CHRONOMETER

timekeeping device of great accuracy, particularly one used for determining longitude at sea. The word was originally employed to denote any time-measuring instrument. Early weight- and pendulum-driven clocks were highly inaccurate at sea due to temperature changes and the ship's motion. It was not until the 18th century that John Harrison, a self-taught English carpenter, invented and constructed four practical marine timekeepers, the fourth of which won him the reward of 20,000 offered in 1714 by the British government for any means of determining a ship's longitude within 30 nautical miles (34 miles) at the end of a six week's voyage. A timekeeper fulfilling this condition would have to keep time within three seconds per day, a standard that, at the date the reward was offered, had not been attained by the best pendulum clocks on shore. Though Harrison's original invention was complicated, delicate, and costly, his success led to further investigations by others and eventually to the modern marine chronometer. The modern chronometer is, broadly speaking, a large, heavy, well-made watch, suspended in gimbals (a set of two rings connected by bearings) 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 the ordinary watch, the spiral balance spring and lever escapement of the latter being replaced by a helical balance spring and a spring detent, or chronometer escapement. This form of escapement is mechanically superior to any other and requires no oiling other than at the pivots, but it is not suitable for use in pocket watches, because if given a more or less circular twist, it will "trip," causing a rapid gain. For the purpose of equalizing the force of the mainspring, almost all chronometers are fitted with a fusee (no longer used in watches) and chronometer compensation balances, by which the effects of heat and cold upon their timekeeping are practically nullified. By these devices the chronometer may provide timekeeping accurate to within 0.1 second per day.