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 proposals for that purpose, and to grant sums of money not exceeding 2000L.

to aid in such experiments.