Astronomical Regulator: Dent 2

Made by the clockmaker Edward Dent and thought to date from around the 1820s, Dent 2 is now in the collections of the National Maritime Museum (Object Number: ZBA1706). Dent 2 has an unclear early history. No records have been found that record when it was made or when it first arrived at the Observatory. Unlike the later Dents, the dial does not carry a serial number. Nor, according to Vaudrey Mercer (1977), does it carry one elsewhere.

The first Observatory clock by Dent to have a serial number on the dial was Dent 1906, which arrived at the Observatory in 1871. Prior to 1835, the clocks listed in the Observatory’s inventories were not named and only a description was given, sometimes with and sometimes without a makers name. It was the sixth Astronomer Royal, George Airy, who, soon after his arrival in 1835, first assigned names to some of the clocks. Still known by the names Airy gave them, they include the three clocks made by George Graham; ‘Graham 1’, ‘Graham 2’ and ‘Graham 3’. Where there was only one clock by a given maker it tended to be known by the makers name; for example Hardy. Likewise, the clock by Dent was referred to by Airy as a ‘clock by Dent’ or a ‘clock marked Dent’. This remained the case even after the arrival of other Dent clocks in the 1870s. The earliest Observatory record yet discovered which refers to the clock as Dent 2, is dated 1894. After that date, this is the name by which it has always been known.

The earliest records show that Dent 2 was used for the daily comparisons of chronometers. Although the clock may have been at the Observatory since the Premium Chronometer Trials began in 1822, its presence can only be dated back to 1834 with any degree of certainty. In Airy’s time, Dent 2 was kept and used in the Chronometer Room, where it remained until at least 1899.

Dent 2 (centre) in the Clock Room on 7 May 1928. Dent 2016 is on the left and the slave clock of Shortt 11 on the right. Humphry Smith photographic archive



An eight-day astronomical regulator, the movement with dust covers and end caps throughout with Harrison's maintaining power, jewelled deadbeat escapement and Mercury pendulum, now with electrical later contacts. Twelve-inch circular engraved dial with signature, in a Grecian-Egyptian Revival pale mahogany case with levelling screws to the gadrooned moulded foot, tapered truck door and hood with eared pediment top and brass framed dial aperture. Reported dimensions: 1960 (excluding pediment?) x 480 x 265 mm.

Like other astronomical regulators, the dial is laid out with an outer minute circle enclosing the seconds and 24 hour subsidiaries. The seconds dial is located centrally in the upper half of the dial plate and the 24 hour dial in the lower half. The dial is signed across the middle with the words ‘DENT. London.’ on either side of the centre, suggesting that it was made before Dent went into partnership with John Roger Arnold on 30 September 1830. The winding aperture is located in the 20 minute position about a third of the way out from the centre to the edge.

The case is stamped (in several places?), with a vertical arrow surmounted by an eight pointed star, the arrow indicating that it was goverment property and the star that it belonged to the Obervatory.


Brief historical summary

Location & role
Before 1840 Date of manufacture, date of arrival and initial role at Greenwich a matter of speculation. Probably in the Chronometer Room at Greenwich by 1834 at the very latest.
1840 - 1898 In the Chronometer Room where it was used for the daily comparisons of chronometers. Became the reserve clock in 1852 following the installation in the Chronometer Room of a slave dial connected to the newly installed Shepherd Master Clock.
1899 - 1905 Unclear.
1906 - 1919 Converted from a mean-time clock to a sidereal clock and placed in the Record Room, where is was used as the reserve sidereal standard. Pendulum replaced in 1909.
1919 - 1937 In the Clock Room (west end of the Meridian Building) under the control of a master clock. This was probably Riefler 50 initially, but later Cottingham (the clock that became the new sidereal standard in 1922 but was replaced in this role by Shortt 3 at the start of 1925). It seems likely that Dent 2 could easily be switched from one master clock to another.
1937 - 1998 Original pendulum refitted, converted back into a mean-time clock and running as a decorative clock, initially in the Octagon Room at Greenwich. In store during the War Years. By 1954 installed in the Observatory Library at Herstmonceux. Following the Observatory’s relocation to Cambridge in 1990, installed in the Library there.
1998 - present Transferred to the National Maritime Museum in 1998 following the closure of the Observatory by PPARC.


History prior to 1840

In July 1821, the Post of Superintendent of Chronometers was transferred by the Admiralty from the Hydrographer Thomas Hurd to the Astronomer Royal John Pond and with it, the Government’s stock of chronometers (RGO5/229/2). Although no records have been found, the matter must have been discussed with Pond beforehand as arrangements seem to have been made in advance to accommodate the chronometers. To this end, the Observatory buildings had been extended to accommodate a new library, and the old library fitted out as a Chronometer Room. The evidence for this comes from notes made by Airy from a journal kept by the Second Assistant, John Belville (RGO6/1/55). The earliest published record of the room’s existence seems to be a letter from Belville published on 30 January 1830.

The Premium Chronometer Trials began at the Observatory in February 1822. Instigated by the Board of Longitude, (who set the rules), they were advertised by the Admiralty, who promised to ‘purchase, at the end of each year, the chronometer which shall have kept the best time, at the price of £300, and the second best at the price of £200’. It is not known if the clock by which their rates were measured was supplied by the Observatory, the Admiralty or the Board of Longitude.

The three consecutive inventories made at the Observatory in 1824, 1830 and 1831, list the contents of the Observatory on a room by room basis, but do not include the Chronometer Room and its contents. This strongly suggests that like the chronometers themselves, the clock in the chronometer room belonged to the Admiralty. According to Mercer, ‘there is no explanation’ as to why the clock by Dent is known as Dent 2. Mercer also states that Dent supplied a clock to the Admiralty in 1814, and that this is probably the clock now known as Dent 1. If the clock used at the Observatory belonged to the Admiralty, not only might this explain why it was later referred to as Dent 2, it could also explain why neither it nor the Chronometer Room were mentioned in the inventories. A less likely explanation for its omission is that Dent 2 was originally owned by the Board of Longitude: but if so, there is no record of it being transferred to the Observatory following the winding up of the Board in 1828. The list of items transferred does include two clocks whose makers are not specified (RGO6/54/21A), but a list of those instruments being stored in the Octagon Room when the 1831 inventory was taken, suggests very strongly that these were journeyman clocks rather than astronomical regulators (RGO6/54/38).

The first mention of a clock by Dent being at the Observatory comes in 1829, when a ‘clock by Dent’ and one of the clocks by Graham was used by Edward Sabine for pendulum experiments at the western end of Meridian Building (Experiments on the Length of the Seconds Pendulum at the Royal Observatory of Greenwich. Edward Sabine and Thomas Glanville Taylor. Philosophical Transactions of the Royal Society of London, Vol. 121 (1831), pp. 459-488). When a similar set of experiments were carried out in the same location at the Observatory in 1828, the only clock that was used was the one by Graham. Click here to read more. Unfortunately, Sabine gave no informations as to where the Dent clock had come from.

The next reference to a clock by Dent comes in the 1830 volume of Greenwich Observations when Hardy, the Transit Clock, was out of action between 13 February and 30 March while a dead-beat escapement was being fitted and a clock belonging to Dent used as a replacement. Interestingly, on 24 March, Pond made the following observation about the clock, which suggests that it may have been the same clock:

‘On winding up the clock this morning, in order to prevent the weight again coming into contact with the arm of brass alluded to on the 24th instant, which had been applied to the pendulum for the purpose of observing coincidences in the pendulum experiments, for which the clock had previously been employed …’

The first mention of a clock being present in the Chronometer Room comes in the inventory of 4 June 1824 (RS MS/371/70). An undated note in the inventory records that Graham 3 had been moved to the Chronometer Room. From other notes in the inventory, it would appear that this occured before the inventory was taken. How long it remained there is not known, but it was clearly no longer there in 1830 as the 1830 inventory records all the Graham Clocks in other rooms at the Observatory (RS MS/371/71). Graham 3 had been used as the Transit Clock until 10 September 1821 when it was dismounted and replaced with a clock by Molyneux & Cope (Greenwich Obs). It is entirely possible therefore that it was used in the Chronometer Room from the time the trials first started. It is also possible that it was used as a back-up for Dent 2.

The next reference to a clock in the Chronometer Room comes ten years later when one was mentioned in the 1834 volume of Greenwich Observations, no maker being specified. The first record of a Clock made by Dent being in the Room does not occur until 1840 when not only were the contents of theChronometer Room listed for the first time, but ‘A clock with mercurial pendulum, by Dent; adjusted to Mean solar Time’ was recorded as being there by Airy. This is the clock that later became known as Dent 2. What we can’t be sure of is if the three clocks mentioned in 1829, 1830 and 1834 were three different clocks, two different clocks or one single clock.

The inventory of 1840 appears to have been the first new inventory made since 1831. 1840 was also the year when chronometer trails were restarted at the Observatory after a four-year break. The eighth Premium Chronometer Trial took place between September 1829 and August 1830 and would have taken place in the Chronometer Room at the eastern end of the Meridian Building. Given that Sabine was carrying out his experiments in the Quadrant Room at the opposite end of the building at the same time, two different clocks must have been in use. It is possible that the clock used by Sabine was Dent 2 and that it was moved to the Chronometer Room after his depature: but there is no evidence that this was the case.

Although it was normal practice for new items acquired by the Observatory to be listed each year in the minutes taken at the visitation of the Board of Visitors, no such record has been found for Dent 2. The minutes for the years 1813–1830 appear to be missing and there is no mention of Dent 2 in the minutes from 1831 onwards (ADM190/4). The 1836 minutes (ADM190/4/106) record that the secretary to the Board had copied the inventory into a new book (ADM190/4/106). This has not been located. Interestingly, Airy does not appear to have had a copy as his inventory of 1840 (RGO39/1/16 & RGO6/54/72) specifically references items against the 1831 inventory.

In light of all the above, the two most likely candidates for the clock used in the Chronometer Room in the early 1820s are:

  • Dent 2, the supposition being that it was suppled to the Observatory by the Admiralty in 1821 for use as the Chronometer Clock and that whilst Pond regarded it as Admiralty property and omitted it from the inventories, Airy considered it to be Observatory property and therefore included it in the first inventory that he compiled.
  • Graham 3, the supposition being that it was replaced by Dent 2 some time prior to 1830.


The changing locations of the Chronometer Room

By 1870, the Chronometer Room had had several different locations at the Observatory. Initially, it was on the first floor at the east end of the Meridian Building immediately to the south of the pier originally built for the Shuckburgh telescope. In 1851, when more space was required, it was relocated within the building where it occupied the upper story of the section of the building that had been built for Bradley. This room was abandoned in 1868 when it was decided that the Chronometers should be moved into a fireproof room. As a result, they were moved to the middle floor of the Great Equatorial Building (the South-East Dome), where they were kept behind an iron door accessed via an iron staircase. Dent 2 was used in all three locations. In 1898/9, when more space was required, an additional chronometer room was created in what had originally been Bradley’s Transit Room. No evidence has been found to suggest that Dent 2 ever occupied this room.


Important dates in the history of Dent 2 and the Chronometer Room
1821 John Pond appointed as Superintendent of Chronometers and the Admiralty stock of chronometers transferred to the Observatory where a room was adapted to receive them. No references yet located as to which clock was installed there. On 10 September, the Transit Clock (Graham 3) was taken down and replaced with a clock by Molyneux & Cope (Greenwich Obs). Never to be used as the Transit Clock, an undated note in the inventory of 4 June 1824 (RS MS/371/70) records that Graham 3 had been moved to the Chronometer Room. From other notes in the inventory, it would appear that this occured before the inventory was taken.
1822 Premium Chronometer Trials commence.
1824 No clock by Dent recorded in the 1834 inventory, nor is the Chronometer Room per se listed in the inventory. As noted above however, it does record the fact that Graham 3 was moved into it at some point. It can be inferred from the 1830 inventory that Graham 3 had been removed from the Chronometer Room prior to that date.


A ‘clock by Dent’ used by Sabine for pendulum experiments at west end of Meridian Building (Experiments on the Length of the Seconds Pendulum at the Royal Observatory of Greenwich. Edward Sabine and Thomas Glanville Taylor. Philosophical Transactions of the Royal Society of London, Vol. 121 (1831), pp. 459-488).
1830 No clock by Dent recorded in the 1830 inventory, nor is the Chronometer Room listed in the inventory (RS MS/371/71). Belville’s letter of January 1830 records: ‘Beyond the circle-room is the library, containing a large collection of valuable books, connected with astronomy and mathematics; and the chronometer-room, wherein are deposited the marine time-keepers, belonging to the Admiralty, and the trial chronometers, which are placed there, pursuant to the direction, of the late Board of Longitude ...’
1831 No clock by Dent recorded in the 1831 inventory nor is the Chronometer Room listed in the inventory (RGO6/54).
1834 ‘On the 6th [December], on comparing the ball clock and that in the chronometer room with the Transit clock, the two comparisons indicated that the Transit clock had lost about 2,3 seconds’ (Greenwich Observations, 1834). This is the first concrete evidence of a clock being present in the Chronometer Room.
1836 ‘Two comparisons of the Chronometer Clock with the Transit Clock are to be made, one before the comparisons of chronometers and one after. The senior assistant will make these himself or direct the junior to make them, as he may think fit. The entries of these comparisons are [not?] to be made in the Chronometer Books.’(RGO6/574/119).
1840 ‘A clock with mercurial pendulum, by Dent; adjusted to Mean solar Time’ recorded in the 15 May 1840 inventory as being in the Chronometer Room (RGO39/1/16 & RGO6/54/88). Referred to by Airy as the Chronometer Clock on 21 Dec1840 (RGO6/574/130).


First mention in the introduction to the volumes of Greenwich Observations comes in 1848, when it is recorded as being in Chronometer Room, ‘marked Dent’, and ‘used for the daily comparisons of chronometers’.
1851 Moved to new Chronometer Room in the upper part of the former Bradley Observatory (1851 Report).
1868 ‘The chronometers were removed from the room above the Principal Computing Room to the middle story of the South-East Dome, at the end of July 1868’ (1869 Report). ‘There is a clock marked "Dent" placed in the Chronometer-room and used occasionally for comparisons of chronometers in case of accidental failure of the galvanic clock placed there.’ (Intro. to Greenwich Observations, 1868), The same form of words was used in the introduction to each volume up to and including 1890 after which the reason for it being there was omitted.
1891 1891 Intro. to Greenwich Observations: ‘There is a clock marked "Dent" placed in the Chronometer-room’.
1893 1893 Intro. to Greenwich Observations: ‘There is a clock marked "Dent" placed in the Chronometer-room’.
1894 Inventory (RGO39/10/50): ‘Mean Time Clock by Dent with mercurial pendulum’ recorded as being in Upper Chronometer Room. 1894 Intro. to Greenwich Observations: ‘The following clocks are in use in the Observatory as Mean Solar Clocks: "Dent 2" with a mercurial pendulum in the Upper Chronometer Room ...’. This appears to be the first reference to the clock as Dent 2.
1896 1896 Intro. to Greenwich Observations: ‘The following clocks are in use in the Observatory as Mean Solar Clocks: "Dent 2" with a mercurial pendulum in the Upper Chronometer Room’. The same information is recorded until the volume for 1898.
1905 Not mentioned in the 1899 Intro. to Greenwich Observations nor those up to and including the 1905 Intro. to Greenwich Observations.
1906 1906 Intro. to Greenwich Observations: ‘The clock "Dent 2" has been fitted with springs and placed in the Record Room for use when the Sidereal Standard is under repair.’ Until 1905, Arnold 1 had been used as a reserve for both the transit clock Hardy and the Sidereal Standard. From 1906, it was used only as a reserve for Hardy. Much the same information was given in the 1907 Report, suggesting that the alterations took place after 10 May. The Report did not however state the location. When researching his book Greenwich Observatory Vol.3 (1975), Howse only seems to have consulted the second of these sources and appears to have assumed, incorrectly, that in its new role, Dent 2 was placed in the basement of the Magnetic Observatory (which is where the sidereal standard Dent 1906 was housed).
1907 1907 Intro. to Greenwich Observations: 'The clock "Dent 2" has been fitted with springs and placed in the Record Room for use when the Sidereal Standard is under repair.’ 1907 Report (p11.): ‘The clock Dent 2, fitted with a mercurial pendulum, has been thoroughly overhauled, the contact springs improved, and a new contact spring for a minute signal added. The clock is intended to be  used when the Sidereal Standard [Dent 1906] is under repair.’
1908 1908 Report p.12: ‘The Sidereal Standard clock [Dent 1906] was taken away by E. Dent & Co. for repair on July 16, and was returned on July 22’. Dent 2 was presumably used as the standard during these few days.
1909 Inventory 1 Jan 1909 (RGO7/44): ‘Fitted with springs to give contact at alternate seconds; also to give signal at the minute [.] Intended to replace the Sidereal Standard when required.’ Location: ‘Record Room’. A list of alterations on the last page of the inventory records ‘Mercurial pendulum taken off and replaced by the zinc & steel belonging to Earnshaw (Sheepshanks) - 1909, March 12.’ On the same day, Earnshaw was refitted with its original gridiron pendulum.

During the period September 8 to December 4 used as the Sidereal Standard while Dent 1906 being overhauled (1912 Report).
1919 1911 Inventory (RGO39/4/60): ‘Cleaned, new contact wheel. 1919 Oct.’ The entry also has the location of the Record Room struck out (undated) and replaced with Clock Room (secondary to Cottingham or Riefler 50), the first of which to arrive at the Observatory was Riefler 50 on 23 June 1920. Dent 2 was presumably moved to the Clock Room at the west end of the Meridian Building for use with the Cottingham at around this time.

1924 Report: ‘The clock Dent 2 used as the controlled clock with the Cottingham Standard Clock is now under repair.’ 1911 Inventory (RGO39/4/60): ‘Cleaned & overhauled order 244 of 26/9/23.’

1926 1926 inventory (RGO39/5/222): Location still recorded as Clock Room.
1937 1937 Report: The clock Dent 2 is being renovated and fitted with its original mercurial mean time pendulum. When finished, it will be placed in the Octagon Room. This clock was converted from mean time to sidereal time and for many years was used as a reserve sidereal standard and was later used as a subsidiary clock, controlled by the Cottingham-Riefler sidereal clock. 1938 Report: The renovation of the clock Dent 2 has been completed and it has been installed in the Octagon Room. The 1933 Inventory (RGO39/6/208) implies that the clock had been moved at some point to the Upper Record Room before being renovated and placed in the Octagon Room. It also records ‘movement & case repaired by Kullberg. 941. 1937 Nov 1.’

Dismantled (RGO71/1/3/1).
1948 Recorded in October as being in store at Greenwich with the case in the Octagon Room and the movement in the Time Department (1954 clock inventory, RGO71/1/3/1).

At check in Nov 1954, recorded as going and in the library in the castle at Herstmonceux (1954 clock inventory RGO71/1/3/1).
1990 By 1990 moved to the Observatory’s new home at Cambridge. Some records at the National Maritime Museum record it as having been in the library, whilst others record it as having being in the Common Room.
1998? Transferred to National Maritime Museum.