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The origin and development of themarine chronometerEric Gray Forbes M.Sc. Ph.D. F.R.A.S. aa Department of History , University of EdinburghPublished online: 02 Jun 2006.

To cite this article: Eric Gray Forbes M.Sc. Ph.D. F.R.A.S. (1966) The origin and development of themarine chronometer, Annals of Science, 22:1, 1-25, DOI: 10.1080/00033796600203005

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ANNALS OF SCIENCE A QUARTERLY REVIEW OF THE HISTORY OF

SCIENCE AND TECHNOLOGY SINCE THE RENAISSANCE

VOL. 22 March, 1966 No. 1

(Published September, 1966)

THE OI~IGIN AND DEVELOPMENT OF THE MARINE CHRONOMETER

By EHIc GHAY FOHBWS, M.Sc., Ph.D., F.R.A.S.*

HARRISON'S TIMEKEEPERS WHEN the Parliament of Great Britain, under the terms of the act 12 Anne (1714), offered bounties of up to £20,000 to anyone who might invent a practical and general method for accurately predicting longitude at sea, the young Yorkshire clock-maker John Harrison (who was destined to receive this maximum award) had just come of age, and was fired with the ambition of constructing a timekeeper which would answer this purpose. In principle, the problem was very simple: the timekeeper had only to be set to some standard t ime--for a specified meridian, such as that through Greenwich--and regulated to keep mean time; the difference between the Greenwich mean time and the local mean time for a nautical observer being a direct measure of the latter's separation in longitude from the Greenwich meridian.

The local mean time was generally obtained in the following manner. Using a quadrant or a sextant, the mariner observed the altitude of either the sun's lower or upper limb, noting the time of observation on his pocket watch, and applied the necessary corrections for the dip of the horizon and the atmospheric refraction. He then either added or subtracted (as was appropriate) the tabulated value of the sun's semi-diameter, to obtain the al t i tude--hence the zenith distance---of the centre of the solar disc. The sun's declination at the approximate time and place of observation was next deduced by interpolation from tables that had been prepared for Greenwich apparent noon, allowing for the time interval corresponding to the estimated value of the longitude at sea by account; any errors incurred through the uncertainty in the latter value being negligible, owing to the small daily variation of the calculated quantity. By adding algebraically the sun's declination to its meridian Mtitude, the mariner found his co-latitude, and a straightforward trigonometrical calculation yielded the value of the local hour angle and hence the local apparent time. The required local mean time was finally obtained by applying the equation of time. This quanti ty is a tabulated correction which

q* Depar~raent of History, University of Edinburgh.

Ann. of S~.--Vol. 22, No. 1. a

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2 E . G . Forbes on

compensates for periodic differences in the length of the solar day (i.e. in the intervM between successive transits of the sun across the meridian) produced by the Earth 's variable orbital speed, and by the latter 's inclination to the equatorial plane on which our time-measurement is based.

Implicit in the above observations was the question of the reliability of the mariner's watch. A common way of regulating this instrument was to note the times at which the sun was at the same (arbitrarily selected) altitude before and after it crossed the observer's meridian; local apparent noon, according to the watch, being found simply by averaging the two times. For example, if such equal altitude observations were made at 8.40 a.m. and 3.16 p.m. by the watch, the time of noon would be ½(08-40÷ 15-16)=11.58 a.m., so the watch would be two minutes slow. Of course, if the ship were under way during the interval between the observations, allowance would first have had to be made for the effect of the ship's run before the watch's loss (or gain) could be calculated. I t was generally considered to be satisfactory if the watch-error were found to be less than four minutes per day; bu t if it were more consider- able, daily observations of this nature were normally made to establish the rate. The standard timekeeper set to Greenwich mean time, on the other hand, had to satisfy a much more stringent requirement: it had to maintain an accuracy of within four minutes at all times (even after several weeks at sea) if this method were to be generally useful and practicable for the determination of longitude at sea even to the lower limit of accuracy (1 °, or 60 geographical miles) specified in the 1714 act. The real difficulty was therefore the practical one of constructing a mechanism capable of doing this, in the face of such unpredictable factors as the effect upon the going of a timekeeper of irregular motions of the ship at sea and variations in the climatic conditions. Changes in the force of gravity at different latitudes were also known to affect the time of oscillation of a pendulum clock.

John Harrison was among the first to realize that for use at sea, a clock should be driven by a spring instead of a weight, since the action of the former is independent of gravity; while to preserve accuracy b y counteracting the effects of the motion of the ship at sea, the customary pendulum had to be replaced b y interconnected balances acting in contrary directions. In his first marine timekeeper (It 1) which he com- pleted in 1735 after six years of hard work, he applied four cylindrical springs to the upper and lower extremities of these two balances, to regulate their times of vibration. A secondary spring, which he called the 'maintaining power ', kept the machine going at a constant rate while the main spring was being wound up (once each day). He greatly reduced the effects of temperature changes and variations in friction

2 E . G . Forbes on

compensates for periodic differences in the length of the solar day (i.e. in the interval between successive transits of the sun across the meridian) produced by the Earth 's variable orbital speed, and by the latter's inclination to the equatorial plane on which our time-measurement is based.

Implicit in the above observations was the question of the reliability of the mariner's watch. A common way of regulating this instrument was to note the times at which the sun was at the same (arbitrarily selected) altitude before and after it crossed the observer's meridian; local apparent noon, according to the watch, being found simply by averaging the two times. For example, if such equal altitude observations were made at 8.40 a.m. and 3.16 p.m. by the watch, the time of noon would be ½(08-40+ 15.16)= 11.58 a.m., so the watch would be two minutes slow. Of course, if the ship were under way during the interval between the observations, allowance would first have had to be made for the effect of the ship's run before the watch's loss (or gain) could be calculated. I t was generally considered to be satisfactory if the watch-error were found to be less than four minutes per day; but if it were more consider- able, daily observations of this nature were normally made to establish the rate. The standard timekeeper set to Greenwich mean time, on the other hand, had to satisfy a much more stringent requirement: it had to maintain an accuracy of within four minutes at all t imes (even after several weeks at sea) if this method were to be generally useful and practicable for the determination of longitude at sea even to the lower limit of accuracy (1 °, or 60 geographical miles) specified in the 1714 act. The real difficulty was therefore the practical one of constructing a mechanism capable of doing this, in the face of such unpredictable factors as the effect upon the going of a timekeeper of irregular motions of the ship at sea and variations in the climatic conditions. Changes in the force of gravity at different latitudes were also known to affect the time of oscillation of a pendulum clock.

John Harrison was among the first to realize that for use at sea, a clock should be driven by a spring instead of a weight, since the action of the former is independent of gravity; while to preserve accuracy by counteracting the effects of the motion of the ship at sea, the customary pendulum had to be replaced by interconnected balances acting in contrary directions. In his first marine timekeeper (H 1) which he com- pleted in 1735 after six years of hard work, he applied four cylindrical springs to the upper and lower extremities of these two balances, to regulate their times of vibration. A secondary spring, which he called the 'maintaining power ', kept the machine going at a constant rate while the main spring was being wound up (once each day). He greatly reduced the effects of temperature changes and variations in friction

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The Origin and DeveIo1~ment of the Marine Ghronometer 3

(associated with the climatic conditions) by incorporating two novel features which he had previously embodied in his ' regulator u; namely, the ' grid-iron ' pendulum and the ' grass-hopper ' escapement. The whole instrument was mounted in gimbals in a large wooden case suspended at its corners by spiral springs, and tested successfully in a barge on the g iver Humber.

Harrison's invention was approved by five eminent members of the Royal Socie ty--Edmond Halley, Robert Smith, James Bradley, John Machin, and George Graham--who signed a certificate to the effect that its principles promised a great and sufficient degree of accuracy for the purpose intended, and that it ought to be given a thorough trial. Their recommendation was put before the Board of Longitude with the result that arrangements were made for Harrison to sail with his timekeeper to Lisbon in H.M.S. Centurion in May 1736. The captain of this man-of-war, George Proctor, who unfortunately died in Lisbon shortly after the ship's arrival, was sceptical of the accuracy with which such an instrument could be expected to go; bu t the master of H.M.S. Orford, on which Harrison was obliged to return to England, was able to certify tha t observations made with the timekeeper had corrected an error of almost 1½ ° in his own reckoning of the ship's longitude.

This trial was not intended to enable Harrison to qualify for the reward stipulated in the 1714 act, since the latter specified a voyage to the West Indies; nevertheless, its success demonstrated that he was working on sound principles, and the Board therefore resolved that he should be given £500 to assist him in the construction of a second machine of the same kind incorporating several improvements (H 2), which Harrison duly completed in 1739. This instrument was never tested at sea, because at the time of its completion Britain was at war with Spain; and Harrison was already constructing a third machine (K 3) which, being simpler in construction, was less liable to disorder, easier to adjust, and easier to copy.

The construction of H 3 differed significantly from the other two: the two balances (which had consisted of straight rods with brass globes at their ends) were now replaced by circular flat wheels, thus allowing a single cylindrical spring to do what four had previously done. Moreover, this spring was kept at the same tension at different temperatures by a ' metalline thermometer ' composed of two flat brass and steel rulers joined firmly together, whose changes in curvature at different tempera- tures provided a means of compensating for the varying effects of hotness and coldness, which Harrison predicted would ensure an accuracy at sea to within three seconds per week at all t imes and in all climatic conditions.

i This was an accurate timekeeper completed before 1726, which has been described by E. T. Cottingham, M e n . Not . R . Astr . Soy., 1909, 7{}, 25-28 (see also plates 3 and 4).

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4 E . G . :Forbes on

This third timekeeper took him over seventeen years to construct, bu t it was finally completed and adjusted in 1758.

Harrison had, however, already begun making a fourth timekeeper (H 4) resembling externally a very large watch of about five inches in diameter, which he compared for some time with his regulator and found to be ' ha rd ly inferior ' to the third machine. This was the opinion expressed in his Memorial of 26 February 1761 to Lord Anson, chairman of the Board of Longitude, in which Harrison requested a trial of both these machines at sea; at the same time he outlined the procedure which he felt necessary to ensure a reliable check being made upon the accuracy of his timekeepers. Harrison's request was readily granted by the Board a few weeks later, and arrangements were quickly made for his son William to sail to Jamaica in April with Captain John Campbell in H.M.S. Dorsetshire. Action had perhaps been taken too quickly, for this ship was required for another service and a substitute was not obtained until October. By this time, John Harrison had definitely decided to stake his claim upon the performance of his watch, so t t 3 was never tested at sea. Another consequence of the delay was that, owing to the lateness of the season, the plan originMly devised by the Royal Society for ' an adequate Trial of Mr. Harrison's Time-Keepers '~' could no longer be adopted.

TI~E FIRST SEA TRIAL O1~ t t 4

The proposed method for testing the accuracy of Harrison's watch involved the use of observations of the eclipses of Jupiter 's satellites made at Portsmouth and Jamaica as a means of obtaining the ' t rue ' difference of longitude. This choice was ill-considered, in view of the fact tha t less than ten years previously a comparison of the times of two eclipses of Jupiter 's first satelli te--the brightest and most easily observ- a b l e - b y the Abb4 de la Caille had revealed systematic discrepancies in longitude determinations of approximately 2½ minutes, ~ implying an uncertainty in the method of well over 30 geographical miles (the limit within which a timekeeper had to perform for its maker to qualify for the £20,000 reward). Thus it made little difference to the outcome of this sea trim that the time of year was too late to permit observations of such a phenomenon to be made at the two ports, for it would have been impossible in any case to distinguish between the errors inherent in the respective methods. An alternative and quite reliable method already existed in previous determinations of the difference in longitude from

s A n Account of the l~roceedings, in order to the Discovery of Longitude at Sea; relating principally to the Time-Piece of Mr. John Harrison, etc., 2rid ed., London, 1763, Appendix IX.

8 Ibid. , Appendix X I

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The Origin and Development of the Marine Chronometer 5

observations of the transit of Mercury on 25 October 1743, 4 by James Short, at George Graham's house in Fleet Street, London, and Alexander Macfarlane in Kingston, Jamaica. After due allowance was made for the effects of parallax and for the differences in longitude between Greenwich and Mr. Graham's house, and between Kingston and Port Royal, the difference in longitude between Greenwich and Port Royal was found to be 5 h 07 TM 02S--just over 20 s (or five nautical miles) less than the modern value, s

A proposal made by Harrison, with a view to testing the consistency-- as opposed to the accuracy--of his watch, was tha t his son William might be sent on H.M.S. Deptford, which was assigned to carry Governor Lyttleton to Jamaica, and make equal altitude observations of the sun in Jamaica upon arrival and just before departure. William Harrison's results could then be used in conjunction with the timekeeper readings and with similar astronomical observations that were to be made before and after the voyage by Mr. Robertson, Head Master of the l~oyal Academy at Portsmouth, for the purpose of obtaining two independent determinations of the required difference in longitude.

The :Board agreed in principle to this plan, but assigned the equal altitude observations in Jamaica to Mr. Robinson, a person ' well skilled in the Mathematicks and particularly Astronomy ,6: they also issued instructions regarding the setting of the watch in Portsmouth and the time given by it on arrival in Jamaica. The watch was put in a case with four different locks, the keys of which were entrusted to William Harrison, Governor Lyttleton, Captain Digges, and his first lieutenant. I t was also stipulated that all four gentlemen had to be present before the case could be opened, even for winding, but this was found to be inconvenient in practice: nevertheless, a certificate given by these three officials to William Harrison at the end of the trial certified tha t he had never had access to H 4 except in the presence of one or more of them. 7

Only one set of equal altitude observations was made in Jamaica, at Port I~oyal on 26 January 1762, a week after the Deptford's arrival and only two days before William Harrison returned to England in H.M.S. Merlin. Copies of these were subsequently checked independently by Matthew Raper, John Howe, and Gael Morris, three suitably-qualified individuals appointed by the Board. This experiment was judged to be

a O14 S t a n d a r d (O.S.) TLme: the re was a r ad i ca l c h a n g e in t h e ca l enda r in Sep t embe r 1752. Fo r deta i ls , see t h e ' E x p l a n a t i o n ' in t h e Nautical Almanac for 1938.

A n Account of the Proceedings . . . , L o n d o n , 1763, A p p e n d i x X I I . 6 B o a r d of L ong i t ude , Confi rmed Minutes , 13 October 1761, col . v , p. 16. Those I~ISS.

a re in t h e l%ecord l%oom of t he l%oyal Greonwmh O b s e r v a t o r y , t I e r s t m o n e e u x Castle. J o t m Har r i son , A Naxrative of the Proceedings relative to the discovery of the ]Longitude

at ~ea; by Mr. John Harrison's Time-Keeper; subsequent to these published in the Year 1763, L o n d o n 1765.

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6 E . G . Forbes on

inconclusive, however, on two grounds: first, errors could have been introduced b y the fact that the astronomers moved their instruments before making the afternoon observations; and secondly, such observa- tions, made on one day only, were not sufficient to determine the time at any one place. I t is interesting to note tha t these objections, which were raised at the meeting of the Board on 3 June 1762, could also have been construed as a reflection both upon the integrity of Messrs. Robertson and Robinson and upon that of the members of the Royal Society who had been responsible for drafting the original scheme of observing eclipses of Jupiter 's satellites (which was surely subject to the second criticism). Positive evidence favouring the accuracy of Harrison's watch, on the other hand, was that the differences in longitude found from the use of the timekeeper, and independently from the above- mentioned observations of the transit of Mercury differed by only 53.1 which, at the latitude of Jamaica, corresponds to about one geographical mile.

Doubts were raised at the next meeting of the Board concerning the accuracy of the latter method of longitude determination, in spite of the fact that at the transit of Mercury in 1753 six Paris observers had obtained times for the moment of total egress which differed by only 23 ', while five observers in London had obtained results which were more than twice as consistent thereby implying that t t 4 must have indeed per- formed with an accuracy well within the limits prescribed in the 1714 act. The Board, however, were not fully satisfied that the watch's apparently excellent performance was not accidental, a view which was doubtless enhanced b y John Harrison's refusal to divulge the principles of its construction, but which was flatly contradicted by the accuracy of William Harrison's predictions of H.M.S. Deptford's position at different times during the course of the outward voyage, s which had greatly impressed the ship's officers.

Despite very bad weather at sea on the return journey, the total error shown by the watch from Portsmouth and back was a loss of 1 TM 54~.5, which in the latitude of that port is equivalent to about eighteen geographical miles. This result, which is independent of the longitude of Jamaica (whose value was also questioned!), undoubtedly constituted evidence of the accuracy of the watch, bu t could not be used in Harrison's favour since the official trial authorized by act of Parliament ended at Jamaica; moreover, there was also the uncertainty of the rate (2½ sees. per day) at which William Harrison had estimated the watch to be losing, and this would affect the result quoted above. Accordingly, the Board

8 A n e x a m p l e is quo ted b y W i l l i a m H a r r i s o n in a l e t t e r to his f a the r , w r i t t e n on b o a r d t h e Deptford w h e n i t was docked a t Made i r a on 14 D e c e m b e r 1761 (eft An account of the l~roceeding~..., L o n d o n , 1763, A p p e n d i x X V I , pp . 82-3).

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The Origin and Development of the Marine Chronometer 7

refused to certify tha t Itarrison had complied with the requirements of the 1714 act until either he, or someone appointed by him, should accompany the watch on another voyage to the West Indies; but they agreed to grant him an interim reward of £1,500 in virtue of the promising performance of his invention, to be deducted from any reward to which he might afterwards become entitled, plus the guarantee of another £1000 after this proposed further trial was completed.

Viewed objectively and in the light of the facts related above, this decision was a just one, although it is easy to sympathize with Harrison, who thought he had been unfairly treated since he knew that the per- formance of his timekeeper was no fluke, bu t a feat tha t it was always capable of performing. The faults incurred by the Board in its dealings over this matter were primarily a lack of foresight in the planning and execution of the trial, and subsequently an over-rigid interpretation of the terms of a not too carefully worded act. The first of these might be ascribed to their lack of experience (at that time) in handling projects of this kind, and to their lack of knowledge of the workings of the time- keeper; the second merely reflects their unwillingness to surrender such a large sum of public money without the most decisive proof of the worthiness of the claim, an at t i tude which the public itself would undoubtedly commend.

T E E S~CO~CD S ~ A T R I A L OF Tr 4

After an unsuccessful a t tempt by John Harrison to obtain more money by petitioning Parliament, arrangements were made for William Harrison to take H 4 with him on a second voyage to the West Indies, this time to Bridgetown in the Barbados on board H.M.S. Tartar. Elaborate precautions were taken to ensure that the difficulties which had arisen in the previous trial would now be avoided; and the necessary instructions were sent to John Harrison on 9 August 1763. Two days before the Tartar sailed from Portsmouth, he prepared a written declaration for the Board of Longitude of the rate of going of his timekeeper, in which he stated that he expected it to keep perfect time at a temperature of 72°F, and to gain or lose one second per day for a decrease or increase of 10°F respectively. He also declared that he would abide by the rate of its gaining on the average one second per day for the forthcoming voyage2

On this occasion, Mr. Bradley, purser of H.M.S. Dorsetshire, was ordered to make equal altitude observations of the sun and observations of the eclipses of Jupiter 's satellites at Portsmouth on at least three occasions, while similar observations in Jamaica were entrusted to the

Rev. Nevil ]VIaskelyne, An Account of the going of Mr. John Harrison's Watch, at the Royal Observatory, from May 6th, 1766 to March 4th, 1767. Together with The Original Observations and Calculations of the same, London, 1767.

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8 E.G. Forbes on

Astronomer Royal's assistant Charles Green and the Rev. Nevfl Maskelyne, the latter having just returned from an unsuccessful expedi- tion to St. Helena in connection with the 1761 transit of Venus. The reflecting telescopes and equal altitude instruments to be used by these observers were carefully examined at the Royal Observatory by William Harrison, Peter Bliss (the Astronomer Royal), and Professors Anton Shepherd and Thomas Hornsby, and were reported to be fully sufficient for the purpose of determining longitude, or the true time at a land observing station.

Maskelyne and Green sailed to the Barbados on board H.M.S. P r i n c e s s L o u i s a several months ahead of William Harrison: their respective observations of Jupiter's satellites and occultations of fixed stars and planets, made during the voyage between 15 November 1763 and 3 March 1764, were later compared with corresponding observations made at Portsmouth and at Greenwich during that same period. The former data were reduced to apparent time, and the longitudes at sea determined independently by three gentlemen nominated by the Board, and a fourth, James Short, nominated by William Harrison. The results obtained from a comparison of Maskelyne and Green's observations at Bridgetown of nine emersions of Jupiter's first satellite with five emersions of the same satellite observed by John Bradley at Portsmouth and two observed at the Royal Observatory, together with those deduced for the time- keeper from the declared clock-rate and the equal altitude observations at Portsmouth and Bridgetown, 1° are recorded in the following form in the minutes of the meeting of the Board on 19 January 1765:

Computers' names

Captain Campbell Dr. Bevis Mr. WitcheU Mr. Short

Difference of Meridians between Portsmouth and Barbadoes Error of

by Observation by Timekeeper Timekeeper ~¢ t • • s n H t i~

3 54 19-1 3 55 00.6 0 00 41.5 3 54 22½ 3 54 56 0 00 33~ 3 54 12.7 3 55 00.05 0 O0 47.3 3 54 18½ 3 54 53 0 oo 34½

Three weeks later, when it was known that these figures were corro- borated by further independent calculations by five other well-qualified members of the Board, the commissioners unanimously agreed that H 4 had kept time beyond the nearest limit required by the 1714 act, the mean error of + 39~.2 being equivalent to a difference in longitude of only 9.8 nautical miles, while the greatest discrepancy (shown in Mr. Witehell's

lo I b i d . The a p p e n d i x inc ludes these e q u a l a l t i t u d e observa t ions , t o g e t h e r w i t h o the r s

made a~ Jamaica dur ing t he firs~ voyage .

8 E . G . ~orbes on

Astronomer Royal 's assistant Charles Green and the Rev. Nevil Maskelyne, the latter having just returned from an unsuccessful expedi- tion to St. Helena in connection with the 1761 transit of Venus. The reflecting telescopes and equal altitude instruments to be used by these observers were carefully examined at the Royal Observatory by William Harrison, Peter Bliss (the Astronomer Royal), and Professors Anton Shepherd and Thomas Hornsby, and were reported to be fully sufficient for the purpose of determining longitude, or the true time at a land observing station.

Maskelyne and Green sailed to the Barbados on board H.M.S. Princess Lonisa several months ahead of William Harrison: their respective observations of Jupiter 's satellites and occultations of fixed stars and planets, made during the voyage between 15 November 1763 and 3 March 1764, were later compared with corresponding observations made at Portsmouth and at Greenwich during that same period. The former data were reduced to apparent time, and the longitudes at sea determined independently by three gentlemen nominated by the Board, and a fourth, James Short, nominated by William Harrison. The results obtained from a comparison of Maskelyne and Green's observations at Bridgetown of nine emersions of Jupiter 's first satellite with five emersions of the same satellite observed by John Bradley at Portsmouth and two observed at the Royal Observatory, together with those deduced for the time- keeper from the declared clock-rate and the equal altitude observations at Portsmouth and Bridgetown, 1° are recorded in the following form in the minutes of the meeting of the Board on 19 January 1765:

Difference of Meridians between Computers' names Portsmouth and Barbadoes Error of

by Observation by Timekeeper Timekeeper H o • I I 0 n H ~ 1,

Captain Campbell 3 54 19.1 3 55 00-6 0 00 41.5 Dr. Bevis 3 54 22½ 3 54 56 0 00 339 Mr. Witehell 3 54 12.7 3 55 00.05 0 00 47.3 Mr. Short 3 54 18½ 3 54 53 0 00 34½

Three weeks later, when it was known that these figures were corro- borated by further independent calculations by five other well-qualified members of the Board, the commissioners unanimously agreed that H 4 had kept time beyond the nearest limit required by the 1714 act, the mean error of + 39s.2 being equivalent to a difference in longitude of only 9.8 nautical miles, while the greatest discrepancy (shown in Mr. Witchell's

lo 1bid . The appendix includes these equal altitude observations, together with others made at $amaiea during the first voyage.

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The Origin and Development of the Marine Chronometer 9

values) was only 11.8 nautical miles, lVforeover, the total error found by comparing H 4 with the clock at Portsmouth, after William Harrison's return in H.M.S. 2~ew Elizabeth, was only a fraction of a second per day. Thus John Harrison's pride in his masterpiece was fully justified by its second trial at sea.

THE CONDITIONS OF AWARD

Although the capabilities of H 4 could no longer be disputed, the Board did not feel authorized to grant Harrison a certificate until he had clarified the principles of the construction of his watch to people whom they would appoint. They therefore resolved to apply to Parliament for permission to pay Harrison £7,500 as soon as he complied with this requirement. Together with the £2,500 that he had already received, n this made up half the sum to which he was legally entitled, and the remaining £10,000 was to be paid to him as soon as it was proved that his method would be of general utility for finding longitude to within 30 geographical miles. These resolutions were soon embodied in the act of 1765 (5 Gee. III, e.20).

The precise conditions with which the Board expected Harrison to comply in order to obtain the first half of the reward were formulated at the next meeting, held on 28 May 1765. He had to supply, on oath, drawings and a written explanation of the principles of construction of his watch, and give a verbal explanation and experimental exhibitions if judged necessary--producing the watch and taking it to pieces in the presence of three gentlemen skilled in mechanics and three watchmakers. He was also to hand over his other three timekeepers to the Board. Harrison objected strongly to the clause obliging him to explain his invention to the satisfaction of the commissioners and rival watchmakers, and aired his general grievances on the matter in an indignant letter composed immediately after being informed of the resolutions of the Board.

At their next meeting, the Board's attempts to persuade him were in vain; for Harrison left abruptly, declaring 'Tha t he never would consent to it, so long as he had a drop of English Blood in his Body ,.1, Despite this display of temperament, he was finally persuaded to take the necessary oath five weeks later and was duly awarded the certificate authorising him to receive £7,500. His watch was sealed up in its box and placed in the custody of Sir Philip Stephens, Secretary to the Board of Admiralty; his written explanation and drawings were given to the

n Exc lu s i ve of ~ho £3000 which had boon awarded ~o him in six equal instalments during the years between 1737 and 1761 while he had been experimenting on improving his invention.

II Board of Longitude, Confirmed Minutes, 13 June 1765, eel . v, p. 50.

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10 E.G. Forbes on

Astronomer Royal (Maskelyne), who had 500 copies of them carefully copied, engraved and printed. An additional safeguard for the preserva- tion of Harrison's discovery was the preparation of a joint written account of the watch by the Astronomer Royal and the six other gentlemen who actually received Harrison's explanation, one of whom (Larcum Kendall) was ordered to make a duplicate of H 4 which he finally surrendered to the Board in January 1770.

After an inspection which the Harrisons were invited to attend, Keudall's copy (K 1) was judged to be completely satisfactory, and was then sent to the Royal Observatory for trial, during which, on 10 January 1772, it stopped even though it had been regularly wound up. It was therefore sent back to Kendall's house and examined there in the presence of the Astronomer Royal and two other watchmakers (John Holmes and James Monk), when the defect was quickly diagnosed. The Board, encouraged by the previous performance of this timekeeper, had already persuaded Kendall to attempt to construct a second watch (K 2), omitting the most expensive parts so that the price would be low enough for general purchase. For this they paid him £200, and at the same time approved Kendall's own suggestion to make yet another (K 3) for only half that sum. This was completed just over two years later and produced at the meeting of the Board on 25 June 1774. As had happened in the case of K 1, however, K 3 also developed a fault during its trial at the Royal Observatory, but it was soon rectified by Kendall. This instrument was used on H.M.S. Discovery during Captain Cook's fateful voyage of 1776-79 and was subsequently at sea with Captains Vancouver and Flinders. However, neither K 2 nor K 3 performed nearly as well as K 1.

TH~ GR~WIC~ TRIAL Or ~ 4

Meanwhile, with a view to obtaining more insight into the behaviour of Harrison's masterpiece under varying conditions of temperature and differences of position, and more detailed knowledge of the general reliability of its mechanism, the Board resolved that H 4 should be subjected to a prolonged trial at the Royal Observatory in the hands of the Astronomer Royal--a decision which did not please Harrison since Maskelyne was, in his opinion, a biased judge of the merits of the watch. On 5 May 1766, H 4 was duly handed over to the Astronomer Royal by the Secretary of the Admiralty in the presence of Captain Thomas Baillie of the Royal Hospital at Greenwich, John Ibbetson, Secretary of the Board of Longitude, and Lareum Kendall himself, who accompanied Maskelyne to the Royal Observatory and witnessed the watch being put in motion and locked up in a box provided for it. On most days from then until the end of its period of trial on 4 March 1767, H 4 was

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The Origin and Development of the Marine Chronometer 11

wound up and compared with the transit clock at Greenwich b y Maskelyne himself; at other times, this du ty was performed by his assistant Joseph Dymond, and afterwards by William Bayly, always in the presence of one of the officers of the Greenwich Hospital.

To begin with, two or three comparisons were made during the day, but from 3 August onwards, with few exceptions, only one such observa- tion was made. The watch was placed in different positions--horizontal with face upwards, tilted, vertical, and horizontal with face downwards-- during the first two months of its trial, in order to test its isochronism; after this, it was restored to its original position (horizontal with face upwards). The report of this trial, published by Maskelyne in the name of the Board, was most unfavourable. After giving an analysis of the magnitudes of the gains and losses, and discussing the irregularities in the variation of these quantities from day to day, Maskelyne concluded:

Mr. Harrison's watch cannot be depended upon to keep the longitude within a degree in a West India voyage of six weeks; nor to keep the longitude within half a degree for more than a fortnight, and then it must be kept in a place where the thermometer is some degrees above freezing?

This verdict was completely opposed to the results of the two sea-trials, bu t Harrison was not slow to point out good reasons for the apparently paradoxical result, la In the first place, the temperature inside the box containing H 4 could have been much greater than that measured on a thermometer in a shady part of the same room, b y reason of its being close to a window facing South-East and having glass in the lid and on one side; moreover, the watch was never designed for temperatures as low as the freezing point, the limits of heat and cold to which his ' thermometer k i rb ' (compensation curb) was adjusted being 82 ° and 42 ° :Fahrenheit respectively. Similarly, H 4 had not been adjusted to keep time accurately in positions into which the motion of a ship could never put it, such as were adopted during the trial. Another reason for the apparently bad behaviour of H 4 during the thirty-six weeks while it was tested with its face upwards, was that Maskelyne had compared its readings with mean time without making due allowance for its rate; for as Harrison pointed out:

it is not necessary that a Watch should perform it's Revolutions precisely in that Space of time which the Earth takes to perform her's; it is only required that it should invariably perform it in some known Time, and then the constant Difference between the Length of the one l~evolution and the other, will appear as so much daily gained or lost by the Watch, which constant Gain or Loss, is called the Rate of its going, and which being added to or deducted from the Time shown by the Watch, will give the true Time, and consequently the Difference of Longitude. la

~a Jo h n Harrison, Remarks on a Pamphlet lately published by the Rev. .Mr. Maskelyne, under the Authority of the Board of Longitude, London, 1767.

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12 E . G . Forbes on

Indeed, when due account is taken of this fact, and if the antics of H 4 in different positions and at extreme temperatures are neglected, the general performance of this watch was amply sufficient--on the basis of the same observational evidence that Maskelyne had presented-- to satisfy the requirements of the 1714 act to within the nearest limits.

THE RECOGNITION OF HAJ~RISON'S RIGHT TO THE GREAT REWARD

After their meeting on 26 April 1766, the Board intimated to Harrison tha t in order to qualify for the second half of the reward he would have to construct another two watches on the same principles, yet when he later requested the loan of his own watch for a period of six months in order to enable him to fulfil this new requirement more quickly than he could otherwise do, he was refused it on the ground that it would soon be required b y Kendall to assist him in making fine adjustments to his own model (K 1). Understandably, this decision annoyed Harrison, who gave vent to his feelings in the form of a libellous at tack on Maskelyne, ~3 whom he had long regarded, with some justification, as prejudiced against timekeepers. Maskelyne planned, but never published, a reply. To make matters worse, the Board were planning more elaborate trials than a West Indies voyage for the watches that the Harrisons were expected to make; and resolved that these two timekeepers, when completed, should be subjected to trials at the Royal Observatory for ten successive months, followed by a further two months in the Downs. 14

Harrison's fifth timekeeper (H 5) was completed in 1770, and its adjustment finished during the following year. On hearing that the Board intended to send K 1 with Captain Cook, then preparing for his second voyage to the South Seas, Harrison appealed to them to send instead H 4 and H 5, offering to rest his claim to the remainder of the reward upon their performance, or to submit ' t o any Mode of Trial, by men not already proved partial, which shall be definite in its nature, conclusive as to the reward in case of success, and, in any degree, near the Limitation of the Act of Queen Anne in point of duration and exactness ,.15 The Board coldly replied that they did not wish H 4 to leave the kingdom, and that they saw no reason to depart from the manner of trial which they had already laid down.

Harrison then asked King George I I I ' s permission to test H 5 at His Majesty's private observatory at Kew, a request which was readily granted. The King was extremely sympathetic to Harrison's situation and had previously granted him two audiences since the publication of accounts of the Tartar's voyage: he took a great interest in the per- formance of this watch, and attended the daily comparisons during the

1, Board of Longitude, 0onfirmec[ iVIinutos, l l April 1767, vol. v, p. 78. 1~ I b i d . , 14 December 1771, vol. v, p. 107.

12 E.G. Forbes on

Indeed, when due account is taken of this fact, and if the antics of H 4 in different positions and at extreme temperatures are neglected, the general performance of this watch was amply sufficient--on the basis of the same observational evidence that Maskelyne had presented-- to satisfy the requirements of the 1714 act to within the nearest limits.

TIIE RECOGNITIOIq OF HARRISON'S RIGHT TO THE GREAT REWARD

After their meeting on 26 April 1766, the Board intimated to Harrison tha t in order to qualify for the second half of the reward he would have to construct another two watches on the same principles, yet when he later requested the loan of his own watch for a period of six months in order to enable him to fulfil this new requirement mere quickly than he could otherwise do, he was refused it on the ground tha t it would soon be required by Kendall to assist him in making fine adjustments to his own model (K 1). Understandably, this decision annoyed Harrison, who gave vent to his feelings in the form of a libellous at tack on Maskelyne, 1~ whom he had long regarded, with some justification, as prejudiced against timekeepers. Maskelyne planned, but never published, a reply. To make matters worse, the Board were planning more elaborate trials than a West Indies voyage for the watches that the I-Iarrisons were expected to make; and resolved that these two timekeepers, when completed, should be subjected to trials at the Royal Observatory for ten successive months, followed by a further two months in the Downs. 14

Harrison's fifth timekeeper (H 5) was completed in 1770, and its adjustment finished during the following year. On hearing that the Board intended to send K 1 with Captain Cook, then preparing for his second voyage to the South Seas, Harrison appealed to them to send instead H 4 and H 5, offering to rest his claim to the remainder of the reward upon their performance, or to submit ' to any Mode of Trial, by men not already proved partial, which shall be definite in its nature, conclusive as to the reward in case of success, and, in any degree, near the Limitation of the Act of Queen Anne in point of duration and exactness ,.15 The Board coldly replied that they did not wish H 4 to leave the kingdom, and that they saw no reason to depart from the manner of trial which they had already laid down.

Harrison then asked King George I I I ' s permission to test H 5 at His Majesty's private observatory at Kew, a request which was readily granted. The King was extremely sympathetic to Harrison's situation and had previously granted him two audiences since the publication of accounts of the Tar tar ' s voyage: he took a great interest in the per- formance of this watch, and attended the daily comparisons during the

1~ Board of Longitude, Confirmed Minutes, 11 April 1767, vol. v, p. 78. 1~ I b i d . , 14 December 1771, vol. v, p. 107.

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The Origin and Development of the Marine Chronometer 13

course of its ten-week trial. The total error on mean time over this period was only 4½ seconds; yet the communication of this result and the circumstances of the trial to the Board produced the same negative response.

Harrison, now in his eightieth year, petitioned the House of Commons, describing the circumstances of his claim and requesting its payment. This was backed by the King, and strongly supported by the Prime Minister, Fox, so when a money bill was drawn up in consequence of the petition it immediately received the royal assent, and Harrison was finally granted a sum of £8,750 which, including the assistance that he had received while constructing and improving his earlier timekeepers, made up the second £10,000. Harrison lived just long enough to hear of the impressive performance in the South Seas of Kendall 's copy of his masterpiece, and died at his home on 24 March 1776. His four famous marine timekeepers, now preserved in the National Maritime Museum at Greenwich, are impressive memorials to the man who, by virtue of his application, determination and perseverance, made the greatest and most significant individual contribution to the development of accurate navigation during the eighteenth century.

MUDGE'S CLAIM :FOR AN AWARD

The recognition of John Harrison's right to the £20,000 reward and the payment of the remainder of this sum to him, coupled with the success of Kendall 's watch which proved that the employment of a time- keeper based on Harrison's principles was generally practicable and useful for the purpose of accurate longitude determination at sea, necessi- ta ted the repeal of all previous acts concerning the discovery of longitude, and the formulation of a new one. The terms of the act of 1774 (14 Geo. III , c. 66) were certainly not designed to give much encouragement to others interested in the further development of the marine timekeeper (or chronometer, as it was now being called). The respective rewards offered to anyone who might invent a timekeeper based on principles other than those which had been made public were halved, and were not to be paid until two timekeepers of the same construction had been successfully tried at the same time for twelve months at the Royal Observatory, then in two voyages round Great Britain in opposite direc- tions (and possibly other voyages), followed by a further trial at Green- wich. Moreover, the principles and practice of construction, together with descriptions, theories, and explanations, had all to be made public and the two timekeepers handed over to the Board of Longitude. A two-thirds majority of the commissioners had to approve the utility and practicability of the method.

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14 E . G . Forbes o n

Among those adversely affected by the passing of this act with its unduly severe restrictions was Thomas Mudge, who had been a member of the committee appointed by the Board of Longitude to report on the mechanism of Harrison's watch and had subsequently retired from business a t the comparatively early age of fifty-six in order to devote himself entirely to the construction of a better timekeeper, which his examination of Harrison's watch (H 4) had convinced him he could achieve. When the act was passed, he had just completed his first watch which he submitted for trim at Greenwich in December of the same year (1774). On two occasions during the next four months it stopped, the cause in the second instance being a broken mainspring. This was at tr ibuted by Mudge to the extra tension produced by its being wound up daily at the Royal Observatory whereas it was designed to go for over a week without rewinding. Mudge was convinced that the earlier stoppage had been caused by his watch being roughly handled while being carried from one room to another for the observation of its daily variation with the pendulum clock in the transit room at the RoyM Observatory, and he lodged a complaint against its t reatment. This accusation was hotly denied by Maskelyne's assistant, Hellins, but to avoid further criticism the watch was kept in the transit room during its second trial which began in November 1776.

At the meeting of the Board in March 1777, Maskelyne gave a favour- able report of the going of the watch, which had been found to gain only 79 seconds in 109 days, remarking tha~ it was ' greatly superior in point of accuracy to any Timekeeper which hath come under his inspection'. 16 The continuation of the trial forced him to alter his opinion, however, since the rate of the watch increased considerably until in February 1778 it again stopped with a broken mainspring. I t then passed into the hands of the Duke of Saxe Gotha and his astronomer Dr. Zach. The latter made ' a multi tude of observations ' with it, the results of which are contained in the Berlin ephemerides for the years 1789 and 1790 and testify to its high accuracy; later, in the same publica- tion for 1794, Zach strongly criticized the method by which Maskelyne had assessed the reliability of timekeepers on trial at Greenwich, remarking tha t it was ' contrary to every principle of reason and justice '. The watch was next taken by Admiral Campbell on two voyages to Newfound- land, on *he first of which, from 4 Ju ly to 21 October 1784, it served to determine longitude with an accuracy well within the most stringent limits required by the 1774 act. The mainspring again broke (for the third time) before the second voyage, from 9 June to 25 October 1785, and had to be replaced; this may have been partly responsible for the

le I b i d . , 1 March 1777, eel. v, p. 156.

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The Origin and Development of the Marine Ghronometer 15

poorer performance of the watch during this voyage, 17 but even then, it was found to be about seven times more accurate than one of Kendall 's watches is which was also on board.

Meanwhile, assisted by the £500 reward which he had received from the Board in 1777 as a result of the Astronomer Royal 's recommendation, Mudge had completed two more watches--known as ' Blue ' and ' Green ' on account of the colours of their respective cases--which had been officially tested at the Royal Observatory on three occasions during 1779-80, 1783-84, 1789-90. TM Although the results of these trials were all judged to be outwith the limits of the 1774 ac t - -on each occasion because ef the progressive acceleration and irregularities in the rates of the watches which Gould has attributed to the defective design of the com- pensation2°--their closeness to those required encouraged Mudge to request a reward for his chronometers on the grounds of their utili ty and the time spent on improving them for general use. This request was, however, rejected, largely on account of the unfair advantage that he would then have had over rival watchmakers whose timekeepers were judged to be of equal merit.

ARNOLD'S POCKET CHRONOMETERS

One such person was John Arnold, who had completed a marine timekeeper which had proved itself to be very accurate during a pre- liminary trial at the Royal Observatory and which he had exhibited to the Board in November 1771. During the next ten years, Arnold received five imprests amounting to £1,400 from the Navy Board in order to enable him to perfect his invention, on the understanding tha t these sums would be regarded as partial payments towards whatever number of watches the Board might eventually think fit to purchase from him at an estimated cost of no more than 60 guineas each. Three of Arnold's watches were tried at sea, on board H.M.S. Resolution and H.M.S. Adventure, with little success. One stopped at the Cape of Good Hope owing to being roughly handled, and when restarted it behaved most erratically; its companion aboard the Adventure stopped before the Cape was reached. The third watch also stopped, because the maintaining gear jammed.

17 The figures for the daily rates during both voyages quoted by Thomas Mudgo, jun., m his Narratiw of Facts (see note 24) ~ro not reliable, since no correction had boon made for the clock-rate.

10 Mudge's son s ta ted (see note 24) tha t this was Kendall 's copy of Harrison's watch (K 1); but the Roy. :Novll Maskelyno in his Answer (see note 25) pointed out t ha t the timekeeper in question was, in fact, Kendall 's third (K 3), which was three times loss accurate than K 1.

1~ A detailed history of these trlals is contained in Maskelyno's Answer to Mudgo (see note 25).

20 Ruper t T. Gould, The Marine Uhronometer, London, 1923, p. 80.

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16 E . G . ~Forbes o n

The Board therefore refused to give Arnold further financial assistance ' 'tfl they shall have better proofs of the merits of the Watches, which they have already had of him, or be satisfied that he has already made some very considerable improvements ,.21

Perhaps stimulated by the need to effect some significant improve- ments, Arnold began making pocket chronometers fitted with a helical (i.e. cylindrical) spring and a compensation balance, which he patented (No. 1113, 1776). He sent one such watch (No. 36) to the Royal Observa- tory for trial in March 1779. At the end of its official thirteen-month trial, its total error--allowing a mean rate based on its performance during the first month of the t r ia l - -had accumulated to only 2 m 33s.2, while its daily rate never varied by more than 3 S on two consecutive days. His success with certain other improvements in the construction of such watches encouraged Arnold to take out another patent (No. 1328, 1782), which he then offered to surrender to the Board on suitable terms, along with a complete description a n d explanation of the construction and manner of adjustment of his chronometers. 2~

Consideration of Arnold's memorial was postponed indefinitely, however, until the Board automatically dismissed it several years later by resolving that they would give no assistance to any inventor who secured his invention by a patent. There was no question of Arnold obtaining a reward under the terms of the 1774 act, since he never complied with the requirement that two watches had to be submitted simultaneously for trial at the Royal Observatory. Nevertheless, the successful performances of his timekeepers on land and at sea were well known at tha t time; and it was largely because of this, and the promising work of another watchmaker, Josiah Emery, tha t the Board decided not to support 1Vfudge's claim.

THE QUESTIOl~ OF T HE C H R O N O M E T E R E R R O R

At this stage Thomas ~udge, junior, who was a lawyer, decided to draft a petition to Parliament, describing his father's work and hoping that it might be suitably rewarded--an action which caused Arnold to issue a pamphlet stating his own claims for further remuneration. 2a In order to gain public support for his petition, Mudgo also published a pamphlet the following year (1792), 34 in which, among other things, he

,1 B o a r d o f L ong i t ude , Confi rmed 1VIinutos, 25 J u n o 1774, vol. v, p. 131. 22 T h e w ord ' c h r o n o m e t e r ' occurs for t h e f irst t ime in t h e B o a r d ' s m i n u t e s of

7 D e c e m b e r 1782. ~a Uertifieates and Circumstances relative to the going of Mr. Arnold's Uhronomcters,

L o n d o n , 1791. ~ T h o m a s l~ludge, j un . , A Narrative of ~aets relating to some Time.Keepers, constructed

by Mr. Thomas Mudgc .for the discovery of longitude at sea: together with Observations upon the conduc~ o.f the Astronomer Royal respecting them, L o n d o n , 1792.

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The Origin and Development of the Marine Chronometer 17

openly accused the Astronomer Royal of deliberately falsifying the registers containing the accounts of the trials of his father's watches. This brought forth a hurried, but dignified, reply from Maskelyne ~5 in an a t tempt to justify his position. Unable to let his case rest, Mudge wrote a further pamphlet and published it before the end of the year, declaring that the only motive behind his at tack on the Astronomer Royal and the presentation of his ' facts ' to the public was ' to vindicate the reputation of an aged Father, who is entitled to all the zeal and activity I can exert upon him; to state his claims impartially to the Public; and to obtain for him that countenance, and support, to which his long, and successful, labours have so justly entitled him. '26

The most significant of Mudge's objections was his criticism of the criterion which Maskelyne had adopted for assessing the accuracy of the performance of his father's watches--namely, tha t of basing the mean daily rate upon the first month's observations and applying the value thus obtained to the whole of the following year. He claimed that this was unfair and exaggerated the errors of these timekeepers. In the case of Harrison's watch, Maskelyne had determined the accuracy by first finding the mean gain or loss over six-week periods immediately succeeding each other, and considering the error to be the difference in the rate of going in these two periods. On the basis of this criterion Mudge's son claimed that the ' Blue ' and ' Green ' chronometers were respectively seven and nine times more accurate than Harrison's watch, He also argued that since the 1774 act specified a voyage of nine months instead of six weeks as required by the (repealed) 1714 act, a six-month period of comparison might have been used in testing his father's watches; had this been done, the difference between the amounts gained during the two six-month periods while it had been on trial (i.e. the error) would have been less than two minutes and consequently within the most extended limits of the act. Another possibility, which Mudge would also have found acceptable, was to define the error as the deviation between the mean daily rate based upon the whole year 's observations and the observed daily rate.

An alternative method adopted by Maskelyne for determining the degree of perfection of Harrison's watch had been to define the error as the difference between the greatest and least quant i ty of time measured by the watch on any two days during the course of the trial. On one day

35 Rev. Noml Maskolyne, A n Answer to a pamphlet entitled ' A Narrative of Facts ', lately published by Mr. Thomas Mudge, Junior, relating to some Time-Keepers constructed by his Father Mr. Thomas Mudge, etc., London, 1792.

~a Thomas Mudge, jun., A Reply to the Answer of the Roy. Dr. Maslcelyne, Astronomer Royal, to A Narrative of JFacts, eta., London, 1792, p. 169.

Ann. of Bd.--Vol . 22, No. 1. b

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18 E . G . Forbes o n

Harrison's watch had gained 21s-5, and on another lost 6s.5, on mean time, thus giving a total difference of 28s; whereas the corresponding differences for Mudge's two timekeepers (in the third trial) were 9s.1 and 5~.3, again testifying to their superior accuracy.

Maskelyne, writing in his own defence and backed by the Board of Longitude, stated that he had subjected Mudge's watches to more severe tests than Harrison's since Mudge himself had previously claimed tha t this ought to have been done with Harrison's watch, and the Board had resolved in favour of so doing. 2~ His reasons for choosing one month as the period of time from which the clock-rate was deduced were, how- ever, mainly conjectural, ~s and not at all convincing; on the other hand, objections could also be made against the methods suggested by Mudge. The intrinsic difficulty really lay in the ambiguity of the words ' accuracy ' a n d ' e r ror ' when applied to timekeepers.

Maskelyne dismissed as ' preposterous ' Mudge's proposM to base the mean rate on the two worst errors, and considered his other proposals to be impracticable. Be that as it may, Mudge's main cri t icism--that a long period of time ought to be used for obtaining the mean rate of chronometer rather than a short one--was undoubtedly valid. Another difference of opinion between these gentlemen arose over the allowance of a clock-rate, which Maskelyyne regarded as an ' indulgence '. Mudge indignantly asserted that, on the contrary, his father's watches would have appeared to have gone better w i t h o u t the rate which the Astronomer Royal had used than w i t h it; so that the rate had been adopted without any consideration as to whether it was favourable or unfavourable.

The above evidence was duly taken into account by a select sub- committee set up by the House of Commons Committee ~9 appointed to consider Mudge's petition. This sub-committee consisted of three scientists (the Bishop of St. David's, George Atwood, J. A. de Luc), two mathematicM instrument makers (Jesse Ramsden and Edward Troughton), and three watchmakers (John Holmes, Charles Haley, William Howells). In spite of the opposition of the Board of Longitude, and the fact tha t their sittings were regularly at tended by l~evil l~askelyne, Sir Joseph Banks, John Arnold, etc., who were all in a position to give evidence against Mudge's timekeepers, the sub-committee in a report issued on 20 May 1793 recommended the petition to the House, praising Mudge's invention but remarking t h a t ' no judgment can be formed of the exactness of any time-keeper by theoretical reasoning upon the principles of its construction, with such certainty as with safety to be relied upon, except

~' Board of Longitude, Confirmed Minutes, ii April 1767, vol. v, p. 78. 3s !VIask~lyno, op. cir. (see note 9), pp. 85-89. ~* Pitt, Fox, Elliott, Shuckburgh, Bragge, Gregor, Ryder and Wbadham.

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The Origin and Development of the Marine Ghronometer 19

it be confirmed by experiments of the actual performance of the machine'.a° In consequence of this, the House awarded Thomas Mudge, senior, £2,500 on the understanding that a further sum of money would be forthcoming if future experience of his invention should merit it.

M U D G E ' S COMMERCIAL E N T E R P R I S E

In an a t tempt to supply chronometers on a commercial basis, Thomas Mudge, junior, set up a manufactory of his father's timekeepers; and several months after his father's death he wrote to the Secretary of the Admiralty (Evan Napean) proposing tha t he should enter into a contract with the Admiralty to supply the Navy with timekeepers, stating the conditions and price which he thought should apply. 81 Before any action was taken on this original request, he was asked by Lord Spencer, First Lord of the Admiralty, to send him some specific proposals. On this occasion, Mudge asked for a fourteen-year contract to supply time- keepers made upon the same principles as, and after the invention of, those constructed by his late father, a2 He did not specify the total number, but guaranteed the delivery of eight per year for the first three years, and thereafter twelve per year until the contract expired. His prices, which tended to be higher than his original estimates, ranged from 150 to 180 guineas. The watches were to be delivered to the Master of H.M. Academy at Portsmouth (at that time, William Bayly), who would test them for six weeks from the time of delivery and certify them as fit for use if their total error at the end of that t ime--af ter allowing for the maker's rate aa given with the ins t rument--did not exceed 1 m 20' (or ½o). The Navy Board agreed to these terms and ordered eighteen timekeepers.

By the end of the eighteenth century, Mudge had supplied six chrono- meters to the British navy and two to the Danish navy, and had sold three more. A dozen more had been made, but not sold, since by this time timekeepers made by other artists, including Arnold & Son, were rapidly becoming available at lower costs and the Navy were loath to keep to their agreement without first obtaining a surety, by means of comparative trials, that their money was being spent to the best advant- age. Mudge was forced to abandon the scheme, having lost heavily by

30 T h o m a s Mudgo. j un . , A Deser*ption, with plates, of the Time-Keeper invented by the late Mr. Thomas Mudge, etc . , L o n d o n , 1799, p. xxxf i .

31 Ibid., pp. lvi i i - lxi . *a Ibid., pp . lxx i - lxx i i . 8a A l t h o u g h n o t expl ic i t ly s t a t ed , one can infer a f t e r r ead ing Mudgo ' s or iginal p roposa l s

t h a t t h i s was t h e m o a n va l ue o f t h e errors f o u n d b y c o m p a r i n g t h e t imes s h o w n on t h e wa¢ch w i th t r a n s i t s o f t h e s u n for t h e first t w o w e e k s of t h e w a t c h ' s going.

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20 E . O . l~orbes on

it, since it had become increasingly evident that no one could make a timekeeper of his father's design perform nearly as well as the original ones.

EARNSHAW'S CHRONOMETERS

Another watchmaker who, before that time, was making timekeepers which were not only much cheaper but also deemed by a committee of the Board of Longitude to be ' incomparably bet ter ,3a than those of Mudge, was Thomas Earnshaw. Earnshaw's major improvements to chronometer design were his detached escapement and compensation balance, these being the most essential features of this instrument and practically the only ones which differentiate it from a common watch. According to his own account, 35 Earnshaw was first introduced to Maskelyne when he left a chronometer for a private trial at the Royal Observatory on 1 Ju ly 1789, as the man who had invented the escapement for which Arnold had taken out a patent. The results of this trial, which lasted six weeks, were extremely consistent, the greatest and least daily rates differing b y less than four seconds. Two years later, as a result of a semi-official comparative trial at Greenwich of five pocket timekeepers constructed by Earnshaw, three box timekeepers by Arnold and a time- keeper by Brockbank, Captain William Bligh (at whose invitation the trial had been arranged) chose one of Earnshaw's chronometers as being the best, and took it with him on his voyage to the West Indies in H.M.S. Providence.

After two unsuccessful a t tempts at petitioning the Board for assistance, Earnshaw was finally offered £200 for two watches which were to be sent to the Royal Observatory for trial, on the understanding that a more substantial sum would be given him if the results of that trial should merit it. He at first declined this offer, since he regarded the sum as insufficient, but subsequently submitted another two watches (Nos. 1 and 2) for trial on 1 January 1798 under similar conditions. In the meantime, however, he had constructed a first-class regulator for the observatory at Armagh, which was tested at the Royal Observatory for one year (during which time it went extremely well), before Earnshaw personally installed it in the Irish observatory, where it continued to keep excellent time and almost invariably enabled the right ascension of stars to be computed with an accuracy of a quarter of a second. ~6 But perhaps of greater importance to the expansion of his business were the results of three sea trials of his chronometers, in competition with others made by Mudge, Arnold, and other makers, in H.M.S. Sans Pareil during 1796, 1797, and 1798; these were decidedly in his favour.

8~ Board of Longitude, Confirmed iViinutes, 11 June 1796, vol. vl, p. 265. a5 Thomas Earnshaw, Longitude. An Appeal to the Public, etc., London, 1808. a6 Ibid., Appendix E: letter from Dr. Hamil ton to Earnshaw, dated 26 December 1807.

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The Origin and Development of the Marine Ghronometer 21

The results of the trial of his No. 1 and No. 2 timekeepers were very good indeed but, according to Maskelyne's method of applying the mean rate of the first month to successive periods of six months, outside the limit of error imposed by the 1774 act (4 minutes). Had Earnshaw's proposed method of applying to each period of six months the mean rate of the preceding month 3~ been adopted, however, No. 1 would have satisfied the requirement of the act for all six, and No. 2 for all but two, of the periods.

At his own request, both watches were returned to Earnshaw for further adjustment, and then submitted for a second trial which lasted from 17 October 1799 to 17 November 1800. 8s The figures for the going of the two watches according to both methods of computation are given in the Board's minutes of 6 December 1800; but again, the results based on this period of trial were not quite accurate enough. Yet over six months, which was the maximum length of trial definitely required by the act, both were decidedly within the required limit. Accordingly, although the Board felt tha t Earnshaw was not entitled to the large reward, they decided to grant him £500 for his endeavour, on the understanding that this sum would be deducted from any such reward which might subsequently be made.

A few months later, they agreed to give the same watches a third trial at the Royal Observatory, which duly took place between 24 Ju ly 1801 and 24 November 1802; 39 but Earnshaw had over-corrected the temperature compensations, and the final figures based upon this sixteen- month trial 4° show that their errors in periods of six months each had again exceeded the prescribed limit. I t was therefore resolved that no further trial of them should be made, and Earnshaw's request for their trial at sea was refused despite the fact tha t Captain Flinders had reported that another two of his watches (Nos. 520 and 543) had gone ' v e r y regular '.

The commissioners were nevertheless sympathetic to Earnshaw's claim for a reward not less than that given to other mechanics (i.e. watch- makers), and recognized the fact that his watches had ' gone better than any others that have been submitted to trial at the Royal Observatory'. 41

8~ This was first suggested by E~rnshaw in his pet~tmn to the Board dated 2 June 1798, which is printed in his Longitud~ (see note 35), p. 80, and again in his petition dated 30 November 1798, a handwri t ten copy of which is to be found in the Board of Longitude, Confirmed Minutes, 1 December 1798, vol. vi, p. 298.

8s Gould (op. c/t., p. 122) quotes Sane 1800 as the last month of the trial. The dates given in the present text , however, were taken directly from the Board of Longitude's Minutes of 6 December 1800, which are likely to be the most authentic source of reference.

8~ Gould (op. c~'$., p. 122) quotes September instead of November. 40 Board of Longitude, Confirmed Minutes, 2 December 1802. vol. vfi, p. 18. ~ Ibid., 3 March 1803, vol. vii, pp. 21-22.

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22 E . G . Forbes on

They were generally, though not unanimously, of the opinion that he deserved at least as much as Parliament had given to Mudge, provided tha t he submitted the required descriptions, etc., and resolved to ask for an additional £2,500 on Earnshaw's behalf when petitioning Parlia- ment for more money. The Chancellor of the Exchequer, however, preferred the Board to receive their customary award from Parliament first and then decide how much of it should be awarded to Earnshaw, thus placing the whole responsibility on the Board. a2 This was felt to necessitate the introduction of a new condition in the 1803 act (43 Gee. I I I , c. 118), namely, that any single award amounting to more than £1,000 would now have to be certified by the First Commissioner of the Admiralty, the l~irst Commissioner of the Iqavy, the President of the Royal Society, the Royal Astronomer at Greenwich, and the Controller of the I~avy-- not merely any five of the commissioners, as when lesser sums were to be distributed. Any one of the above five gentlemen was therefore in a position to veto Earnshaw's award; and unfortunately for him one did, obliging him to wait for over two years before finally receiving the money to which he and almost every other member of the Board considered he was justly entitled.

BANKS'S PROTEST

Earnshaw's antagonist was none other than Sir Joseph Banks, the President of the Royal Society and one of the most influential men of tha t time. Banks, along with many others, was not convinced of the originality of Earnshaw's claim to be the inventor of the improvement of Le Roy's detached escapement, for which Arnold had taken out a patent in 1782. The Board accordingly investigated the matter, and interrogated a number of watchmakers. The balance of evidence collected during the course of a year seemed to favour the t ru th of Earnshaw's repeated accusation tha t Arnold had known of Earnshaw's escapement for eight days before taking out his patent, which throughout the course of over twenty years Arnold had never a t tempted to deny. In order tha t the nature of their respective improvements would be better appreciated, Arnold and Earnshaw were asked to make five-times enlarged models, and drawings, of the escapements of their respective timekeepers, together with short descriptions for the inspection of the Board.

While all this was going on, Sir Joseph published a printed protest against Earnshaw's being granted an award, in which he exhibited the comparative going of seven watches: two of Earnshaw's, four of Arnold's, and one of Emery's, and claimed tha t ' a bare inspection of the errors

a2 Let ter of Sir Joseph Banks to an anonymous person (probably the Astronomer Royal)~ dated 4 April 1803, Royal Society, Misc. MS. 8.48.

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The Origin and Development of the Marine Chronometer 23

found by the method of using the first month 's rate for all the periods of six, is sufficient to show the considerable advantage that the Watches of Arnold and Emery possess over those of Earnshaw ,.43 He also deplored the tendency of Earnshaw's watches to accelerate.

I t has been stated by Gould, following the strongly biased account of these proceedings given by Earnshaw in his book, aa tha t Sir Joseph's views on this subject were coloured by his friendship with Arnold and his son to such an extent tha t he would ' oppose, tooth and nail, anything tending to exalt or reward another chronometer maker to the detriment of their reputat ion ,.as However, Sir Joseph's motives for lodging his protest and for opposing Earnshaw's receiving a reward, as judged from the content of the following letter writ ten by him to an anonymous person, appear to be of a nobler nature, as one might expect from a gentleman of his social standing and scientific reputation.

Sir As the whole time of the last Board of Longitude was scarcely

sufficient for the Reading of Certificates of the accurate going of Mr. Earnshaws time keepers and the examination of witnesses brought forward to prove that Mr. Earnshaw was the inventor of the detached escapement for which Mr. Arnold took out a Patent twenty two years ago, I have thought it proper, in order to save to the Public the charge and to the Professors the time taken up by extraordinary Boards, to send to you for your leisure consideration the opinions I wishd at that time to have brought forward which I shall beg leave, in case the majority of the Board persist in their intention of Rewarding Mr. Earnshaw, to deposit it among the papers of the board as a protest on my part; I have eausd it to be printed in order to secure accuracy which I could hardly expect from common manuscribers and to spare the expcnee of employing such a one acquainted with the subject.

Yours etc. etc. etc. P.S.

I beg leave however to add, that I feel perfectly conscious of the superior knowledge of my friends the Professors on this subject and of course entertain a lively hope, that in consequence of a leisure perusal of the paper, you Sir or some other of them will discover Errors into which I have fallen and convince me that my opinion on the subject is not well founded and I take the liberty Sir to assure you I shall in such case have ten times more of pleasure in withdrawing the paper entirely and voting for the Reward, than I can have of pleasure in fulfilling the dictates of my conscience by lodging it as a Protest against the opinion of the majority who do so. as

as, Sir Joseph Banks ' s Protes t agains t a Vote of the Board of Longitude, grant ing to Mr. Earnshav¢ a Reward for the Merit of his Time-Keepers , ' 19 March 1804, Royal Society, Misc. MS. 8.55a.

'~ See note 35. ha Gould, op. cir., p. 123. 46 Let ter of Sir Joseph Banks to an untmown person, wri t ten at Soho Square on 19 l~Iareh

1804, Roya l Society, Misc. MS. 8.55b.

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24 E . G . Forbes o n

THE VII~DICATIOIq OF EARNSHAW'S CLAIM

From the content of the above letter, it seems reasonable to infer tha t it may have been written to the Astronomer Royal, and a copy of the ' . . . P r o t e s t . . . ' enclosed with it. At any rate, by the end of tha t year Maskelyne had written an answer to Banks's pamphlet, in which he pointed out several errors and misconceptions and gave arguments for giving a reward to Mr. Earnshaw. 47 He claimed that the Board's method of taking one month's rate and applying it to the succeeding six months favoured Earnshaw's watches, and should be used in preference to the other two discussed by Banks, which revealed any accelerative tendency and caused Earnshaw's watches to appear to be the least accurate. After due consideration had been given to all the above factors, the chairman of the Board of Longitude, Lord Melville, accepted ~¢iaskelyne's and the Professors' judgment in favour of Earnshaw, but tactfully ruled tha t Arnold's reward should be made equal to Earnshaw's. Thus, after these watchmakers had disclosed on oath the whole of the construction and method of adjustment of their respective timekeepers, and delivered the specifications to the Board, it was duly resolved at a meeting on 12 December 1805 tha t each should be rewarded the balance of £3,000, as promised--Earnshaw receiving £2,500 and Arnold's son £1,678 in recognition of his father's services.

Earnshaw was far from satisfied with this arrangement, and only two months after receiving his reward he wrote to the public papers, under the heading 'Longi tude ', relating his version of the circumstances described, as This included a libellous at tack upon Banks and Gilpin, the Board's secretary, which caused the former to threaten to sue the writer and elicited an ably written reply on Banks's behalf by Alexander I)alrymple, the hydrographer of the navy. a9 During the same year 250 copies of Arnold's and Earnshaw's specifications, along with questions tha t had been put to them by other watchmakers, etc. and their answers, were printed and published together, and copies sent to those consulted. After two requests in writing to the Board and a private letter to their chairman, the I-Ion. Thomas Grenville, First Lord of the Admiralty, Earnshaw was eventually (on 5 ~/Iarch 1807) authorized to use copies of the Board's resolutions and evidence relating to him and his timekeepers to assist him in preparing a petition to Parliament requesting more money. The Board did not approve of Earnshaw's application, but granted their co-operation merely to allay any suspicion of their conduct in the matter .

aT The Astronomer l%oyal's answer, da ted 1 December 1804, is quoted by Earnshaw, op. c'it. (see note 35), pp. 169-182.

as Morning Chronicle, 4 Februa ry 1806, and The Times, 13 February 1806. a9 Longitude--a ful l answer to the Advertisement, concerning Mr. Earnshaw's Time-

Icee/per..., London, 1806.

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The Origin and Development of the Marine Chronometer 25

This was the information with which Earnshaw was armed when he composed his book, to which he appended no less than 74 ' Certificates of the Going of Mr. Earnshaw's Timekeepers ' as additional proof of the practicability and general utility of his invention. 50 Despite these glowing reports, his petition to Parliament proved unsuccessful since the committee which examined it considered the previous reward authorized by the Board to be adequate. But whatever financial value his con- temporaries may have placed upon his contribution, it remains to Earnshaw's credit that in his hands the chronometer was transformed into essentially the same machine that it is to-day. Not only should we praise him and the other great pioneers in the making of chronometers for achieving enormous advances in performance and design, but also for producing these instruments in large numbers and at relatively low prices, a development of the utmost practical importance to the future of navigation.

A C K N O W L E D G M E N T S

I should like to record my thanks to the Astronomer Royal, Sir Richard Woolley, F.R.S., for permission to examine the Board of Longitude papers at the Royal Greenwich Observatory, which provided a valuable guide to the other references that I have consulted. I am indebted to Mr. Philip Laurie at the Royal Greenwich Observatory, and to Mrs. Barbara Harrington in her former capacity as librarian at the Royal Astronomical Society, for their kind and generous assistance, and to Lt.-Cdr. D. W. Waters, R.N., Curator of Navigation and Astronomy, National Maritime Museum, Greenwich, for reading the manuscript and making several helpful suggestions which have been incorporated into the text.

50 See note 28 .

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