Ship Chronometer is a portable time keeping device of great accuracy, particularly one used for determining longitude at sea. Marine Chronometers are key-wound, spring driven which uses more precise mechanism than others. It uses fusee chain which winds on a cone shape drum to equalize the force of the spring as it unwinds. The balance wheel has a special combination of two metals that expands and contracts at different rate to compensate for temperature changes. It is housed in a very protective box with gimbal which holds the chronometer level with the horizon to prevent position changes that can alter its ability to keep precise time.