Rates of chronometers and watches on trial at the Observatory, 1766–1915

The Observatory’s involvement with Chronometers dates back to 26 April 1766 when the Board of Longitude resolved that Harrison’s longitude watch (H4) along with his three earlier seagoing clocks should be tested there. Over the next 50 years, a small number of other chronometers were also tested mainly for the Board (these included Kendal No.1, Mudge “Green” and Mudge “Blue”).

In 1818, a new Longitude Act was passed, as a result of which, the post of Superintendent of Chronometers was created with a salary of £100 a year. Initially this was bestowed upon the Hydrographer of the Navy, Captain Thomas Hurd. On 23 July 1821 it was transferred to John Pond, the Astronomer Royal.

A month earlier, on the 25 June, following up a suggestion from the chronometer maker WJ Frodsham, the Admiralty instigated a series of annual trials at the Observatory ‘for the purpose of further encouraging the improvement of chronometers’. To encourage makers to submit their instruments, it was announced that the Admiralty would ‘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’.

The first trial began in 1822, and was followed by twelve others. From 1828 instead of agreeing to purchase the two best chronometers, the Admiralty instead purchased the best three for £200, £170, and £130 respectively. Each of the trials, which became known as the ‘Premium Trials’, lasted for a period of 12 months. They were discontinued in 1836 at the end of the thirteenth trial as no useful purpose seemed likely to be served by continuing them. Over the course of the trials, there had been no marked improvement after the first four trials nor had there been any new inventions or discoveries. Worse still, some individuals had abused the system by entering chronometers that they had not made.

A new series of trials in a different format was begun by Airy in 1840. In 1888/9 separate trials for watches were instigated. These trials continued in much the same format until the outbreak of war in 1914 at which point, they were suspended. Although the formal trials ceased, the testing of watches and chronometers for the Navy did not. As late as 1926, it was still the Astronomer Royal’s intention to reinstate the formal trials, but this never happened, the work of the Observatory’s Chronometer Department becoming largely confined to rating of chronometers and watches that the Admiralty already owned.


Rates of chronometers on trial at the Observatory (1766–1811)

A partnership between Cambridge University Library, the National Maritime Museum (part of Royal Museums Greenwich) and the AHRC-funded Board of Longitude Project, has resulted in fully digitised versions being made available via the Cambridge Digital Archive.of the Board of Longitude papers together with associated material from the papers of Nevil Maskelyne and John Pond held in the Royal Greenwich Observatory Archives held at the University. The links below are to those papers specifically relating to the trial of chronometers at the Observatory.


From Maskelyne’s papers

Trials of Harrison’s chronometers. Observations and comparisons during the trials of John Harrison’s chronometers (H1, H2, H3 and H4) and Kendall I, with mean solar time, 1766–1767, and the rating of the Royal Observatory transit clock, 1771. The volume is comprised largely of tables of measurements, with some descriptions of how the results were achieved (RGO4/311).


Books on chronometer trials (RGO 4/312)

Nine exercise books bound together, containing the rating of chronometers and watches.

1. Comparison of Harrison 4 and Kendall I from 19 March 1770 until 15 February 1773, when Kendall I was not used. John Arnold’s watch was used as a comparison from 27 July to 13 December 1774, and Kendall I was used once more from 1 August 1775. The Harrison 4 observations end on 1 September 1775. Kendall I and III are compared for 22 April – 11 June 1776.

2. Daily rating of Thomas Mudge’s ‘Green’ and ‘Blue’ time-keepers, 21 July 1783 - 12 September 1784.

3. Daily rating of Mudge’s watch, 14 December 1774 - 26 February 1778, and comparison of Mudge’s ‘Green’ and ‘Blue’ time-keepers, 20 April 1779 – 17 July 1780.

4. Comparison of Mudge’s ‘Green’ and ‘Blue’ time-keepers, 22 June 1789 - 20 June 1790, calculated by John Crosley.

5. Rating of Coombe’s watch, 29 June 1779 – 26 January 1780.

6. Daily comparison of Mudge’s ‘Green’ and ‘Blue’ time-keepers, 21 July 1783 – 12 September 1784. The rates have been kept separate for the two watches.

7. Rates for Mudge’s first time-keeper from 14 December 1774, and kept daily from 11 November 1776 to 16 February 1778.

8. Separate daily rates of Mudge’s ‘Green’ and ‘Blue’ time-keepers, 20 April 1779 – 17 July 1780.

9. Computed variation in Mudge’s ‘Green’ and ‘Blue’ time-keepers, 24 June 1789 – 29 June 1790, with a note on how the tests were performed.


An Account of the going of Dr Maskelyne's new Chronometer N.309 made by Mr T. Earnshaw made between 1800 & 1811(RGO 4/153)

Click here for an image of the chronometer (from the Royal Museums Greenwich website)


From the Board of Longitude Papers

Click here to view the Board of Longitude Papers.


Rates of chronometers entered for the ‘Premium Trials’ held at the Observatory between 1822 and 1836

Links to most of these trial results are not currently available from this website. A bound volume (ref: 10/5/995) of the published results of all 13 trials (part of the official records of the Hydrographic Department of the Admiralty, but missing some sections) is held in the Horology Office at the National Maritime Museum.

1822 Feb – 1823 Jan
1823 Mar –  1824 Feb
1824 Apr  –  1825 Mar
1825 May – 1826 Apr
1826 Jun  – 1827 May
1827 Jul   – 1828 Jun
1828 Aug – 1829 Jul
1829 Sep – 1830 Aug
1830 Oct –  1831 Sep
1831 Nov –  1832 Oct
1832 Dec –  1833 Nov
1834 Jan  –  1834 Dec
1835 Mar –  1836 Feb


Rates of chronometers on trial at the Observatory (1840–1915)

The rates were published as an appendix to Greenwich Observations, (normally in the volume for the preceding year). From 1840 to 1902/3, the instruments were described as 'Chronometers'. From 1903/4 onwards, they were described as for 'Box and Pocket Chronometers'. Although the rates were published, no commentary of explanation was published alongside them. It seems to have taken until 1866 for an explanation to appear in print. This is the account that Airy's Asssistant, William Ellis, gave in a lecture on the treatment of chronometers at the Royal Observatory that was subsequently published in The Horological Journal. 

‘Three tables are given. The first contains, for each chronometer, the weekly rates arranged in the order of time. The second gives the same rates, but with the weeks arranged in the order of temperature of the different weeks, as determined by an instrument which we call a “chronometrical thermometer,” and which I will presently explain. The week of greatest cold is placed first, the next warmer week follows, and so on, the last week being that of greatest heat. By the first table we discover whether there exists acceleration of rate to any great extent, or any general irregularity such as might be caused by some imperfection in the machine or by bad oil. The second table gives more especially information as to the efficiency of the compensation, though this may be sometimes masked when large faults of the first-mentioned kinds exist. A third table gives, for each chronometer, the least weekly rate during the time of trial, and the greatest weekly rate, the difference of these numbers gives the greatest variation of weekly rate during the trial; this is found in the printed table in the column headed “Difference between the greatest and least.” In another column is given the greatest difference between any two successive weekly rates, or as it is called in the table the “Greatest Difference between one week and the next.” Of these two numbers, the “Difference between the Greatest and Least,” and the “Greatest Difference between one week and the next,” the latter indicates a sudden change of rate, the former may also result from a sudden change of rate, but is usually a measure of some gradual change of rate. Of the two evils, that indicated by the latter of the two numbers is evidently the worst, and greater importance is therefore attached to that number being small. And the practical rule which the Astronomer Royal has adopted for determining the order of merit of the chronometers (that is the order in which they appear in the printed tables) is, that it shall correspond to the order of magnitude of certain numbers, called “trial numbers,” that for each chronometer being found as follows;—

Trial No. = Difference between the Greatest and Least weekly rates, + 2 x Greatest Difference between one week and the next.

In this formula it will be remarked that the change which is the more obnoxious of the two has double weight given to it, so that a chronometer which shows a large change of this kind will have its trial number greatly increased, and its position in the table will in consequence be correspondingly low. This formula is considered, on the whole, to afford, in a simple way, a fair estimation of the relative merits of the different chronometers at the time of trial. Should a chronometer, from whatever cause, not undergo the complete trial, it is not placed with the other chronometers in order of merit, but is put into a separate division at the foot of the printed tables. I may remark that, in all the tables, the sign + indicates, as usual, a gaining rate, and the sign - a losing rate.’

Although the chronometers were listed in order of merit in the tables, the trial numbers themselves were not published. Nor was there any mention of them in the Reports of the Astronomer Royal until 1887, when the following account was given of how trial numbers were calculated for the recently commenced series of trials of Deck watches:

‘In order to compare the performances of the several watches, "trial numbers," representing deviations in weekly rates, have been formed on the same general principles as for the chronometer trials. The trials in different positions introduce, however, a new element, and an arbitrary weight must be assigned to them in combining them with the trials "dial up." It has been considered that when the watch is worn in the pocket the pendant will generally be "up," and that not more than one-third of the deviation" pendant right" or "pendant left" is likely to have practical effect.


           a = Difference between greatest and least weekly rates "dial up,"
           b = Greatest difference between one week and the next "dial up,"
           c = Difference between weekly rates "pendant up" and "dial up,"
           d = Difference between weekly rates "pendant right" and "dial up,"
           e = Difference between weekly rates "pendant left" and "dial up,"

the quantity c + d/3 + e/3 may be taken as the measure of deviation in weekly rate due to positions in ordinary wear. Half weight has been given to this quantity in combining it with the trial number "dial up" (a +2b), on the assumption that the deck-watch would be usually lying "dial up" and that it would not be carried in the pocket more than eight hours a day on average.. Thus the quantity a +2b +1/2(c +d/3 +e/3). has been adopted as the trial number for deck-watches.’

The dates below are the year of the trial.


1841 1841/2 1842/3 1844 1845

1846 1847 1848 1849 1850

1851 1852 1853 1854 1855

1856 1857 1858 1859 1860

1861 1862 1863 1864 1865

1866 1867 1868 1869 1870

1871 1872 1873 1874 1875

1876 1877 1878 1879 1880

1881 1882 1883 1884 1885

1886 1887 1887/8 1888/9 1889/90

1890/1 1891/2 1892/3 1893/4 1894/5

1895/6 1896/7 1897/8 1898/9 1899/900

1900/1 1901/2 1902/3 1903/4 1904/5

1905/6 1906/7 1907/8 1908/9 1909/10

1910/11 1911/12 1912/3 1913/4 1914/5


Rates of watches on trial at the Observatory (1888–1914)

The rates were published as an appendix to Greenwich Observations, (normally in the volume for the preceeding year). From 1888/9 to 1905, the instruments were described as 'Deck Watches'. From 1906 onwards, they were described as for 'Chronometer Watches'. The dates below are the year of the trial. 

1888/9 1889/90

1890/1 1891/2* 1892/3 1893/4 1894/5

1895/6 1896/7 1897/8 1898/9 1899/900

1900/1 1901/2 1902/3 1903/4 1904/5

1905 1906 1907 1908 1909

1910 1911 1912 1913 1914

# not published
*  incomplete