Early regulation adopted that of the original imported lantern clocks, namely a single foliot with two weights. These clocks had to have compound adjustment every morning and evening to follow the Japanese day and night hours, as well as taking account of the changing daylight hours/seasons.
Single foliot clocks were produced from the earliest dates from 1620 onwards, through to the abandonment of the traditional Japanese clock industry in 1873 – including use in conjunction with the Namagata dial (see later).
During the “middle” period (from 1780) the Japanese introduced an ingenious design that had two foliots. Different foliots were used during the daylight hours to that used at night.
There was an evolution to the mechanism that changed the foliots. At first the levers were moved by pins in two arms, later the pins vanished and the work was done by the two arms themselves. The arms carried a square cam located by a spring. Finally the lifting was done by triangular cams and the spring eliminated, the weight of the arm resting on one side of the triangles being enough to stop it turning.
In diagram opposite, pin at b on the count wheel a turns the wheel c once each revolution. Shaft (d) which is integral with wheel (c) carries the three sided cams which first engage one foliot (a) and then the other foliot. It will be seen that the weights of foliot (g) are right in for the short hours of the summer night and the weights on the other foliot are right out for the long day hours. These weights were normally shifted each fortnight as the seasons progressed. The upper foliot is that used during the day and the lower foliot that used at night.
The double foliot was phased out with the introduction of the Namagata dial.
Foliots of brass and steel usually have some form of decoration at the roots of the arms and are a push fit onto a square section of the pallet arbor. This makes it easier for repair and replacements.
It is probable that the double foliot was introduced about the end of the 17th century. At first the change-over levers were moved by pins in two arms, later the pins vanished, the work being done by the ends of the arms themselves; in both these cases the arms were located by a spring acting on a square cam. Finally the lifting was done by triangular cams and the spring was done away with, the weight of one arm resting on the side of one of the triangles being enough to stop it turning.