ory at Greenwich we
can, by comparing the two local times, determine the difference of
local times, or, what is the same thing, the difference of longitude
between the two places. It was necessary therefore for the navigator to
be in possession of a first-rate watch or chronometer, to enable him to
determine accurately the position of his ship at sea, as respected the
longitude.

Before the middle of the eighteenth century good watches were
comparatively unknown. The navigator mainly relied, for his
approximate longitude, upon his Dead Reckoning, without any observation
of the heavenly bodies. He depended upon the accuracy of the course
which he had steered by the compass, and the mensuration of the ship's
velocity by an instrument called the Log, as well as by combining and
rectifying all the allowances for drift, lee-way, and so on, according
to the trim of the ship; but all of these were liable to much
uncertainty, especially when the sea was in a boisterous condition.
There was another and independent course which might have been
adopted--that is, by observation of the moon, which is constantly
moving amongst the stars from west to east. But until the middle of
the eighteenth century good lunar tables were as much unknown as good
watches.

Hence a method of ascertaining the longitude, with the same degree of
accuracy which is attainable in respect of latitude, had for ages been
the grand desideratum for men "who go down to the sea in ships." Mr.
Macpherson, in his important work entitled 'The Annals of Commerce,'
observes, "Since the year 1714, when Parliament offered a reward of
20,000L. for the best method of ascertaining the longitude at sea, many
schemes have been devised, but all to little or no purpose, as going
generally upon wrong principles, till that heaven-taught artist Mr.
John Harrison arose;" and by him, as Mr. Macpherson goes on to say, the
difficulty was conquered, having devoted to it "the assiduous studies
of a long life."

The preamble of the Act of Parliament in question runs as follows:
"Whereas it is well known by all that are acquainted with the art of
navigation that nothing is so much wanted and desired at sea as the
discovery of the longitude, for the safety and quickness of voyages,
the preservation of ships and the lives of men," and so on. The Act
proceeds to constitute certain persons commissioners for the discovery
of the longitude, with power to receive and experiment upon pr