Full name: John Harrison Born: 3rd April 1693 Invention/Achievement: The Marine Chronometer Date of introduction/Achievement: 1762 (Date of proving voyage to Jamaica) Died: 24th March 1776

In the 18^th century, Britain's empire was expanding as its naval power grew.  But navigation was still an inexact science.  Longitude could be established by comparing local time with Greenwich Mean Time, but for this a chronometer was needed that would perform accurately and reliably in the often turbulent environment of a sailing ship.  

In 1707 four Royal Navy ships were wrecked off the Scilly Isles with the loss of 1,400 lives, due to the navigator's inability to accurately calculate their position.  Parliament offered a prize of £20,000 for a solution to the problem of determining longitude.  

John Harrison, a carpenter and clockmaker, set about producing a clock sufficiently accurate and robust to allow mariners to know the time at Greenwich.  In 1736 he presented his first "Sea Clock" (later designated "H1") to the Royal Society and it was given a sea trial to Lisbon and back, during which it performed well.  

However the requirement was for a clock that would perform on the more arduous transatlantic voyage to Jamaica.  Harrison was granted £500 to develop a more compact and robust version.  After several years work he discovered a serious flaw in his method, concerning the effect of a ship's yawing on the oscillation period and thus the accuracy.  

Harrison spent another 17 years working on the problem but in the end was still not satisfied with its performance.  In London in 1758 he saw small watches that were as accurate as his much larger "sea clocks" (probably due to high grade steels that had become available.  He realised that a watch could do the job perfectly well if it could be made to perform under rugged conditions, in different temperatures.  

This led to him to produce H4, his masterpiece, only a little larger than a pocket watch, and would allow mariners to establish longitude all over the globe.  

In 1762 H4 was found to be only 5 seconds in error after a voyage to Jamaica.  It took Harrison some years to obtain the prize from the government, but eventually with the support of King George III, he received a total of £23,065.