“Ghastly weather“, railed the Admiral, Sir Clowdisley Shovell, glaring at the fog that had been annoying him for twelve days at sea. After victoriously battling the French Mediterranean fleet, he set off to England from Gibraltar with 21 Royal Navy ships on 29th September, 1707. Plagued by the fear that his ships could slam against rocky ledges, the Admiral ordered his navigation officers to deliberate on the matter. The consensus was that the fleet was at a safe distance west of the island of Ushant; however, the officers had miscalculated their position. The Isles of Scilly, located 20 miles off the south-western tip of England, became a nameless grave for 2,000 soldiers.

The “Longitude Act” 
For centuries, scientists, mariners and politicians had been absorbed in the search for a method of determining one’s position at sea. The governments of the seafaring nations had pledged huge prizes. In its Longitude Act of 8th July, 1714, the British Parliament offered the highest sum of prize money. The first person to find a method for establishing geographic longitude was to be awarded 20,000 Pounds.

John Harrison (1693-1776)
John Harrison, who was born into a family of simple means on 24 th March, 1693 in the county of Yorkshire, had learned the carpentry trade. In 1713, not quite having reached the age of twenty, he had made his very first clock without having had any training as a clockmaker. More clocks were to follow.
No one knows when or how Harrison had first learned of the Board of Longitude’s prize. Perhaps he had already dealt with the problem before. To make a clock that would work reliably at sea, he dispensed with the pendulum and replaced it with two dumbbell balances linked together.  
In 1737, he presented his clock to the Board of Longitude. Although his clock had only been off by a few seconds per day on the trial voyage from London to Lisbon, he declared that his timekeeper still had a few deficiencies that he intended to rectify. In the 20 years that followed, he perfected his ideas and completed his „sea watch“ in 1759. The drawings were published in 1767.
Yet many years passed before Harrison received his prize. Not until King George III intervened on Harrison’s behalf did Harrison receive the prize in 1773, three years before his death.

John Arnold (1736-1799)
John Harrison’s clocks were good; however, they were complicated to make and, consequently, expensive. John Arnold designed a less expensive model. In 1782 he had a new type of escapement and a compensation balance patented. The most significant aspect of Arnold’s design was that the balance would be in contact with the gear train for only a very short period of time in order to trigger the impulse for the movement.
The first marine chronometer to have this new escapement was the Number 13. The German Clock Museum is in possession of Number 16, which is thought to have been made in 1783.
Arnold’s invention changed the way position was determined on the high seas. Thanks to the sextant and the chronometer, mariners would now know where they were, even after weeks of voyaging on the open sea.
Such was the case in 1789 with Captain Thomas Welladvice, commander of the Indiaman, Barwell. By determining their position with his Arnold chronometer in the middle of the night, Welladvice found out that he had to be near the Isles of Scilly. He had the ship moored at midnight. By daybreak he saw the rocks before him. The tragedy of 1707 did not repeat itself.