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 20,000_l._ 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 5000_l._ 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 10,000_l._ more;
at the same time Euler of Berlin and the heirs of Mayer of Göttingen
received each 3000_l._ 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 ” 8 17 ” 2 4
Sept. 17)
In the 3d period (from ” 10 5 ” 2 31
Sept. 17 to Oct. 29)
In the 4th period (from ” 12 26 ” 3 6
Oct. 29 to Dec. 20)
In the 5th period (from ” 5 42 ” 1 25
Dec. 20 to Jan. 21)
In the 6th period (from ” 10 54 ” 2 43
Jan. 21 to Mar. 4)
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 10,000_l._ 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.