- Museum number
- 1958,1006.2132
- Description
-
Weight-driven regulator with true solar time indication and calendar with equation of time dial.
Longcase regulator in a solid mahogany case, the door now glazed.
Month-going weight-driven movement with Graham dead-beat escapement and mercurial compensated pendulum; 'bolt-and-shutter' maintaining power;
Silvered-brass dial with applied silver spandrels. Chapter ring with hours I-XII and subsidiary seconds dial above VI. The central area is matted and gilded. Blued-steel hour and minute hands, the minute hand indicates mean solar minutes, the gilt minute hand shows true solar minutes.
To set the clock, the hands are turned until the gilt hand indicates the same time as that shown on a sundial, at this point the steel hand will automatically indicate mean time. The aperture in the dial shows the date and the large dial in the arch rotates once in a year. The scales are read against the vertical index in the centre of the arch; reading upwards the scales are for: time of sunrise, sun's position in zodiac, sun's declination, equation of time, month and date.
TRAIN-COUNT.
Great wheel 96
2nd wheel 84 pinion 14
Centre wheel 75 pinion 12
4th wheel 72 pinion10
Escape wheel 30 pinion 9
- Production date
- 1740-1750
- Dimensions
-
Height: 230 centimetres
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Width: 48 centimetres
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Depth: 26 centimetres
- $Inscriptions
-
- Curator's comments
-
Compare with 1958,1006.2164
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Text from 'Clocks', by David Thompson, London, 2004, p. 114.
George Graham
Longcase precision regulator with equation of time indicator
London, c. 1745
Height 230 cm, width 48 cm, depth 26 cm
By the early years of the eighteenth century it was realized that temperature change was the enemy of accurate timekeeping. In any pendulum clock, in general terms, it is the length of the pendulum that determines the rate at which it swings. This in turn controls the rate at which the clock runs and thus its accuracy. It is not helpful, therefore, that a pendulum gets longer when heated and shorter when cooled. George Graham, a Fellow of the Royal Society and one of the leading clock-, watch- and instrument-makers in eighteenth century London, was the first successfully to apply temperature compensation to a pendulum clock in the form that it appears in this example of his work. In 1726 he published a paper entitled, 'A contrivance to avoid irregularities in a clock's motion' in Philosophical Transactions of the Royal Society, in which he says that in about 1715 he tried to see whether there was any considerable difference in the expansion of different metals when subjected to temperature change. The result of his investigations was the mercury pendulum.
Here, the brass pendulum rod carries a glass jar containing mercury so that, as the brass rod expands and contracts, the mercury expands and contracts in the opposite direction, in order to keep the effective length of the pendulum constant. This weight-driven, month-going longcase regulator has Graham's dead-beat escapement, an innovation he invented in about 1719, which also improved the accuracy of clocks, and bolt-and-shutter maintaining power.
Following the invention of the pendulum in 1657, clocks became sufficiently accurate to need a table showing the difference between the time shown by a sundial and that shown by a mean-time clock throughout the year. In practice this difference, known as the equation of time, means that the clock and the sundial or solar observation will only be the same on four occasions in the year. The problem is caused by the fact that the earth's orbit around the sun is elliptical rather than circular and that the earth's axis is tilted in relation to the celestial equator. The added sophistications on this clock are the secondary gilt minute hand which shows true solar minutes (sundial time) and the dial in the arch which, as well as being a calendar, shows the equation of time in minutes and seconds for each day of the year. With all these sophistications, this regulator represents the highest degree of accuracy available at the time and, being made by one of the most celebrated makers, would have been a highly-prized possession.
Formerly in the Ilbert Collection.
- Location
- On display (HSR/northstore/dru1/dr2)
- Condition
-
Latest: 2 2018/JAN
-
1 (1994) (case)
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3 (1993) (movement)
- Acquisition date
- 1958
- Acquisition notes
- The Ilbert Collection of clocks, prints and other related material was destined to be sold at Christie's auction house on 6th-7th November 1958. As a result of the generous donation of funds by Gilbert Edgar CBE the sale was cancelled and the material purchased privately from the beneficiaries of the Ilbert Estate.
Ilbert's watches were then acquired with further funds from Gilbert Edgar CBE, public donations and government funds. These were then registered in the series 1958,1201.
- Department
- Britain, Europe and Prehistory
- Registration number
- 1958,1006.2132
- Additional IDs
-
Previous owner/ex-collection number: CAI.2132 (Ilbert Collection)
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Previous owner/ex-collection number: Q301 (Ilbert Ledger)
