A Mariner's ‘equinoctial dial’ of 1634?

Extract

Recently the author was asked to suggest how an old (Scottish?) sun-dial, inscribed on a slate slab about 50 cm across and 3 cm thick, could be put into working order and set up correctly to tell the time. Upon examination the dial was found to be of considerable historic interest. Although it was quite a well-known type of dial—an equatorial—the furniture and inscriptions suggested a definite connection with the sea. Moreover, no other identical instrument is known to exist; possibly the publication of this note may help to bring other specimens to light. For this reason it is felt that a description of the various features of this multi-purpose dial will be of some interest.

Both surfaces of the dial are shown in the accompanying photographs. For clarity, a fine talcum powder was sprinkled on the dial to fill all the incised markings which would otherwise be invisible. Unfortunately the gnomon is missing. This gnomon would have passed vertically through the dial extending on either side for not less than 10 cm. Probably two circular discs with pointers were mounted centrally on the upper surface, with one of these discs bearing an arm with the ‘dioptera’, but unfortunately these items are also missing. The discs would have been capable of rotating around the gnomon. It would appear that some modifications became necessary after its construction as the four holes around the inner circle have spoilt the lettering.