- Museum number
SOLAR/SIDEAREAL LONGCASE REGULATOR
Mahogany case with raised panels at the sides of the hood and on the plinth.
Silvered brass dial with an outer ring for sideral minutes and three subsidiary rings down the middle with sidereal hours minutes and seconds with a separate rotating disc in the middle of each for the mean solar equivalents.
Weight-driven month going movement with dead-beat escapement with jewelled pallets and high-quality gear train.
- Production date
Height: 2.10 metres
Width: 48.50 centimetres
Depth: 25.50 centimetres
- Curator's comments
- Text from 'Clocks', by David Thompson, London, 2004, p. 122.
John De Lafons
Royal Exchange, London, 1780
Height 201 cm, width 48.5 cm, depth 25.5 cm
The sidereal day is marked by the successive passing of the fixed stars and is shorter than the twenty-four-hour, mean solar day. This phenomenon results from the fact that the earth follows the path of its orbit and in consequence has to turn slightly further round to present the same aspect to the sun each day. This does not happen with much more distant fixed stars. The sun passes overhead 365 times in a solar year but the fixed stars pass overhead 366 times in the same period, a difference of one day per year, or three minutes and fifty-six-and-a-half seconds per day.
Observatory regulators showing sidereal time were commonly used by astronomers in their observations. It is therefore not surprising that clocks which indicate sidereal time were popular with amateur astronomers and gentlemen of science in an age when all matters scientific fascinated.
George Margetts of Cheapside, London (1748-1808) is best known for his astronomical watches and a small group of regulators which indicate both mean solar time and sidereal time on a single dial. When this clock, signed 'John De Lafons Royal Exchange London', is compared with those by Margetts there is virtually no difference between them. De Lafons had his business at 66 Threadneedle Street in London between 1793 and 1816, initially working on his own but later in partnership with his son Henry. In 1801, he received an award of thirty guineas from the Royal Society of Arts for a remontoire mechanism, and in 1805 was granted a patent for a marine chronometer with an alarm. The similarity between the series of Margetts regulators and the example by De Lafons suggests that they were all made by the same maker, probably Margetts, but possibly De Lafons or, indeed, an unknown trade maker who was supplier to them both. In this context, it is most interesting that an original signature on the dial of this clock has been punched out from behind, before that of De Lafons was engraved.
The regulator is month-going and housed in a finely-made mahogany case with raised panels at the sides of the hood and on the plinth. The movement is mounted on a massive cast iron frame attached to the very substantial back-board of the case. The dead-beat escapement has a steel escape wheel and jewelled pallets and is controlled by a grid-iron pendulum. The quality of the wheel-work is exceptional. The whole movement is built upside-down with the escapement at the bottom. The geared conversion from mean solar time to sidereal time is so accurate that the theoretical error is only two seconds per year. The two times are shown on a composite dial with an outer ring for sidereal minutes and three subsidiary rings down the middle with sidereal hours, minutes and seconds at the top, middle and bottom respectively. Each of these rings has within it a rotating disc which shows the mean solar equivalent.
Bequeathed by Miss Diana Walker in 1998.
- On display (G39/od)
- Latest: 3 (Jan 2000)
- Acquisition date
- Britain, Europe and Prehistory
- Registration number