Harrison's first four time-keepers are still preserved at the Royal Observatory. He did not, however, receive his reward until a facsimile of the fourth had been made by his apprentice, Larcum Kendall. The latter is preserved at the Royal Observatory. There is a Larcum Kendall at the Royal Institution which is said to have been used by Captain Cook. Harrison's chronometer was sent on a trial voyage to Jamaica in 1761, and on its return to Portsmouth in the following year it was found that its complete variation was under the two minutes for which the Government had stipulated. Since Harrison's day the improvement of the chronometer has been carried on almost to perfection, and now the care and rating of chronometers for the Royal Navy is one of the most important duties of the Observatory. [Illustration: THE CHRONOMETER ROOM.] A visitor who should make the attempt to compare a single chronometer with a standard clock would probably feel very disheartened when, after many minutes of comparison, he had got out its error to the nearest second, were he told that it was his duty to compare the entire army here collected, some five hundred or more, and to do it not to the second, but to the nearest tenth of a second. Practice and system make, however, the impossible easy, and one assistant will quietly walk round the room calling out the error of each chronometer as he passes it, as fast as a second assistant seated at the table can enter it at his dictation in the chronometer ledgers. The seconds beat of a clock sympathetic with the solar standard, rings out loud and clear above the insect-like chatter of the ticking of the hundreds of chronometers, and wherever the assistant stands, he has but to lift his eyes to see straight before him, if not a complete clock-face, at least a seconds dial moving in exact accordance with the solar standard.