We shall close the history of navigation with a brief account of the methods which have been employed for determining the longitude at sea, by means of chront,: meters, or accurate time-keepers. The daily rotation of the earth on its axis being performed with the most per fect uniformity, it is evident that the sun must, in the course or twenty•four hours, be twice in the same plane with the meridian of every place, viz. at noon and mid night ; and, consequently, that the angle formed by any two meridians, or the measure of their inclination, which in fact is their difference of longitude, may be estimated by the thne which elapses between the sun being- thus in succession in each of their planes. Could, therefore, an accurate time-keeper, duly regulated to the noon of some particular place, be transported, without any change in its rate, to the meridian of some other place, the differ ence between the noon given by the time-keeper, and the noon of the place indicated by the sun, would afford, after a proper allowance for the equation of time, a simple method of determining their difference of longitude. Ac cordingly, the British government offered, in 1714, a re ward or 20,000/. to any individual who should construct a time-keeper, capable of being used at sea, and kecping time with sufficient accuracy to determine the longitude within 30 geographical miles ; and, at the same time, to encourage artists of ingenuity to engage in the compe tition, smaller sums were promised, in cases where the approximation might deviate more widely from the truth. This proposal, so worthy of a generous and enlightened govermnent, was somewhat modified by a new act of parliament, passed in 1774, by which a reward of 5000/. was offered for the invention of any time-keeper, or other method, capable of determining the longitude within 60 geographical miles, and double that sum, if it came within 30 miles of the truth. The commissioners of the board of longitude were at the same time impowered to grant 5000/. to the author of any lunar tables, which should not
deviate more than 15" in assigning the moon's actual place, and such smaller sums, at discretion, as might prove conducive to the main design of the act. In conscquence of these encouragements, Mr. John Har rison, a most ingenious artist, Wi10 had long del oted his attention to the improvement of chronometers, and dis tinguished himself by several important inventions con nected with horology, was induced to attempt the con struction of a chronometer that 'night be entitled to the reward held out by the government of his country. Having finished his first time-keeper, its going was suf ficiently correct to make him aim at higher improve ments ; and, after repeated attempts, prosecuted with the most unwearied perseverance, he at length produced a time-keeper, which satisfied so completely the condi tions of the act of parliament, that, in 1765, the commis sioners of longitude ordered the sum of 10,000/. to be paid to him, on condition of his making a full disclosure of the mechanism of his watch. The principles on which it was consoucted were accordingly laid before the pub lic, in a treatise which Harrison published in 1767, and which has frequently been cited as a proof that great in ventive powers may belong to a mind which is nearly des titute of the faculty of arranging, in a plain and systema tic form, the 'objects of his own conceptions. Notwith standing the obscurity, however, in which Harrison's ac count of his time-keeper was written, it was sufficiently intelligible to enable artists to construct similar watches; and many chronometers have since been made, which have been found to answer the purpose equally well with that of the original inventor. It is only to be regretted, that the high price of those valuable mechanical con trivances has hitherto confined them, in a grcat measure, to the Navy, and the ships employed in the service of the East India Company.