Modern time-reckoning in equal hours was not introduced into Japan until 1870. Before this time, Japan used a 'temporal hours' system, similar to that of Europe's in the middle ages, the period of dawn to dusk being divided into six equal hours and the period of darknes also into six equal hours. A daylight hour was in general of different length from a night hour and both varied according to season. This clock has been provided with two separate timekeeping mechanisms, one to be used during the day and the other at night. The change-over takes place automatically as the hour of twilight strikes at dusk and dawn. The length of day and night hours can be adjusted by moving the weights suspended from the two foliot balances to alter their period of swing.
© Science Museum / Science & Society Picture Library