The relationship between Pilkington and Gibbs was not a happy one.
As the inventor, Gibbs wanted to call the Company, “Gibbs and Pilkington”, but Pilkington insisted that it was the other way round, and, as is usually the case, the one putting up the money prevailed. Gibbs retaliated by having GIBBS INVENTIT engraved on many of the heliochronometers.
Unknown to Gibbs, Pilkington started work on an alternative design, which he patented, and which culminated in the manufacture of the Sol Horometer. He used the existing base, and the resulting dial involved much less engineering, but a lot more engraving.
Unfortunately, the First World War intervened, and by the time that was over, the need for heliochronometers had passed into history. Only about 50 Sol Horometers were ever made. If you know the whereabouts of one (or more), please let me know. Unlike the original P&Gs, not all Sols have serial numbers.
Fig 1 shows the upper part of the dial in pieces. There is an inner date wheel, which sits inside the outer date wheel. The inner wheel is engraved with the year’s dates. The outer wheel has both dates, and time markings, see Fig 2. The outer wheel can be rotated, and to use the dial, it is moved so that today’s date is matched with the same date on the inner wheel. This action causes the time scale on the outer rim of the dial to be moved the correct amount to account for the Equation of Time on the date selected.
There is a light box mounted on the top of the dial, which has a thin slit in each end. To read the time, the light box is rotated until the sun’s light shines through both slits.
The pointer at one end of the light box indicates the time on the outer time scale, and can be offset via a screw and slot to allow for the longitude of the site - Fig 3