John Shelton Regulator c.1764
John Shelton was a famous London clockmaker. He made five astronomical regulators for the Royal Society for timing the transits of Venus in 1761 and 1769. Regulators were accurate clocks used specifically for timing transit observations to the exact second.
This regulator, in a veneered mahogany case, dates from c. 1764 and measures 1905 x 440 x 240 mm. The brass face measures 300 x 300 mm and is signed 'John Shelton London'. It comprises a large outer dial, of 285 mm diameter, with minutes marked from 5 to 60, a smaller inner dial, of 130 mm diameter, with seconds marked in the same way, and a rotating inset display from which the hour is read. The hands are of lacquered, blued steel.
The following description of a typical Shelton regulator, taken from D. Howse and B. Hutchinson The clocks and watches of Captain James Cook 1769-1969 (London: Antiquarian Horological Society, ) pp. 289-291, is based upon the St John's clock.
The basic movement consists of a pair of shaped brass plates ... The average dimensions in the series are approximately 10.25 x 6.25 in. The six pillars are riveted to the back plate; the front plate is fastened by latches.
The train has five wheels and is constructed for a month's duration. Where feasible, the pivots bear on end-plates which minimise friction and preserve the oil. One large end-plate covers all pivot holes on the back plate ...
The escapement is dead-beat, and has a 30 tooth brass wheel with relieved teeth; the anchor is steel and has a long shank and curved arms which terminate in the pallets. The collets for the escape wheel and anchor are characteristically long ...
Bolt-and-shutter maintaining power is fitted, with an additional device to prevent the clock stopping while the mechanism is being engaged. Although the movement is weight driven, stopwork (acting on a principle similar to that used in fuzee clocks) is fitted to prevent overwinding.
The motion work is conventional; the hour-wheel pipe carries a friction-mounted hour circle instead of an hour hand. The cannon pinion and minute wheel are only partially crossed out, and thus counterpoise the minute hand. The minute wheel and pinion are pivoted on a cock screwed to the front plate, and the arbor is extended to pivot on the back plate.
The dial is a square plate of brass - engraved, waxed, and silvered ... The dial is mounted on four pillars and can be removed by the screws ... The pillars are turned with wide feet, and are located with steady pins and screwed to brass plates which in turn are fastened to the front plate ...
The pendulum is of gridiron construction and is suspended from the back cock by a steel strip. The crutch is steel and has a brass pin which engages in a slot in the central rod ...
Shelton's punch-mark on the Cambridge movement is on the front plate ...
The movement is rigidly mounted, and is secured to the seat-board with four brass holdfasts and screws. Two more holdfasts are screwed to the back plate, and line up with brackets screwed to the back-board.
This clock was given to the College, with other instruments for the College observatory, by the astronomer and surveyor Richard Dunthorne (1711-75), who was employed as butler of Pembroke College and assistant to Dr Roger Long, Master of Pembroke and Lowndean Professor of Astronomy and Geometry. It appears to have come to the College in late 1764 or early 1765. William Ludlam's Astronomical observations made in St John's College, Cambridge, in the years 1767 and 1768, with an account of several astronomical instruments(Cambridge, 1769) states that 'Before the clock at St John's College was placed in the observatory, it stood in a private room fixt to the wainscot only: during that time, from Jan. 8, 1765, to July 19, 1766, a register of it's going was constantly kept' (p. 40). The College observatory was situated atop the Shrewsbury tower in Second Court. Ludlam describes the fixing of the clock in the observatory as follows:
'Two pieces of oak (5 ½ inches broad, 3 thick and 24 long) were fixed to the wall of the observatory, by screwed bolts passing quite through it. The upper of these pieces, crossed the case of the clock, just above the rising-board [the board on which the wheel-work stand], the lower just above the bottom of the door of the case. The back of the case was screwed to these transverse pieces, by screws 0,4 inches diameter, and 3 inches long, which (having square heads) were forced by a long spanner, so as to cut their own threads quite through the oak. As the back of the case is a single board only half an inch thick, and the frame-plates on which the pendulum hangs, are fastened to it, to strengthen that part, a mahogany board an inch thick, and 6 inches wide, was put cross the back (but not glued to it) just above the rising board, and the screws before mentioned, passing through the mahogany board, pinch the whole back between that board and the upper oak piece. Nor does the clock stand upon the floor, but on a stone lying on the arch described in paragraph 6, and consequently is not liable to be disturbed by treading on the floor.' (p. 37).
Ludlam's register of the clock in the observatory starts in July 1767. It performed excellently and Ludlam noted that 'the clock at St John's College has been more regular than that at Greenwich' (p. 40).
The clock remained in the observatory until its closure in 1859, and according to a writer in the College magazine was used 'with complete satisfaction to all the observers' (The Eagle VII (1871), 335). Where the clock was housed for the next century is uncertain, but in 1966 it was deposited on loan at the National Maritime Museum. Here it remained until its return to the College in 1992. It is now housed in the Library Exhibition Area.