Harrison, John (1693–1776)

English horologist and instrumentmaker. He made the first chronometers that were accurate enough to allow the precise determination of longitude at sea, and so permit reliable (and safe) navigation over long distances.

Harrison was born in Foulky, Yorkshire, and learned his father's trades of carpentry and mechanics. In 1726, he made a compensated clock pendulum, which remained the same length at any temperature, making use of the different coefficients of expansion of two different metals.

In 1714, the British government's Board of Longitude announced a prize of up to £20,000 for anyone who could make an instrument to determine longitude at sea to an accuracy of 30 minutes (half a degree). Between 1735 and 1760, Harrison submitted four instruments for the award. When his fourth marine chronometer was tested at sea, it kept accurate time to within 5 seconds over the duration of two voyages to the West Indies, equivalent to just over one minute of longitude. Harrison was eventually awarded the prize money.

A unique feature that contributed to the chronometer's accuracy was a device that enabled it to be rewound without temporarily stopping the mechanism. This was subsequently incorporated into other chronometers.

His marine instruments are now exhibited at the National Maritime Museum, Greenwich.

In 1996 Dava Sobel's book about Harrison, Longitude, became a best-seller.