Tidal/ High water clocks
This is a rarer form of moon-phase clock where calibration is also included to show times of high water at a particular location. These are probably around 1 in 10 of moon dial clocks so move into the realms of genuinely rare items. Tidal dials are seen through all eras of clocks, from the earliest penny moons through to later Victorian painted dials. The normal format is to have a series of roman numerals around the edge of the moon disk to show the time of high water when the moon is in a certain position. There are also some rarer exceptions showing high water times at several different locations, or displaying high water time on a different subsidiary dial.
Early types/ penny moons
With the first type of penny moon a separate inset in the dial was sometimes used to show high water time. This required extra calibration on the moon disk. Tidal dials with penny moons in the arch generally had the high water time engraved on a chapter ring along with the moon’s age. With this arrangement a hand fixed to the moon disk which points to the high-water time and enables adjustment. Occasionally two hands are provided, one for the moon’s age and one for the high water time, quite rare though.
Single pointer, arch moon, tidal dials
These have a single movable pointer, or hand, going across the moon disk and identifying the time of high water. Often the location (tidal river or port) is not specified although would probably be close to where the clock was made. In theory the pointer could be adjusted to show high water at any UK location, the moon’s age then identified from the position of the moon face. This arrangement is seen more often on Northern clocks, Newcastle, Durham etc.
Multiple pointer, arch moon, tidal dials
Similar to single pointer dials but may have two or more adjustable hands. These could be used to show high water at two different places, though often one is left in a vertical position to identify the moon’s age.
High water engraved for specific location
Instead of a movable pointer the dial has an engraved plaque identifying the port or river and this is read in conjunction with the moon’s age as the moon disk rotates. Earlier examples have the engraving on the disk itself, later ones on the main dial. Probably the most common is “high water at Bristol key” although I have encountered many different ones.
Rarer types of tidal dial
Sometimes high water for multiple locations are engraved on the arch above the moon disk, this is very rare.
Another one I have has a separate subsidiary dial which rotates to show when high water occurs at several different locations around the UK.
