Marine chronometer
H4 departs on its first Jamaica sea trial
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18 November 1761: H4 goes to sea
On 18 November 1761, William Harrison sailed from Portsmouth aboard HMS Deptford with H4, beginning the watch's first transatlantic sea trial. The Commissioners of Longitude had authorized a Jamaica voyage, and William took charge of the timekeeper because his father was elderly.
The marine chronometer was not invented that year. H4 was already dated and recorded as completed in 1759; what 1761 marks is a live test aboard a rolling Royal Navy ship.
Under four locks
For the trial the watch was kept in a box secured by four different locks, and could be handled for winding only in the presence of the keyholders.
Its job was to carry Portsmouth reference time across the ocean, for comparison with local solar time in Jamaica. A chronometer does not display longitude; the navigator gets it from the difference between the two.
Madeira, and an argument about seconds
On the outward voyage William used H4 to predict that Madeira would be sighted earlier than the crew expected. The correct prediction impressed Deptford's captain.
The ship reached Port Royal on 19 January 1762, after 80 days. The celebrated 5.1-second result was Harrison's own calculation after applying a daily rate; without that correction the reported error was 3 minutes 37.9 seconds, and in August 1762 the Commissioners ruled the experiments insufficient to determine longitude at sea.
Sources
Researched 23 Aug 2026 4 sources 2 audit passes Date corrected
How this was checked
Researched from the web into a fact sheet, rewritten from that sheet, then audited against it by a different model. How the pipeline works →
Date corrected. The research places this at 1761-11-18.
What the sources leave uncertain
- The departure date is well supported as November 18, 1761. The often-repeated statement that H4 'lost only five seconds' requires qualification: 5.1 seconds was Harrison's calculation after applying a daily rate.
- Without that rate correction, the reported error was 3 minutes 37.9 seconds. Official figures were not published, and in August 1762 the Commissioners ruled that the experiments were insufficient to determine longitude at sea.