Harrison's No.4 chronometer.
A chronometer is an extremely accurate clock, especially one used in connection with celestial navigation at sea (see also celestial sphere). It differs from an ordinary clock in that it has a fusee, by means of which the power transmission of the mainspring is regulated such that it remains approximately uniform at all times; and a balance made of metals of different coefficients of expansion to minimize the effects of temperature changes. The device is maintained in gimbals to reduce the effects of rolling and pitching. A chronometer's accuracy is checked daily and its error noted; the daily change in error is termed the daily rate.
Chronometers are always set to Universal Time (Greenwich Mean Time). The first chronometer was invented by John Harrison (1735).