John Harrison was born in Yorkshire, England in 1693. As a youth he learned clock-making technique by himself based on principles of physics and mechanical engineering while helping his father in the family woodworking shop. In his career as a clockmaker he devoted himself to the development of a marine chronometer, a timepiece with the accuracy to determine longitude for safe sailing. His inventions contributed greatly to the accuracy of mechanical clocks.
The frequency of maritime accidents and collisions between ships rose sharply in Europe upon the advent of the Age of Discovery in the middle of the 15th century. While navigators could calculate the latitudes of their ships by the height of the sun or Polaris, no method had yet been devised to calculate longitude. In 1707, four warships from the Royal Navy fleet collided with submerged rocks and sunk under the sea, killing 2,000 sailors. In 1714, the Parliament of Great Britain established a panel of Commissioners for the Discovery of the Longitude at Sea and issued the Discovery of Longitude at Sea Act. The first person to find a practical and effective method to determine a ship’s longitude at sea was to receive an award equivalent to a king’s ransom. (*1)
Harrison, who had developed a wooden pendulum clock at the age of 20, set out to win the award by inventing an accurate sea clock. All of the clocks of the time used pendulums, which rendered them useless on a raging sea. Climate and variations in humidity and temperature variations also affected the teeth of the escapement and the mainspring, which hindered clocks from keeping accurate time at sea. (*2)

In 1730 Harrison met Edmond Halley, a head of the Royal Observatory in London, and presented his idea of longitude clocks. Halley referred him to George Graham, a clockmaker then famous for the Graham escapement and cylinder escapement. Graham loaned him money to build his sea clock.
Harrison is reputed to have invented a grasshopper escapement strong in friction and vibration in the 1720s. With great effort, he built a sea clock in 1735 using a mainspring with a wooden gear on a brass structure. He called it the H-1 (Harrison-1). The high accuracy of the H-1 was demonstrated on a ship heading to Lisbon a year later, in 1736. This H-1 was equipped with two dumbbell balances that absorbed the motion of the ship, in addition to the grasshopper escapement.

In 1739 Harrison developed a more compact sea clock with a mechanism to compensate for temperature variations. After the completion of on-land testing in 1741, the H-2 was ready for testing at sea. But by then Britain was at war in the War of Austrian Succession. The experiment was postponed to ensure that the mechanism would not fall into the possession of a hostile country.

In 1757, after 20 years of great effort, Harrison completed his third sea clock, the H-3, a lighter and more compact machine. In 1761 he developed the H-4, a portable round silver watch with a diameter of only 13 cm. The H-4 remained accurate under temperature variations by resisting the influences of contraction and expansion. Its key components were a new continuous device that worked when the spring was wound and a bi-metal balance with iron and brass. The mechanism, basically a primordial thermostat, was to bring great benefits to engineering in later centuries.
Harrison conducted a round-trip test at sea from Britain to Jamaica through the Caribbean via the Atlantic from 1761 to 1762. The watch lost only 5.1 seconds in 81 days, reaching the level of accuracy required to receive a £20,000 reward.

But just then a rival named Nevil Maskelyne, the head of the Greenwich Observatory and a member of the Commissioners for the Discovery of the Longitude, was vying to receive the same award for his astronomical theory. Maskelyne refused to recognize Harrison’s success. To Harrison’s chagrin, he was granted only a few thousand pounds.

Harrison and his son later developed the H-5, a watch with the same complex internal structure as the H-4 but simpler in appearance, in a bid to acquire the rest of the award. The H-5 achieved excellent results with only 15-second-error over a five-month voyage to the Barbados. At last, in 1773, the 80-year-old Harrison received the rest of the award under the direct patronage of King George III.