HOW MARINE CHRONOMETERS ARE RATED AT THE ROYAL OBSERVATORY, GREENWICH.
The determination of the Longitude at Sea requires simply accurate instruments for the measurement of the positions of the heavenly bodies, and one or other of the two following,—either perfectly correct watches—or chronometers, as they are now called—or perfectly accurate tables of the lunar motions.
So early as 1696 a report was spread among the members of the Royal Society that Sir Isaac Newton was occupied with the problem of finding the longitude at sea; but the rumour having no foundation, he requested Halley to acquaint the members “that he was not about it.”[56] (Sir David Brewster’s Life of Newton.)
In 1714 the legislature of Queen Anne passed an Act offering a reward of 20,000l. for the discovery of the longitude, the problem being then very inaccurately solved for want of good watches or lunar tables. About the year 1749, the attention of the Royal Society was directed to the improvements effected in the construction of watches by John Harrison, who received for his inventions the Copley Medal. Thus encouraged, Harrison continued his labours with unwearied diligence, and produced in 1758 a timekeeper which was sent for trial on a voyage to Jamaica. After 161 days the error of the instrument was only 1m 5s, and the maker received from the nation 5000l. The Commissioners of the Board of Longitude subsequently required Harrison to construct under their inspection chronometers of a similar nature, which were subjected to trial in a voyage to Barbadoes, and performed with such accuracy, that, after having fully explained the principle of their construction to the commissioners, they awarded him 10,000l. more; at the same time Euler of Berlin and the heirs of Mayer of Göttingen received each 3000l. for their lunar tables.
The account of the trial of Harrison’s watch is very interesting. In April 1766, by desire of the Commissioners of the Board, the Lords of the Admiralty delivered the watch into the custody of the Astronomer-Royal, the Rev. Dr. Nevil Maskelyne. It was then placed at the Royal Observatory at Greenwich, in a box having two different locks, fixed to the floor or wainscot, with a plate of glass in the lid of the box, so that it might be compared as often as convenient with the regulator and the variation set down. The form observed by Mr. Harrison in winding up the watch was exactly followed; and an officer of Greenwich Hospital attended every day, at a stated hour, to see the watch wound up, and its comparison with the regulator entered. A key to one of the locks was kept at the Hospital for the use of the officer, and the other remained at the Observatory for the use of the Astronomer-Royal or his assistant.
The watch was then tried in various positions till the beginning of July; and from thence to the end of February following in a horizontal position with its face upwards.
The variation of the watch was then noted down, and a register was kept of the barometer and thermometer; and the time of comparing the same with the regulator was regularly kept, and attested by the Astronomer-Royal or his assistant and such of the officers as witnessed the winding-up and comparison of the watch.
Under these conditions Harrison’s watch was received by the Astronomer-Royal at the Admiralty on May 5, 1766, in the presence of Philip Stephens, Esq., Secretary of the Admiralty; Captain Baillie, of the Royal Hospital, Greenwich; and Mr. Kendal the watchmaker, who accompanied the Astronomer-Royal to Greenwich, and saw the watch started and locked up in the box provided for it. The watch was then compared with the transit clock daily, and wound up in the presence of the officer of Greenwich Hospital. From May 5 to May 17 the watch was kept in a horizontal position with its face upwards; from May 18 to July 6 it was tried—first inclined at an angle of 20° to the horizon, with the face upwards, and the hours 12, 6, 3, and 9, highest successively; then in a vertical position, with the same hours highest in order; lastly, in a horizontal position with the face downwards. From July 16, 1766, to March 4, 1767, it was always kept in a horizontal position with its face upwards, lying upon the same cushion, and in the same box in which Mr. Harrison had kept it in the voyage to Barbadoes.
From the observed transits of the sun over the meridian, according to the time of the regulator of the Observatory, together with the attested comparisons of Mr. Harrison’s watch with the transit clock, the watch was found too fast on several days as follows:
h. m. s. 1766. May 6 too fast 0 0 16·2 May 17 ” 0 3 51·8 July 6 ” 0 14 14·0 Aug. 6 ” 0 23 58·4 Sept. 17 ” 0 32 15·6 Oct. 29 ” 0 42 20·9 Dec. 10 ” 0 54 46·8 1767. Jan. 21 ” 1 0 28·6 March 4 ” 1 11 23·0 From May 6, which was the day after the watch arrived at the Royal Observatory, to March 4, 1767, there were six periods of six weeks each in which the watch was tried in a horizontal position; when the gaining in these several periods was as follows:
During the first 6 weeks it gained 13m 20s, answering to 3° 20′ of longitude. In the 2d period of 6
weeks (from Aug. 6
to Sept. 17)” 8 17 ” 2 4 ” In the 3d period (from
Sept. 17 to Oct. 29)” 10 5 ” 2 31 ” In the 4th period (from
Oct. 29 to Dec. 20)” 12 26 ” 3 6 ” In the 5th period (from
Dec. 20 to Jan. 21)” 5 42 ” 1 25 ” In the 6th period (from
Jan. 21 to Mar. 4)” 10 54 ” 2 43 ”
| h. | m. | s. | |||
| 1766. | May 6 | too fast | 0 | 0 | 16·2 |
| May 17 | ” | 0 | 3 | 51·8 | |
| July 6 | ” | 0 | 14 | 14·0 | |
| Aug. 6 | ” | 0 | 23 | 58·4 | |
| Sept. 17 | ” | 0 | 32 | 15·6 | |
| Oct. 29 | ” | 0 | 42 | 20·9 | |
| Dec. 10 | ” | 0 | 54 | 46·8 | |
| 1767. | Jan. 21 | ” | 1 | 0 | 28·6 |
| March 4 | ” | 1 | 11 | 23·0 |
| During the first 6 weeks | it gained | 13m | 20s, | answering to | 3° | 20′ | of longitude. |
| In the 2d period of 6 weeks (from Aug. 6 to Sept. 17) | ” | 8 | 17 | ” | 2 | 4 | ” |
| In the 3d period (from Sept. 17 to Oct. 29) | ” | 10 | 5 | ” | 2 | 31 | ” |
| In the 4th period (from Oct. 29 to Dec. 20) | ” | 12 | 26 | ” | 3 | 6 | ” |
| In the 5th period (from Dec. 20 to Jan. 21) | ” | 5 | 42 | ” | 1 | 25 | ” |
| In the 6th period (from Jan. 21 to Mar. 4) | ” | 10 | 54 | ” | 2 | 43 | ” |
It was thence concluded that Mr. Harrison’s watch could not be depended upon to keep the longitude within a West-India voyage of six weeks, nor to keep the longitude within half a degree for more than a fortnight; and that it must be kept in a place where the temperature was always some degrees above freezing.[57] (However, Harrison’s watch, which was made by Mr. Kendal subsequently, succeeded so completely, that after it had been round the world with Captain Cook, in the years 1772–1775, the second 10,000l. was given to Harrison.)
In the Act of 12th Queen Anne, the comparison of chronometers was not mentioned in reference to the Observatory duties; but after this time they became a serious charge upon the Observatory, which, it must be admitted, is by far the best place to try chronometers: the excellence of the instruments, and the frequent observations of the heavenly bodies over the meridian, will always render the rate of going of the Observatory clock better known than can be expected of the clock in most other places.
After Mr. Harrison’s watch was tried, some watches by Earnshaw, Mudge, and others, were rated and examined by the Astronomer-Royal.
At the Royal Observatory, Greenwich, there are frequently above 100 chronometers being rated, and there have been as many as 170 at one time. They are rated daily by two observers, the process being as follows. At a certain time every day two assistants in charge repair to the chronometer-room, where is a time-piece set to true time; one winds up each with its own key, and the second follows after some little time and verifies the fact that each is wound. One assistant then looks at each watch in succession, counting the beats of the clock whilst he compares the chronometer by the eye; and in the course of a few seconds he calls out the second shown by the chronometer when the clock is at a whole minute. This number is entered in a book by the other assistant, and so on till all the chronometers are compared. Then the assistants change places, the second comparing and the first writing down. From these daily comparisons the daily rates are deduced, by which the goodness of the watch is determined. The errors are of two classes—that of general bad workmanship, and that of over or under correction for temperature. In the room is an apparatus in which the watch may be continually kept at temperatures exceeding 100° by artificial heat; and outside the window of the room is an iron cage, in which they are subjected to low temperatures. The very great care taken with all chronometers sent to the Royal Observatory, as well as the perfect impartiality of the examination which each receives, afford encouragement to their manufacture, and are of the utmost importance to the safety and perfection of navigation.
We have before us now the Report of the Astronomer-Royal on the Rates of Chronometers in the year 1854, in which the following are the successive weekly sums of the daily rates of the first there mentioned:
| Week ending | secs. | ||
| Jan. | 21, | loss in the week | 2·2 |
| ” | 28 | ” | 4·0 |
| Feb. | 4 | ” | 1·1 |
| ” | 11 | ” | 5·0 |
| ” | 18 | ” | 4·9 |
| ” | 25 | ” | 5·5 |
| Mar. | 4 | ” | 6·0 |
| ” | 11 | ” | 6·0 |
| ” | 18 | ” | 1·5 |
| ” | 25 | ” | 4·5 |
| Apr. | 1 | ” | 4·0 |
| ” | 8 | ” | 1·5 |
| ” | 15, | gain in the week | 0·4 |
| Apr. | 22, | ” | 2·6 |
| ” | 29, | loss in the week | 1·4 |
| May | 6 | ” | 2·1 |
| ” | 13 | ” | 3·0 |
| ” | 20 | ” | 5·1 |
| ” | 27 | ” | 3·3 |
| June | 3 | ” | 2·8 |
| ” | 10 | ” | 1·8 |
| ” | 17 | ” | 2·0 |
| ” | 24 | ” | 3·0 |
| July | 1 | ” | 2·5 |
| ” | 8 | ” | 1·2 |
Till February 4 the watch was exposed to the external air outside a north window; from February 5 to March 4 it was placed in the chamber of a stove heated by gas to a moderate temperature; and from April 29 to May 20 it was placed in the chamber when heated to a high temperature.
The advance in making chronometers since Harrison’s celebrated watch was tried at the Royal Observatory, more than ninety years since, may be judged by comparing its rates with those above.