By this time, John Harrison had finished his
fourth timepiece--the small one, in the form of a watch. At
length William Harrison set sail with this timekeeper from
Portsmouth for Jamaica, on November 18th, 1761, in the Deptford
man-of-war. The Deptford had forty-three ships in convoy, and
arrived at Jamaica on the l9th of January, 1762, three days
before the Beaver, another of His Majesty's ships-of-war, which
had sailed from Portsmouth ten days before the Deptford, but had
lost her reckoning and been deceived in her longitude, having
trusted entirely to the log. Harrison's timepiece had corrected
the log of the Deptford to the extent of three degrees of
longitude, whilst several of the ships in the fleet lost as much
as five degrees! This shows the haphazard way in which
navigation was conducted previous to the invention of the marine
chronometer.
When the Deptford arrived at Port Royal, Jamaica, the timekeeper
was found to be only five and one tenth seconds in error; and
during the voyage of four months, on its return to Portsmouth on
March 26th, 1762, it was found (after allowing for the rate of
gain or loss) to have erred only one minute fifty-four and a half
seconds. In the latitude of Portsmouth this only amounted to
eighteen geographical miles, whereas the Act had awarded that the
prize should be given where the longitude was determined within
the distance of thirty geographical miles.
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