Ferguson Tidal Clock

Originally posted on the RetroMat website, but fits better here.

Source: The Antique Clock Company

In the August 2022 edition of Clocks Magazine, there is an article by Brian Loomes about the astronomical clocks with tidal indications of Phillip Lloyd based on a mechanism devised by astronomer James Ferguson.

One of the clocks shown in the article - the Hotwells one - was for sale as at August 2022 (and still seems to be so as at May 2026). The dial is a thing of great beauty, although perhaps not intuitive to understand.

There is a very good explanation of the dial and mechanism used by Ferguson, albeit in dated language, in the Encyclopaedia Londinensis published in 1811, which I've copied into this document for ease of reading, with some alterations:

• the low-resolution black and white diagrams accompanying the text have been replaced with high resolution images of the original colour plates by J. Pass, 1809, from the Wellcome Collection
• annotations have been made to one of the figures and some footnotes added
• updated spelling of words where the archaic long 's' is used.

With regard to the Hotwells clock by Lloyd: the indicator used by Ferguson to show high water is not present, perhaps having been replaced by the pair of 'spikes' on opposite sides of the central disk representing the Earth. If this is so, then because these are offset to the 'tidal bulge' around the Earth, this depiction corrects the misrepresentation of high water in the Ferguson clock.

However, such a device would seem to be less accurate in use, being some distance from the dial on the sun plate.

Also, the Hotwells clock has two additional, centrally-mounted hands. Presumably the straight hand is for showing seconds, with the other for minutes, the hours being indicated by the short hand with the sun symbol mounted on the sun plate.