s decision to try to make port immediately rather than wait for a more reliable determination of his position was simply foolhardy and cost 2,000 lives, including his own.
Ignoring the fact that the error in latitude in nautical miles had been twice that of the error in longitude, the Admiralty chose not to blame the disaster on the mistake of a highly respected member of the nobility. Instead, it focused on the notoriously difficult problem of accurately determining longitude at sea. In 1714 the British Parliament passed the Longitude Act, which established the Commission for the Discovery of Longitude at Sea, more commonly referred to as the Board of Longitude. The Act provided three prizes based on the accuracy of longitude measurement: £20,000 (equal to more than $3 million today) to anyone who could find a way to determine longitude at sea with an accuracy of one-half degree, £15,000 for two-thirds degree and £10,000 for one degree.