Astronomical regulator by Ellicott & Co

Catalogue description — Royal Museums Greenwich

This regulator was designed for use in temporary field observatories such as those set up to observe the transits of Venus of 1761 and 1768. This clock was purchased by the Museum in 1976 and unfortunately its working history is unknown. However, we do know that Ellicott did supply a precision clock to the Royal Society for observation of the 1761 Transit of Venus at the Cape of Good Hope and in 1768 at Hudson’s Bay. The unusual mahogany case serves a dual purpose. It protects the movement and pendulum when set running and serves as a packing case when in transit. The x-frame base can be unbolted and stowed inside along with the clock movement, pendulum and driving weight. The inner sides of the case are fitted with mouldings that form runners, which would have probably have accommodated shaped planks that, when inserted, locked the loose parts into place. These locating planks are no longer with the clock. The dial has a distinct shape, typical of Ellicott’s best precision clocks, with regulator layout and fine regulation dial to the arch; it is attached to the stoutly constructed eight-day movement by ornately turned latched dial feet. The shouldered plates are also latched and separated by six decorative knopped pillars. The clock has a Graham type dead-beat escapement and bolt and shutter maintaining power. Maintaining power is essential to a weight-driven regulator as it prevents the clock from running backwards during winding, thus protecting the delicate escapement and –most importantly - the clock’s timekeeping. The maintaining power on this clock features a refinement known as a draw-back piece, which prevents the bolt from damaging the teeth on the centre wheel and was probably invented by contemporary regulator maker, John Shelton. This clock has Ellicott’s own form of temperature compensated pendulum, which harnesses the relative expansive properties of brass and steel in a different manner to that found in John Harrison’s gridiron. Ellicott published a description of this invention in the Philosophical Transactions of the Royal Society in 1752. Unlike the gridiron or plain pendulum, Ellicott’s pendulum does not have a rating nut below the bob. Regulation is achieved by altering the effective length of the suspension spring by moving the regulation hand on the dial. The hand is attached to worm gear which drives what is in effect the rating nut and one full turn of the rating hand will turn the rating nut by one fortieth of a turn allowing for fairly precise adjustment against the arbitrary scale on the dial.