CHRONOMETER. A marine timekeeper, used for deter mining longitude at sea. The word was originally used to denote any time-measuring instrument. The first instance of its employ ment in its accepted modern meaning is to be found in Jeremy Thacker's "The Longitudes Examined . . ." (London, 1714) . On the Continent the analogous term "chronometre" is still used to describe any high-class timekeeper, whether for marine purposes or otherwise.
History.—The possibility of determining longitude at sea by the use of a timekeeper was first pointed out by the Flemish astronomer Gemma Frisius, in a work on navigation published at Antwerp in 153o. At that date, however, and for long after wards, the mechanical difficulties in the way of constructing an accurate marine timekeeper appeared to be insurmountable. The first attempt to put Frisius' suggestion into practice was made in 1662-70 by the celebrated Dutch scientist, Christiaan Huygens, of Zulichem, who constructed several marine timekeepers con trolled by pendulums and subjected them to actual tests at sea. It was found that the timekeeping of the machines was quite unre liable, owing to the effects of temperature and of the ship's motion. Many later inventors fared no better, but during 1729-6o John Harrison, a self-taught Yorkshire carpenter, invented and con structed four practical marine timekeepers, with the fourth of which (now preserved, in going order, at Greenwich Observatory) he won the reward of L20,000 offered in 1714 by the British Gov ernment for any means of determining a ship's longitude within 3o nautical miles at the end of a six weeks' voyage. It may be noted that a timekeeper fulfilling this condition would have to keep time within three seconds per day—a standard which, at the date when the reward was offered, had not been attained by the best pendulum clocks on shore.
Harrison's mechanism, although unquestionably efficient, was complicated, delicate and costly—the Board of Longitude paid Larcum Kendall, a London watchmaker, £45o for a duplicate of the No. 4 timekeeper. Accordingly, it had little direct effect upon the evolution of the modern chronometer. But in 1765 Pierre Le Roy, of Paris, invented and constructed a marine timekeeper whose mechanism embodied, in an embryonic but perfectly recog nizable form, practically all the essential features of the modern chronometer. This machine, or a contemporary duplicate, is preserved in the Conservatoire des Arts et Metiers in Paris. Le Roy's work was followed up by Ferdinand Berthoud in France and by John Arnold and Thomas Earnshaw in England. The last-named produced, as early as 1785, several chronometers which, both in appearance and mechanism, are scarcely distin guishable from the machine of to-day.
Description.—The modern chronometer is, broadly speaking, a large, well-made watch, suspended in gimbals (a set of two bearings at right-angles, connected by a ring) and so poised as to remain horizontal whatever the inclination of the ship. It is thus safeguarded from those alterations of position which slightly affect the timekeeping of even the best watches. In addition, it differs somewhat in its mechanism from an ordinary watch, the spiral balance-spring and lever escapement of the latter being replaced by a helical balance-spring and a spring-detent or "chro nometer" escapement. This form of escapement is mechanically superior to any other, and requires no oiling, but it is unsuitable for use in pocket watches, as if stopped it has no tendency to re-start itself. For the purpose of equalizing the force of the mainspring, almost all chronometers are fitted with a device known as a "fusee," which has for some time past been discarded in watches, and their compensation-balances, by which the effects of heat and cold upon their timekeeping are practically nullified, also differ in some respects from the ordinary watch type, as well as being considerably larger and heavier.