attains his meridian, then the place must be in long. 15 degrees west of the meridian of Greenwich. It is not indispensably necessary, that every chronometer used for maritime purposes should keep time exactly with that of the Greenwich observatory, or of any other instrument of known excellence, provided always that its _rate_ as seamen call it, or the daily loss or gain of the chronometer, is well ascertained, and so may be computed in the calculations to be made. The indispensable requisite of a chronometer, however, is that the daily loss or gain shall not vary materially from itself at different periods, or under the changes of temperature of different climates, and these qualities being found in an instrument of any shape or make, constitute a marine chronometer. It will be generally obvious of what immense and universal importance it was for men who 'go down to the sea in ships and do their business on the great waters' to be provided with a chronometer, and so be enabled to calculate with a great degree of nicety,--almost as a traveller by land learns his distances by milestones and finger-posts,--the precise position on the wide ocean of the vessel they are engaged in navigating. So impressed was the British Parliament with the value of such an invention, that as early as 1714, in the reign of Queen Anne, a reward of £10,000 was offered, for any method for determining the longitude within the accuracy of one degree; of £15,000 within the limit of 40 geographical miles; and of £20,000 within the limit of 30 geographical miles, or half a degree, provided such method should extend 80 miles from the coast. In 1736 John Harrison invented the first chronometer, for which, after having added many improvements, he received the gold medal of the Royal Society in 1749. He still continued to persevere in improvements in his instrument, and at last applied to be allowed to test its powers in such a voyage as might permit of proof of its value. After some time his application was granted, and his son, William Harrison, embarked at Portsmouth, Nov. 18, 1761, for Jamaica. After eighteen days sailing the vessel was computed to be 13° 50´ west of Portsmouth, when the distance calculated by the watch was 15° 19´. When the vessel arrived at Madeira, on the 9th of December, it was found that the reckoning was corrected by the time of the piece, about a degree and a half. From Madeira to Jamaica the reckoning was amended 3°; and at the several islands where the ship touched the known longitudes agreed very closely with those indicated by the chronometer. Upon having returned again to England after a very stormy voyage, the instrument underwent examination, and its entire error amounted to 1^m. 53^s. 5. Harrison, on this report being made, obtained from Parliament a reward of £5000. A second experiment was afterwards made in 1764, in March of which year Harrison left Portsmouth with his instrument on board the Tartar for Barbadoes.