An eight-day duration mahogany longcase regulator by George Graham, London, c.1750, known as 'Graham No. 3'
|Description||When the third Astronomer Royal, James Bradley (1692-1762), started work at the Royal Observatory in 1742 he found the instruments to be in poor condition and in 1749 he was granted £1,000 from George II to construct a new observatory building and install new instruments. The money was raised through the sale of old Naval stores and paid by the treasurer of the Navy. Graham 'number 3', as this clock is known, was purchased for £39 in 1750 for use alongside an 8.5 foot transit instrument by John Bird. Though called Graham number 3 it was in fact the fourth clock to be purchased from George Graham for use at the Royal Observatory. The first was of one-week duration, purchased for £5 in 1721, and was initially used by Edmond Halley (1656-1742) as a transit clock but is not recorded after 1818. Graham number 3 had an incredible 174 year working history and during that period it has had many alterations and improvements. When initially supplied to Bradley it was of one-month duration, had a gridiron pendulum suspended from the back cock and the hours would have been displayed on a silvered disk through an upturned lunette. Thomas Earnshaw mentions in his Appeal that John Arnold ‘spent the utmost of his mechanical strength to make the Greenwich clock perfect’. From the observatory records we know that Arnold first fitted ruby pallets in 1771 and in 1779 he fitted Harrison’s maintaining power, jewelled the escapement and later fitted an independent brass suspension when the clock was moved to a stone pier in 1780. Nine years on Larcum Kendall fitted a solid brass pendulum bob and a regulating ball. From the observatory records we know that Thomas Earnshaw adjusted the clock in 1793, reducing the motion work from three wheels to one and also modified the gridiron pendulum. Graham number 3 ceased to be used as the transit clock in 1821 when it was moved to the chronometer room in the equatorial building. This was not the first time the regulator had been used to rate marine chronometers; between 1770 and 1772 it was used to rate K1, Larcum Kendall’s first copy of Harrison’s fourth marine timekeeper. Between 1828 and 1833 this regulator was used in the Quadrant room for pendulum experiments by Captains Sabine and Kater. In 1833 Graham number 3 was adjusted to mean solar time and used for dropping the time ball and in 1856 it was fitted with electro magnetic pendulum control governed by the Shepherd master clock. It was used in this manner until around 1924 when it was used in various offices of the Royal Greenwich Observatory. The movement has typical Graham/Shelton chamfered corners to the tops of the plates which are united by six latched and knopped pillars. The four wheel train is driven by a black painted cylindrical weight suspended on a double line on a narrow diameter going barrel (19 turns) which carries the great wheel and Harrison’s maintaining power. All wheels have four curved crossings except for the light steel escape wheel which has six straight crossings. The escapement is fully jewelled; the long shank pallets have ruby nibs and are attached to the pallet arbor by a long brass collet. The crutch is fitted by a split collet allowing for beat adjustment and runs between two guard pins mounted on a plate screwed to the backplate. All of the rear pivots are fitted with double screwed brass end plates. The movement is screwed down to the seatboard via four brackets, and fixed to the backboard by two further brackets. The large brass suspension block is screwed to the wall with a corresponding cut-out to the backboard of the case. The mercurial temperature compensated pendulum has a steel rod and glass jar held within a stirrup, the lower section of which is signed ‘RICHARDSON, Royal Observatory’ with blued steel pointer to the silvered pendulum scale reading 0-2.5 degrees. The 12 inch square silvered-brass regulator dial has outer minute circle with arabic five-minute marks enclosed by an outer scribe line and inner minute track. The subsidiary seconds dial with observatory marks at five second intervals also has arabic numerals enclosed by an outer line and inner track. Situated above the cursive signature, ‘Geo. Graham, London’, is the twenty-four hour subsidiary dial, recessed within an elliptical aperture, which runs counter clockwise. The blued steel hands to the seconds and hour subsidiaries are counter-poised, the minute hand is round sectioned and tapering and secured by a domed brass collet and taper pin. There is a filled circular hole next to the twenty second mark. The rear of the dial has been refinished, curled and lacquered. The dial is screwed to four dial feet, each riveted to chamfered plates fixed to the front plate by means of a screw and steady pin. The case is veneered in straight grain mahogany. The flat topped hood has a concave moulding to the cornice, panelled sides and locking front door with internal turn buckle locking the hood to the trunk. The trunk has a moulded rectangular door (270 x 845 mm) with three strap hinges, on a rectangular base with moulded double plinth.|
|Date made||circa 1750|
|Credit||National Maritime Museum, Greenwich, London, RGO Collection|
|Measurements||Overall: 1840 x 450 x 270 mm|
Do you know more about this?Share your knowledge