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拍品专文
The present clock is the only extant regulator by Thomas Tompion that achieves the distinction of demonstrating both mean and sidereal time.
In 1691 the Astronomer Royal ordered a pure sidereal clock to be made by Tompion. Now known as The Degree Clock, it is currently on display in the Sextant House, Old Royal Observatory, National Maritime Museum. Designed only to display sidereal time the Degree Clock has an unusual regulator-style dial and very simple movement. It has no need for the complex and delicate gear work that No.483 requires for the far more difficult task of displaying the combination of both mean and sidereal time. The next clockmaker to achieve this feat was George Graham, Tompion's pupil and illustrious successor, who succeeded in combining mean and sidereal time with perpetual calendar (regulator No.634).
INTERPRETING THE DIAL
Of the two silvered chapter rings the outer ring is narrower and each hour is divided by 12 divisions, each with a further 60 sub-divisions making a total of 720 tiny calibrated divisions. The inner dial is stationary and engraved in the usual manner of a mean time ring but with delicate trident half hour markers and small dot half quarter markers. The outer chapter ring revolves clockwise and makes two full revolutions in 366¼ days.
If the pendulum is regulated to beat mean solar seconds, the dial will register mean time on the inner fixed ring and sidereal time on the outer rotating ring. However, with the pendulum regulated for mean time and the present disposition of the hands (reading the hour hand on the fixed ring), 'reading' sidereal time from the rotating chapter ring is an extremely cumbersome affair. It involves the subtraction from the mean time shown of the difference (in divisions of the outer ring counted anti-clockwise) between XII on the rotating ring and XII on the fixed ring, the mean time ring therefore acting as a vernier scale.
In the context of using the clock for practical astronomical use in an observatory, this complicated method of calculating sidereal time would have been impractical. It is unlikely that the dial was ever meant to have been read in this manner, especially as it would have been a simple matter for Tompion to have designed the outer ring to rotate anti-clockwise, thus enabling true sidereal time to be read directly against the inner fixed ring.
The problem appears to have been solved by Todd op. cit., who realised that one had to assume Tompion would never have designed the dial to be complicated and that the outer ring was designed to rotate clockwise specifically to make it easier to read. Todd deduced that the pendulum should be rated to beat sidereal seconds; thus, the fixed chapter ring and seconds ring will register sidereal time ...with the point of the hour hand projected to the outer chapter ring, it will, at any time of the sidereal day, indicate mean time to the nearest minute. This can be easily demonstrated by setting the hands and the outer ring to 12 o'clock and setting the clock going. After 24 hours (sidereal time) it will be seen that the hands again point to 12 o'clock but that the outer ring indicates almost four minutes to 12. This 4 minute daily difference will be apparent at whatever time the observations are made. Concomitantly it can also be suggested that were the blued steel minute hand to be extended up to the rotating ring then mean time minutes could be read within an accuracy greater than 2½ seconds.
The gilt minute hand was suggested by Nielsen op. cit. as possibly a manually set equation hand. It was unexplained by Todd but it may have been intended to indicate sidereal time for a given longitude East or West of the clock's location. The adjustment allowed via the curved slot in the gilt hand's boss could encompass any two longitudes as far apart as Greenwich and Plymouth, i.e. almost any two sites on the British mainland. Were it to be extended to the rotating ring it would register local mean time for the given comparative longitude.
THE ESCAPEMENT
The train between the plates is very well made and typical of Tompion's work. The wheels of the sidereal motion work, however, are particularly finely cut and exceptionally well executed. The deadbeat escapement was always assumed to have been a later alteration, perhaps by Graham, who was traditionally accredited with its invention. However, it is noteworthy that whilst taking credit for the cylinder escapement and gridiron pendulum he never claimed credit for the deadbeat escapement.
In 1983 the movement of present clock and that of a regulator by George Graham (dated to 1722) were subject to a metallurgical test (x-ray fluorescence) by the Research Laboratory for Archaeology at the Department for History of Art, Oxford University, (M. Pollard & C. Heron, The Chemical Study of Metals, Archaeological Chemistry, Royal Society of Chemistry, 1996, ch.6, pp.220-238). The following elements were analysed Ni, Zn, As. Pb, Ag, Sn, Sb, as well as copper.
Tompion 483 was dismantled and thirteen pieces, including the escapement, were tested; the metallurgical content proved to have a remarkably uniform analysis that proved entirely consistent with English scientific instruments from the same period. The composition of the escape wheel was very similar to all the other parts of the Tompion movement. When this result is taken into consideration there seems every likelihood that the escape wheel is contemporary to the rest of the movement. There is no evidence in the plates of the movement to suggest any tampering or redrilling of the escape wheel arbor bearing holes. There is one small hole evident on the inside of the front plate but this does not extend through to the front and it cannot have been used as a pivot hole.
The Graham, dated to 1722, had fifteen separate components analysed and they proved to be less homogeneous. Nine of the fifteen parts had an analysis very similar to that of the Tompion. Interestingly, two elements of the Graham, the escapement and the pendulum had a much higher zinc content (32 - not possible for the early 18th Century and more likely to be of later date.
In conclusion, although no uncontroversial proof can be given to date Tompion No.483's escape wheel categorically, the evidence does appear to point in that direction. It now seems possible that Tompion could well have constructed the first deadbeat escapement. Certainly he appears to have been well aware of the deficiencies of the recoil escapement as early as 1675 when, working in conjunction with Townley, he fitted a form of deadbeat to the year clocks at Greenwich. If the opposite was true, and a new deadbeat escapement had been fitted by Graham, it must surely be a considerable coincidence that the latter used brass of almost identical composition and managed to use the same pivot holes for the escape wheel when the supposed change from anchor to deadbeat was made.
In 1691 the Astronomer Royal ordered a pure sidereal clock to be made by Tompion. Now known as The Degree Clock, it is currently on display in the Sextant House, Old Royal Observatory, National Maritime Museum. Designed only to display sidereal time the Degree Clock has an unusual regulator-style dial and very simple movement. It has no need for the complex and delicate gear work that No.483 requires for the far more difficult task of displaying the combination of both mean and sidereal time. The next clockmaker to achieve this feat was George Graham, Tompion's pupil and illustrious successor, who succeeded in combining mean and sidereal time with perpetual calendar (regulator No.634).
INTERPRETING THE DIAL
Of the two silvered chapter rings the outer ring is narrower and each hour is divided by 12 divisions, each with a further 60 sub-divisions making a total of 720 tiny calibrated divisions. The inner dial is stationary and engraved in the usual manner of a mean time ring but with delicate trident half hour markers and small dot half quarter markers. The outer chapter ring revolves clockwise and makes two full revolutions in 366¼ days.
If the pendulum is regulated to beat mean solar seconds, the dial will register mean time on the inner fixed ring and sidereal time on the outer rotating ring. However, with the pendulum regulated for mean time and the present disposition of the hands (reading the hour hand on the fixed ring), 'reading' sidereal time from the rotating chapter ring is an extremely cumbersome affair. It involves the subtraction from the mean time shown of the difference (in divisions of the outer ring counted anti-clockwise) between XII on the rotating ring and XII on the fixed ring, the mean time ring therefore acting as a vernier scale.
In the context of using the clock for practical astronomical use in an observatory, this complicated method of calculating sidereal time would have been impractical. It is unlikely that the dial was ever meant to have been read in this manner, especially as it would have been a simple matter for Tompion to have designed the outer ring to rotate anti-clockwise, thus enabling true sidereal time to be read directly against the inner fixed ring.
The problem appears to have been solved by Todd op. cit., who realised that one had to assume Tompion would never have designed the dial to be complicated and that the outer ring was designed to rotate clockwise specifically to make it easier to read. Todd deduced that the pendulum should be rated to beat sidereal seconds; thus, the fixed chapter ring and seconds ring will register sidereal time ...with the point of the hour hand projected to the outer chapter ring, it will, at any time of the sidereal day, indicate mean time to the nearest minute. This can be easily demonstrated by setting the hands and the outer ring to 12 o'clock and setting the clock going. After 24 hours (sidereal time) it will be seen that the hands again point to 12 o'clock but that the outer ring indicates almost four minutes to 12. This 4 minute daily difference will be apparent at whatever time the observations are made. Concomitantly it can also be suggested that were the blued steel minute hand to be extended up to the rotating ring then mean time minutes could be read within an accuracy greater than 2½ seconds.
The gilt minute hand was suggested by Nielsen op. cit. as possibly a manually set equation hand. It was unexplained by Todd but it may have been intended to indicate sidereal time for a given longitude East or West of the clock's location. The adjustment allowed via the curved slot in the gilt hand's boss could encompass any two longitudes as far apart as Greenwich and Plymouth, i.e. almost any two sites on the British mainland. Were it to be extended to the rotating ring it would register local mean time for the given comparative longitude.
THE ESCAPEMENT
The train between the plates is very well made and typical of Tompion's work. The wheels of the sidereal motion work, however, are particularly finely cut and exceptionally well executed. The deadbeat escapement was always assumed to have been a later alteration, perhaps by Graham, who was traditionally accredited with its invention. However, it is noteworthy that whilst taking credit for the cylinder escapement and gridiron pendulum he never claimed credit for the deadbeat escapement.
In 1983 the movement of present clock and that of a regulator by George Graham (dated to 1722) were subject to a metallurgical test (x-ray fluorescence) by the Research Laboratory for Archaeology at the Department for History of Art, Oxford University, (M. Pollard & C. Heron, The Chemical Study of Metals, Archaeological Chemistry, Royal Society of Chemistry, 1996, ch.6, pp.220-238). The following elements were analysed Ni, Zn, As. Pb, Ag, Sn, Sb, as well as copper.
Tompion 483 was dismantled and thirteen pieces, including the escapement, were tested; the metallurgical content proved to have a remarkably uniform analysis that proved entirely consistent with English scientific instruments from the same period. The composition of the escape wheel was very similar to all the other parts of the Tompion movement. When this result is taken into consideration there seems every likelihood that the escape wheel is contemporary to the rest of the movement. There is no evidence in the plates of the movement to suggest any tampering or redrilling of the escape wheel arbor bearing holes. There is one small hole evident on the inside of the front plate but this does not extend through to the front and it cannot have been used as a pivot hole.
The Graham, dated to 1722, had fifteen separate components analysed and they proved to be less homogeneous. Nine of the fifteen parts had an analysis very similar to that of the Tompion. Interestingly, two elements of the Graham, the escapement and the pendulum had a much higher zinc content (32 - not possible for the early 18th Century and more likely to be of later date.
In conclusion, although no uncontroversial proof can be given to date Tompion No.483's escape wheel categorically, the evidence does appear to point in that direction. It now seems possible that Tompion could well have constructed the first deadbeat escapement. Certainly he appears to have been well aware of the deficiencies of the recoil escapement as early as 1675 when, working in conjunction with Townley, he fitted a form of deadbeat to the year clocks at Greenwich. If the opposite was true, and a new deadbeat escapement had been fitted by Graham, it must surely be a considerable coincidence that the latter used brass of almost identical composition and managed to use the same pivot holes for the escape wheel when the supposed change from anchor to deadbeat was made.