Object Description
A most rare tidal clock devised by James Ferguson (1710-1776), a driven astronomer and one of the most highly regarded and successful popularisers of natural science in the 18th century. The mechanics are described and illustrated in Ferguson’s 1773 compendium Select Mechanical Exercises: Shewing how to construct different Clocks, Orreries and Sundials. where he notes it requires “…only two Wheels and a Pinion added to the common Movement”. The detail is now available to read online in Encyclopaedia Londinensis for 1811 – see page 332 at this link – https://books.google.co.uk/books?id=SFcMAQAAMAAJ&printsec=frontcover#v=onepage&q&f=false
This clock is unsigned and dates from the third quarter of the 19th century, but adopts Ferguson’s principles.
Movement
The 8-day timepiece fusee movement has four turned and ringed pillars, and in addition to the usual timepiece gearing structure, has an extra wheel for the moon’s aging (see note 1).
Dial
The main dial plate shows a 24-hour sequence showing I to XII twice.
Effectively, the hands for this clock are rotating disks of the same size;
– the forward disk has a slot and a hand decorated as a moon; This disk is decorated with markings for time of HIGH TIDE AT LONDON BRIDGE, TIME OF HER SOUTHING, an additional aperture for the moon’s age, a blue painted ellipse with markings for high and low water, and two wires within an aperture to indicate the time of high water and the age of the moon.
– the second disk sits behind the first and has a hand decorated as a sun. The sun decorated hand indicates the time of day, as shown on the 24-hour dial behind.
Visible through the slot and painted onto the inner disk is the moon phase in days from 1 to 29½, and the Time of High Water I to XII twice (subdivided into quarter hours). One wire is directly in line with the moon hand which extends beyond the disk. This shows the time of the moon’s southing,or the time at which the moon passes the meridian.
In London there is a two and a half hour gap between these two times.
As the sun goes round the dial plate in 24 hours, and the moon in 24 hours, 50½ minutes, the moon makes 28.5 revolutions whilst the sun makes 29.5 revolutions.
There is a further circular aperture showing the age of the moon age graphically.
These functions are created by having two equal diameter wheels, one with 57 teeth, and the other with 59, both being driven by the same 19 tooth pinion. This pinion rotates three times in 24 hours.
Note 1:
There is a systemic inaccuracy inherent in the gearing of the clock. The sun hand (for time) is affixed to the wheel of 57 teeth. The moon hand is fixed to the wheel of 59 teeth. The pinion itself turns once in 8 hours, or three times in 24 hours. Therefore the 59 tooth wheel goes around in (8 x 59)/19=24.842105 Hours.
This equates to 24hrs 50.5263mins which is in excess of that required by 0.0263mins/day, or 16 hours per 100 years.
According to tide / moon tables, the reading given today is out by about 25½ hours; this cumulative error suggests (very crudely) that the clock dates from c.1860.
This is a really interesting horological item. If you want to calculate the time of high tide at London Bridge to negotiate your tall ship through the bridge, this is NOT the one for you. But it has a great deal of charm and character.