Harrison's Prize-Winning Watch, H4
John Harrison (1693 to 1776) · 1759
After three large sea clocks, John Harrison changed direction completely. The object in this case looks like an oversized pocket watch in a silver case, about thirteen centimetres across, and that is exactly what it is. Harrison had realised that a small, fast-beating balance was less disturbed by a ship's motion than the heavy balances of his earlier machines. H4 ticks five times a second, and its engraved silver dial and elegant hands give no hint of the revolution inside.
If the movement is displayed open, look at the richly engraved back plate and the diamond pallets of the escapement, chosen because diamond needed almost no lubrication. A remontoire rewinds a small spring eight times a minute so that the escapement always receives an even push. A bimetallic strip adjusts the balance spring as the temperature changes. Every one of these ideas was new, and together they produced a watch that lost only a few seconds on a voyage of eighty one days to Jamaica in seventeen sixty one.
The Board of Longitude was reluctant to believe that a watch could work so well and demanded more trials and full drawings. Harrison received part of the prize money over the years, but the final award came only after he petitioned King George the Third and Parliament in seventeen seventy three, when he was eighty. His son William helped throughout. The design was copied by Larcum Kendall in the watch K1, which Captain Cook carried on his second and third voyages and praised as his trusty guide; K1 is displayed in the same gallery. H4 is no longer run, to protect its mechanism, so what you see is the finished object at rest.
Did You Know? Harrison's H4 was so accurate on its first trial that the Board of Longitude suspected luck and ordered a second voyage.
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