John Harrison

56
John Harrison bigraphy, stories - English clockmaker, horologist and inventor of the marine chronometer

John Harrison : biography

24 March 1693 – 24 March 1776

John Harrison (24 March 1693 – 24 March 1776) was a self-educated English carpenter and later a clockmaker. He invented the marine chronometer, a long-sought device in solving the problem of establishing the East-West position or longitude of a ship at sea, thus revolutionising and extending the possibility of safe long distance sea travel in the Age of Sail. The problem was considered so intractable that the British Parliament offered a prize of £20,000 (comparable to £ in modern currency) for the solution.

Harrison came 39th in the BBC’s 2002 public poll of the 100 Greatest Britons..Accessed: 10 February 2012.

The longitude watches

After steadfastly pursuing various methods during thirty years of experimentation, Harrison moved to London in late 1758 where to his surprise he found that some of the watches made by Graham’s successor Thomas Mudge kept time just as accurately as his huge sea clocks. It is possible that Mudge was able to do this after the early 1740s thanks to the availability of the new "Huntsman" or "Crucible" steel produced by Benjamin Huntsman sometime in the early 1740s which enabled harder pinions but more importantly, a tougher and more highly polished cylinder escapement to be produced.see Crucible steel Harrison then realized that a mere watch after all could be made accurate enough for the task and was a far more practical proposition for use as a marine timekeeper. He proceeded to redesign the concept of the watch as a timekeeping device, basing his design on sound scientific principles.

The ‘Jefferys’ watch

He had already in the early 1750s designed a precision watch for his own personal use, which was made for him by the watchmaker John Jefferys 1752 – 53. This watch incorporated a novel frictional rest escapement and was not only the first to have a compensation for temperature variations but also contained the first ‘going fusee’ of Harrison’s design which enabled the watch to continue running while being wound. These features led to the very successful performance of the "Jefferys" watch so therefore Harrison incorporated them into the design of two new timekeepers which he proposed to build. These were in the form of a large watch and another of a smaller size but of similar pattern. However only the larger No. 1 (or "H4" as it sometimes called) watch appears ever to have been finished. (See the reference to "H6" below) Aided by some of London’s finest workmen, he proceeded to design and make the world’s first successful marine timekeeper that allowed a navigator to accurately assess his ship’s position in longitude. Importantly, Harrison showed everyone that it could be done by using a watch to calculate longitude. This was to be Harrison’s masterpiece – an instrument of beauty, resembling an oversized pocket watch from the period. It is engraved with Harrison’s signature, marked Number 1 and dated AD 1759.

Harrison’s "Sea Watch" No.1 (H4), with winding crank

Harrison’s First Marine Watch

This watch was referred to by Harrison as his first "Sea watch" but denoted "H4" by Gould in order to differentiate it from the first ‘Sea clock’. Of large size, it is housed in silver pair cases some in diameter. The movement is for the period highly complex but resembling a conventional movement of this period but of much larger size. It has a novel type of ‘vertical’ escapement, which due to its appearance is often incorrectly associated with the ‘verge’ escapement which it superficially resembles. However, its action is completely different to the verge as being a frictional rest escapement it enables the balance to have a large arc. This is quite unlike the verge which is a different escapement altogether having recoil with a limited balance arc, and moreover sensitive to variations in driving torque.

The D shaped pallets of Harrison’s escapement are both made of diamond, a considerable feat of manufacture at the time. For technical reasons the balance was made much larger than in a conventional watch of the period, and the vibrations controlled by a flat spiral steel spring. The movement also has centre seconds motion with a sweep seconds hand. The Third Wheel is equipped with internal teeth and has an elaborate bridge similar to the pierced and engraved bridge for the period. It runs at 5 beats (ticks) per second, and is equipped with a tiny remontoire. A balance-brake stops the watch half an hour before it is completely run down, in order that the remontoire does not run down also. Temperature compensation is in the form of a ‘compensation curb’ (or ‘Thermometer Kirb’ as Harrison called it). This takes the form of a bimetallic strip mounted on the regulating slide, and carrying the curb pins at the free end. During its initial testing, Harrison dispensed with this regulation using the slide, but left its indicating dial or figure piece in place.