John Harrison's Horological Legacy

British Museum

A look at how his inventions changed timekeeping.

Who was John Harrison?
John Harrison (1693–1776) was an English clockmaker, celebrated for developing numerous mechanisms which improved the technology of timekeeping devices.

Harrison invented the gridiron pendulum (illustrated here in the background to the right), grasshopper escapement, bi-metallic strip (as you would find in your kettle) and an automatic form of maintaining power, all of which were adopted by later clockmakers and remain evidence of Harrison's enduring contributions to horology. Harrison is possibly best known for the development of a working marine timekeeper, leading to a revolution in navigation by providing a means to calculate longitude at sea.

Maintaining power
Around the mid-1720s, John Harrison invented a form of maintaining power which would ensure a clock could keep accurate time without stopping or running backward during winding. Harrison’s mechanism was subsequently incorporated into many clocks and watches by other makers, including this grand mahogany cased, weight-driven clock, made by Thomas Earnshaw.
Gridiron pendulum
The pendulum created a problem for clockmakers of the time. Metal pendulums become longer or shorter as the temperature changes, which affects the speed at which they swing. George Graham was the first to invent a solution in about 1711. His new pendulum had mercury at the bottom which expanded in the opposite direction to the pendulum itself, allowing it to swing at a constant rate. Later, in 1727–8, John Harrison invented the gridiron pendulum. This relied on the different expansion rates of brass and steel rods. These new pendulums were widely used for the next 100 years.

Harrison's gridiron pendulum provides compensation for changes in temperature, as here adopted by Abraham Louis Breguet in his astronomer's clock.

Marine chronometers 
Many lives were lost in shipwrecks before sailors had a reliable method of navigating. In 1714 the English parliament offered up to £20,000 to whoever could devise a method of establishing longitude (east–west position). The top prize was finally won by John Harrison. He made a mechanical timekeeper that could keep precise time whilst travelling at sea in changing conditions. Navigators could tell how far east or west they had traveled by comparing the time at home, as shown on the chronometer, with the local time, measured from the sun or stars. The chronometer became an essential tool for anyone travelling out of sight of land. It has now been replaced by GPS satellite navigation.

After John Harrison’s pioneering work, other makers competed to improve and simplify marine chronometers, so putting them into production for the many ships that demanded them. The example here was made by Thomas Earnshaw.
In 1831, Earnshaw’s chronometer was issued to the Beagle for Charles Darwin and Captain Robert Fitzroy’s momentous voyage of discovery to the South Pacific.

Each of the clocks displayed here were produced by other skilled and renowned makers, but their incorporation of elements invented or developed by John Harrison pay testament to his legacy and enduring contribution to horology.

Credits: Story

This exhibit was curated with assistance from Laura Turner and Oliver Cooke, Curators of the Horological Collections at the British Museum.

You can see the objects depicted here by visiting the British Museum Clocks and Watches galleries, Rooms 38-39.

John Harrison's own marine timekeepers are on display at the Royal Observatory Greenwich, in their Time and Longitude gallery.

Credits: All media
The story featured may in some cases have been created by an independent third party and may not always represent the views of the institutions, listed below, who have supplied the content.
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