At the same time, the other method, that by the time-keeper or chronometer, was practically tested by him. The time-keeper constructed by John Harrison had been tested by a voyage to Jamaica in 1761, and now, in 1763, another time-keeper was tested in a voyage to Barbadoes. Charles Green, the assistant at Greenwich Observatory, was sent in charge of the chronometer, and Maskelyne went with him to test its performance, in the capacity of chaplain to his Majesty's ship Louisa. [Illustration: HADLEY'S QUADRANT. (_From an old print._)] The position which Maskelyne had already won for himself as a practical astronomer, and the intimate relations into which he had entered with Bradley and Bliss, made his appointment to the Astronomer Royalship, on the death of the latter, most suitable. At once he bent his mind to the completion of the revolution in nautical astronomy which his _British Mariner's Guide_ had inaugurated, and in the year after his appointment he published the first number of the _Nautical Almanac_, together with a volume entitled, _Tables Requisite to be Used with the Nautical Ephemeris_, the value of which was so instantly appreciated, that 10,000 copies were sold at once. The _Nautical Almanac_ was Maskelyne's greatest work, and it must be remembered that he carried it on from this time up to the day of his death--truly a formidable addition to the routine labours of an Astronomer Royal who had but a single assistant on his staff. The _Nautical Almanac_ was, however, in the main not computed at the Observatory; the calculations were effected by computers living in different parts of the country, the work being done in duplicate, on the principle which Flamsteed had inaugurated in the preparation of his _Historia Coelestis_.