John Harrison (1693 - 1776)

A self-educated English Carpenter and Clockmaker

The Scilly naval disaster of 1707 saw the loss of four warships of a Royal Navy fleet off the Isles of Scilly in severe weather on 22 October 1707. More than 1,400 sailors aboard the wrecked vessels lost their lives, making the incident one of the worst maritime disasters in the history of the British Isles. The main cause of the disaster was the navigators' inability to accurately calculate their positions.

Harrison set out to solve the problem directly, by producing a reliable clock that could keep the time of the given place. His difficulty was in producing a clock that was not affected by variations in temperature, pressure or humidity, remained accurate over long time intervals, resisted corrosion in salt air, and was able to function on board a constantly-moving ship.

In 1730, Harrison designed a marine clock to compete for the Longitude Prize. He presented his ideas to Edmond Halley, the Astronomer Royal, who in turn referred him to George Graham, the country's foremost clockmaker. Graham must have been impressed by Harrison's ideas, for he loaned him money to build a model of his "Sea clock". It took Harrison five years to build his first Sea Clock (or H1). He demonstrated it to members of the Royal Society who spoke on his behalf to the Board of Longitude.

The clock is always set to show what the time is in a location whose longitude is taken as a reference. For the British Navy, this location was London. If you know what time it is in London, and you know what the time is wherever your ship currently is (by calculating the position of the Sun), you can determine how far you are (in terms of longitude) from London. If it's 1pm in London, and it's 4pm wherever you are right now, then you know you're traveling through the Indian Ocean, and by heading straight north, you can get to Calcutta, India.

The clock has to be made to be in balance in all positions, so as the the

ship tilts under sail or is tossed in storms and encounters various temperatures, it remains precise.

The clock was the first proposal that the Board considered to be worthy of a sea trial. In 1736, Harrison sailed to Lisbon on HMS Centurion and returned on HMS Orford. The clock lost time on the outward voyage. However, it performed well on the return trip: both the captain and the sailing master of the Orford praised the design. The master noted that his own calculations had placed the ship sixty miles east of its true landfall which had been correctly predicted by Harrison using H1.

The Board was impressed enough to grant Harrison £500 for further development. Harrison move on to develop H2, a more compact and rugged version. In 1741, after three years of building and two of on-land testing, H2 was ready, but by then Britain was at war with Spain in the War of Austrian Succession and the mechanism was deemed too important to risk falling into Spanish hands. In any event, Harrison suddenly abandoned all work on this second machine when he discovered a serious design flaw in the concept of the bar balances.

The Board granted him another £500, and while waiting for the war to end, he proceeded to work on H3. Harrison spent seventeen years working on this third 'sea clock', but despite every effort it did not perform exactly as he would have wished. Around 1750 Harrison had also come to this conclusion and abandoned the idea of the 'Sea clock' as a timekeeper, realizing that a watch-sized timekeeper would be more successful as it could incorporate a balance which, though smaller, oscillated at a much higher speed. A watch would also be more practicable, another factor required by the Longitude Act of 1714.

About the Scilly naval disaster