THE TIME DEPARTMENT
One day two Scotchmen stood just outside the main entrance
of Greenwich Observatory, looking intently at the great
twenty-four-hour clock, which is such an object of attention to the
passers through the Park. 'Jock,' said one of them to the other,
'd'ye ken whaur ye are?' Jock admitted his ignorance. 'Ye are at the
vara ceentre of the airth.'
Geographers tell us that there is a sense in which this statement as
it stands may be accepted as true. For if the surface of the globe
be divided into two hemispheres, so related to each other that the
one contains as much land as possible, and the other as little, then
London will occupy the centre or thereabouts of the hemisphere with
most land.
This was not, however, what the Scotchman meant. He meant to tell
his companion that he was standing on the prime meridian of the
world, the imaginary base line from which all distances, east or
west, are reckoned; in short, that he was on 'Longitude Nought.'
He was not absolutely correct, however, for the great
twenty-four-hour clock does not mark the exact meridian of
Greenwich. To find the instrument which marks it out and defines it
we must step inside the Observatory precincts, and just within the
gate we see before us on the left hand a door which leads through
a little lobby straight into the most important room of the whole
Observatory--the Transit Room.
[Illustration: THE GREAT CLOCK AND PORTER'S LODGE.
(_From a photograph by Mr. Lacey._)]
This room is not well adapted for representation by artist or
photographer. Four broad stone pillars occupy the greater part of
the space, and leave little more than mere passage room beside. Two
of these pillars are tall, as well as broad and massive, and stand
east and west of the centre of the room, carrying between them the
fundamental instrument of the Observatory, the transit circle. The
optical axis of this telescope marks 'Longitude Nought,' which is
further continued by a pair of telescopes, one to the north of it,
the other to the south, mounted on the third and fourth of the
pillars alluded to above.
This room has not always marked the meridian of Greenwich, for it
stands outside the original boundary of the Observatory. But it is
only a few feet to the east of the first transit instrument which
was set up by Halley, the second Astronomer Royal, in the extreme
N.-W. corner of the Observatory domain, a distance equivalent to
very much less than one-tenth of a second of time, an utterly
insensible quantity with the instruments of two hundred years ago.
It would be a long story to tell in detail how the Greenwich transit
room has come to define one of the two fundamental lines that
encircle the earth. The other, the equator, is fixed for us by the
earth itself, and is independent of any political considerations,
or of any effort or enterprise of man. But of all the infinite
number of great circles which could be drawn at right angles to the
equator, and passing through the north and south poles, it was not
easy to select one with such an overwhelming amount of argument in
its favour as to obtain a practically universal acceptance. The
meridians of Jerusalem and of Rome have both been urged, upon what
we may call religious or sentimental grounds; that of the Great
Pyramid at Ghizeh has been pressed in accordance with the fantastic
delusion that the Pyramid was erected under Divine inspiration and
direction; that of Ferro, in the Canaries, as being an oceanic
station, well to the west of the Old World, and as giving a base
line without preference or distinction for one nation rather than
another.
The actual decision has been made upon no such grounds as these.
It has been one of pure practical convenience, and has resulted
from the amazing growth of Great Britain as a naval and commercial
power. Like Tyre of old, she is 'situate at the entry of the sea,
a merchant of the people for many isles,' and 'her merchants are
the great men of the earth.' To tell in full, therefore, the
steps by which the Greenwich meridian has overcome all others is
practically to tell again, from a different standpoint, the story
of the 'expansion of England.' The need for a supreme navy, the
development of our empire beyond the seven seas, the vast increase
of our carrying trade--these have made it necessary that Englishmen
should be well supplied with maps and charts. The hydrographic and
geographic surveys carried on, either officially by this country, or
by Englishmen in their own private capacity, have been so numerous,
complete, and far-reaching as not only to outweigh those of all
other countries put together, but to induce the surveyors and
explorers of not a few other countries to adopt in their work the
same prime meridian as that which they found in the British charts
of regions bordering on those which they were themselves studying.
Naturally, the meridian of Greenwich has not only been adopted
for Great Britain, but also for the British possessions over-sea,
and, from these, for a large number of foreign countries; whilst
our American cousins retain it, an historic relic of their former
political connection with us. The victories of Clive at Arcot and
Plassy, of Nelson at the Nile and Trafalgar, the voyages and surveys
of Cook and Flinders, and many more; the explorations of Bruce,
Park, Livingstone, Speke, Cameron, and Stanley; these are some of
the agencies which have tended to fix 'Longitude Nought' in the
Greenwich Transit Room.
There are two somewhat different senses in which the meridian of
Greenwich is the standard meridian for nearly the entire world. The
first is the sense about which we have already been speaking; it
constitutes the fundamental line whence distances east and west are
measured, just as distances north and south are measured from the
equator. But there is another, though related sense, in which it has
become the standard. _It gives the time to the world._
There are few questions more frequently put than, 'What time is
it?' 'Can you tell me the true time?' A stickler for exactitude
might reply, 'What kind of time do you mean?' 'Do you mean solar
or sidereal time?' 'Apparent time or mean time?' 'Local time or
standard time?' There are all these six kinds of time, but it is
only within the last two generations, within, indeed, the reign of
our Sovereign, Queen Victoria, that the subject of the differences
of most of these kinds of time has become of pressing importance to
any but theorists.
In one of the public gardens of Paris a little cannon is set up
with a burning-glass attached to it in such a manner that the
sun itself fires the cannon as it reaches the meridian. This, of
course, is the time of Paris noon--apparent noon--but it would be
exceedingly imprudent of any traveller through Paris who wished,
say, to catch the one o'clock express, to set his watch by the gun.
For if it happened to be in February, he would find when he reached
the railway station that the station clock was faster than the sun
by nearly a full quarter of an hour, and that his train had gone;
whilst towards the end of October or the beginning of November, he
would find himself as much too soon.
Until machines for accurately measuring time were invented, apparent
time--time, that is to say, given by the sun itself, as by a
sun-dial--was the only time about which men knew or cared. But when
reasonably good clocks and watches were made, it was very soon seen
that at different times in the year there was a marked difference
between sun-dial time and that shown by the clock, the reason being
simply that the apparent rate of motion of the sun across the sky
was not always quite the same, whilst the movement of the clock was,
of course, as regular as it could be made.
This difference between time as shown by the actual sun and by a
perfect clock is known as the 'equation of time.' It is least about
April 15, June 15, August 31, and December 25. It is greatest,
the sun being after the clock, about February 11; and the sun
being before the clock, about November 2. Flamsteed, before he
became Astronomer Royal, investigated the question, and so clearly
demonstrated the existence, cause, and amount of the equation of
time as entirely to put an end to controversy on the subject.
We had thus, early in the century, the two kinds of time in common
use, apparent time and mean time, or clock time. But as the sun can
only be on one particular meridian at any given instant, the time
as shown by the clocks in one particular town will differ from that
of another town several miles to the east or west of it. It is thus
noon at Moscow 1 hr. 36 min. before it is noon at Berlin, and noon
at Berlin 54 min. before it is noon in London.
This was all well enough known, but occasioned no inconvenience
until the introduction of railway travelling; then a curious
difficulty arose. Suppose an express train was running at the rate
of sixty miles an hour from London to Bristol. The guard of the
train sets his watch to London time before he leaves Paddington,
but if the various towns through which the train passes, Reading,
Swindon, etc., each keep their own local time, he will find his
watch apparently fast at each place he reaches; but on his return
journey, if he sets to Bristol time before starting, he will in a
similar way find it apparently slow by the Swindon, Reading, and
Paddington clocks as he reaches them in succession.
It became at once necessary to settle upon one uniform system of
time for use in the railway guides. Apart from this, a passenger
taking train, say, at Swindon, might have been very troubled to
know whether the advertised time of his train was that of Exeter,
the place whence it started, or Swindon, the station where he was
getting in, or London, its destination. 'Railway time,' therefore,
was very early fixed for the whole of Great Britain to be the same
as London time, which is, of course, time as determined at Greenwich
Observatory. At first it was the custom to keep at the various
stations two clocks, one showing local time, the other 'railway,' or
Greenwich time, or else the clocks would be provided with a double
minute hand, one branch of which pointed to the time of the place,
the other to the time of Greenwich.
It was soon found, however, that there was no sufficient reason
for keeping up local time. Even in the extreme West of England the
difference between the two only amounted to twenty-three minutes,
and it was found that no practical inconvenience resulted from
saying that the sun rose at twenty-three minutes past six on March
22, rather than at six o'clock. The hours of work and business were
practically put twenty-three minutes earlier in the day, a change of
which very few people took any notice.
Other countries besides England felt the same difficulty, and solved
it in the same way, each country as a rule taking as its standard
time the time of its own chief city.
There were two countries for which this expedient was not
sufficient--the United States and Canada. The question was of no
importance until the iron road had linked the Atlantic to the
Pacific in both countries. Then it became pressing. No fewer than
seventy different standards prevailed in the United States only some
twenty years ago. The case was a very different one here from that
of England, where east and west differed in local time by only a
little over twenty minutes. In North America, in the extreme case,
the difference amounted to four hours, and it seemed asking too much
of men to call eight o'clock in their morning, or it might be four
o'clock in their afternoon, their noonday.
The device was therefore adopted of keeping the minutes and seconds
the same for all places right across the continent, but of changing
the hour at every 15° of longitude. The question then arose what
longitude should be adopted as the standard. The Americans might
very naturally have taken their standard time from their great
national observatory at Washington, or from that of their chief
city, New York, or of their principal central city, Chicago. But,
guided partly no doubt by a desire to have their standard times
correspond directly to the longitudes of their maps, and partly
from a desire to fall in, if possible, with some universal time
scheme, if such could be brought forward, they fixed upon the
meridian of Greenwich as their ultimate reference line, and defined
their various hour standards as being exactly so many hours slow of
Greenwich mean time.
The decision of the United States and of Canada brought with it
later a similar decision on the part of all the principal States
of Europe; and Greenwich is not only 'Longitude Nought' for the
bulk of the civilized world, but Greenwich mean time, increased or
decreased by an exact number of hours or half-hours, is the standard
time all over the planet.
No; the statement requires correction. Two countries hold out, both
close to our own doors. France, instead of adopting Greenwich time
as such, adopts _Paris time less_ 9 m. 21 s. (that being the precise
difference in longitude between the two national observatories).
Ireland disdains even such a veiled surrender, and Dublin time is
the only one recognized from the Hill of Howth to far Valentia. So
the distressful country preserves her old grievance, that she does
not even get her time until after England has been served.
The alteration in national habits following on the adoption of
this European system has had a very perceptible effect in some
cases. Thus, Switzerland has adopted Mid-European time, one hour
fast of Greenwich; the true local time for Berne being just half
an hour later. The result of putting the working hours this thirty
minutes earlier in the day has had such a noticeable effect on
the consumption of gas, as to lead the gas company to contemplate
agitating for a return to the old system.
Thus, Greenwich time, as well as the Greenwich meridian, has
practically been adopted the world over.
It follows, then, that the determination of time is the most
important duty of the Royal Observatory; and the Time Department,
the one to which is entrusted the duty of determining, keeping, and
distributing the time, calls for the first attention.
Entering the transit room, the first thing that strikes the
visitor is the extreme solidity with which the great telescope is
mounted. It turns but in one plane, that of 'longitude nought,'
and its pivots are supported by the pair of great stone pillars
which we have already spoken of as occupying the principal part of
the transit-room area, and the foundations of which go deep down
under the surface of the hill. On the west side of the telescope,
and rigidly connected with it, is a large wheel some six feet in
diameter, and with a number of wooden handles attached to it,
resembling the steering-wheel of a large steamer. This wheel carries
the setting circle, which is engraved upon a band of silver let
into its face near its circumference, a similar circle being at the
back of the wheel nearer the pillar. Eleven microscopes, of which
only seven are ordinarily used, penetrate through the pier, and are
directed on to this second circle.
The present transit is the fourth which the Observatory has
possessed, and its three predecessors, known as Halley's, Bradley's,
and Troughton's, respectively, are still preserved and hang on
the walls of the transit room, affording by their comparison an
interesting object-lesson in the evolution of a modern astronomical
instrument.
The watcher who wishes to observe the passing of a star must note
two things: he must know in what direction to point his telescope,
and at what time to look for the star. Then, about two minutes
before the appointed time, he takes his place at the eyepiece. As
he looks in he sees a number of vertical lines across his field
of view. These are spider-threads placed in the focus of the
eye-piece. Presently, as he looks, a bright point of silver light,
often surrounded by little flashing, vibrating rays of colour, comes
moving quickly, steadily onward--'swims into his ken,' as the poet
has it. The watcher's hand seeks the side of the telescope till his
finger finds a little button, over which it poises itself to strike.
On comes the star, 'without haste, without rest,' till it reaches
one of the gleaming threads. Tap! The watcher's finger falls sharply
on the button. Some three or four seconds later and the star has
reached another 'wire,' as the spider-threads are commonly called.
Tap! Again the button is struck. Another brief interval and the
third wire is reached, and so on, until ten wires have been passed,
and the transit is over. The intervals are not, however, all the
same, the ten wires being grouped into three sets, two of three
apiece, and the third of four.
[Illustration: THE CHRONOGRAPH.]
Each tap of the observer's finger completed for an instant an
electric circuit, and recorded a mark on the 'chronograph.' This
is a large metal cylinder covered with paper, and turned by a
carefully-regulated clock once in every two minutes. Once in every
two seconds a similar mark was made by a current sent by means of
the standard sidereal clock of the Observatory. The paper cover of
the chronograph after an hour's work shows a spiral trace of little
dots encircling it some thirty times. These dots are at regular
intervals, about an inch apart, and are the marks made by the clock.
Interspersed between them are certain other dots, in sets of ten;
and these are the signals sent from the telescope by the transit
observer. If, then, one of the clock dots and one of the observer's
dots come exactly side by side, we know that the star was on one
of the wires at a given precise second. If the observer's dot comes
between two clock dots, it is easy, by measuring its distance from
them with a divided scale, to tell the instant the star was on
the wire to the tenth of a second, or even to a smaller fraction.
Whilst, since the transit was taken over ten wires, and the distance
of each wire from the centre of the field of view is known, we have
practically ten separate observations, and the average of these
will give a much better determination of the time of transit than a
single one would.
But let the watcher be ever so little too slow in setting his
telescope, or ever so little late in placing himself at his
eye-piece, and the star will have passed the wire, and as it
smoothly, resistlessly moves on its inexorable way, will tell the
tardy watcher in a language there is no mistaking, 'Lost moments can
never be recalled.' The opportunity let slip, not until twenty-four
hours have gone by will another chance come of observing that same
star.
It is the stars that are chiefly used in this determination, partly
because the stars are so many, whilst there is but one sun. If,
therefore, clouds cover the sun at the important moment of transit,
the astronomer may well exclaim, so far as this observation is
concerned, 'I have lost a day!' The chance will not be offered him
again until the following noon. But if one star is lost by cloud,
there are many others, and the chance is by no means utterly gone.
Beside, the sun enables us to tell the time only at noon; the stars
enable us to find it at various times throughout the entire night;
indeed, throughout both day and night, since the brighter stars can
be observed in a large telescope even during the day.
There are two great standard clocks at the Observatory: the mean
solar clock and the sidereal clock. The latter registers twenty-four
hours in the precise time that the earth rotates on its axis. A
'day' in our ordinary use of the term is somewhat longer than this;
it is the average time from one noon to the next, and as the earth
whilst turning round on its axis is also travelling round the sun,
it has to rather more than complete a rotation in order to bring the
sun again on to the same meridian. A solar day is therefore some
four minutes longer than an actual rotation of the earth, _i.e._ a
sidereal day, as it is called, since such rotation brings a star
back again to the same meridian.
The sidereal clock can therefore be readily checked by the
observation of star transits, for the time when the star ought to
be on the meridian is known. If, therefore, the comparison of the
transit taps on the chronograph with the taps of the sidereal clock
show that the clock was not indicating this time at the instant
of the transit, we know the clock must be so much fast or slow.
Similarly, the difference which should be shown between the sidereal
and solar clocks at any moment is known; and hence when the error of
the sidereal clock is known, that of the solar can be readily found.
It is often quite sufficient to know how much a clock is wrong
without actually setting its hands right; but it is not possible
to treat the Greenwich clock so, for it controls a number of other
clocks continually, and sends hourly signals out over the whole
country, by which the clocks and watches all over the kingdom are
set right.
In the lower computing room, below the south window, we find the
Time-Desk, the head-quarters of the Time Department. This is a very
convenient place for the department, since one of the chronometer
rooms, formerly Bradley's transit room, opens out of the lower
computing room; the transit instrument is just beyond; it is
close to the main gate of the Observatory, and so convenient for
chronometer makers or naval officers bringing chronometers or coming
for them, whilst just across the courtyard is the chronograph room,
with the Battery Basement, in which the batteries for the electric
currents are kept, and the Mean Solar Clock lobby, with the winch
for the winding of the time-ball at the head of the stairs above it.
These rooms do not exhaust the territory of the department, since it
owns two other chronometer rooms on the ground floor and first floor
respectively of the S.-E. tower.
At the time-desk means are provided for setting the clock right very
easily and exactly. Just above the desk are a range of little dials
and bright brass knobs, that almost suggest the stops of a great
organ.
Two of these little dials are clock faces, electrically connected
with the solar and sidereal standard clocks, so that, though these
clocks are themselves a good way off, in entirely different parts
of the Observatory, the time superintendent, seated here at the
time-desk, can see at once what they are indicating. Between the
two is a dial labelled 'Commutator.' From this dial a little handle
usually hangs vertically downwards, but it can be turned either
to the right or to the left, and when thus switched hard over, an
electric current is sent through to the mean solar clock. If now
we leave the computing room and cross the courtyard to the extreme
north-west corner, we find the Mean Solar Clock in a little lobby,
carefully guarded by double doors and double windows against rapid
changes of temperature. Opening the door of the clock case, we
see that the pendulum carries on its side a long steel bar, and
that this bar as the pendulum swings passes just over the upper
end of an electro-magnet. When the current is switched on at the
commutator, this electro-magnet attracts or repels the steel bar
according to the direction of the current, and the action of the
clock is accordingly quickened or retarded. To put the commutator
in action for one minute will alter the clock by the tenth of a
second. As the error of the clock is determined twice a day, shortly
before ten o'clock in the morning, and shortly before one o'clock in
the afternoon, its error is always small, usually only one or two
tenths. These two times are chosen because, though time-signals are
sent over the metropolitan area every hour from the Greenwich clock
through the medium of the Post Office, at ten and at one o'clock
signals are also sent to all the great provincial centres. Further,
at one o'clock the time balls at Greenwich and at Deal are dropped,
so that the captains of ships in the docks, on the river, or in the
Downs may check their chronometers.
The Time-Ball is dropped directly by the mean solar clock itself.
It is raised by means of a windlass turned by hand-power to the top
of its mast just before one o'clock. Connected with it is a piston
working in a stout cylinder. When the ball has reached the top of
the mast, the piston is lightly supported by a pair of catches.
These catches are pulled back by the hourly signal current, and
the piston at once falls sharply, bringing the ball with it. But
after a fall of a few feet, the air compressed by the piston acts
as a cushion and checks the fall, the ball then gently and slowly
finishing its descent. The instant of the beginning of the fall is,
of course, the true moment to be noted.
The other dials on the time-desk are for various purposes connected
with the signals. One little needle in a continual state of
agitation shows that the electric current connecting the various
sympathetic clocks of the Observatory is in full action. Another
receives a return signal from various places after the despatch
of the time-signal from Greenwich, and shows that the signal has
been properly received at the distant station, whilst all the many
electric wires within the Observatory or radiating from it are made
to pass through the great key-board, where they can be at once
tested, disconnected, or joined up, as may be required.
[Illustration: THE TIME-DESK.]
The distribution of Greenwich time over the island in this way is
thus a simple matter. The far more important one of the distribution
of Greenwich time to ships at sea is more difficult. The difficulty
lay in the construction of a clock or watch, the rate of which
would not be altered by the uneasy motion of a ship, or by the
changes of temperature which are inevitable on a voyage. Two hundred
years ago it was not deemed possible to construct a watch of
anything like sufficient accuracy. They would not even keep going
whilst they were being wound, and would lose or gain as much as a
minute in the day for a fall or rise of 10° in temperature. This
was owing to the extreme sensitiveness of the balance spring--which
takes the place in a watch of a pendulum in a clock--to the effects
of temperature. The British Government, therefore, in 1714 offered
a prize of the amount of £20,000 for a means of finding the
longitude at sea within half a degree, or, in other words, for a
watch that would keep Greenwich time correct to two minutes in a
voyage across the Atlantic. In 1735, James Harrison, the son of a
Yorkshire carpenter, succeeded in solving the problem. His method
was to attach a sort of automatic regulator to the spring which
should push the regulator over in one direction as the temperature
rose, and bring it back as it fell. This he effected by fastening
together two strips of brass and steel. The brass expanded with heat
more rapidly than the steel, and hence with a rise of temperature
the strip bent over on the steel side. This was the first germ of
the idea of making watches 'compensated for temperature;' watches,
that is, which maintain practically the same rate whether they are
in heat or cold, an idea now brought to great perfection in the
modern chronometer.
[Illustration: HARRISON'S CHRONOMETER.]
The great reward the Government had offered stimulated many men to
endeavour to solve the problem. Of these, Dr. Halley, the second
Astronomer Royal, and Graham, the inventor of the astronomical
clock, were the most celebrated. But when Harrison, then poor and
unknown, came to London in 1735, and laid his invention before them,
with an utter absence of self-seeking, and in the true scientific
spirit, they gave him every assistance.
Harrison's first four time-keepers are still preserved at the
Royal Observatory. He did not, however, receive his reward until
a facsimile of the fourth had been made by his apprentice, Larcum
Kendall. The latter is preserved at the Royal Observatory. There is
a Larcum Kendall at the Royal Institution which is said to have been
used by Captain Cook. Harrison's chronometer was sent on a trial
voyage to Jamaica in 1761, and on its return to Portsmouth in the
following year it was found that its complete variation was under
the two minutes for which the Government had stipulated.
Since Harrison's day the improvement of the chronometer has been
carried on almost to perfection, and now the care and rating of
chronometers for the Royal Navy is one of the most important duties
of the Observatory.
[Illustration: THE CHRONOMETER ROOM.]
A visitor who should make the attempt to compare a single
chronometer with a standard clock would probably feel very
disheartened when, after many minutes of comparison, he had got out
its error to the nearest second, were he told that it was his duty
to compare the entire army here collected, some five hundred or
more, and to do it not to the second, but to the nearest tenth of a
second. Practice and system make, however, the impossible easy, and
one assistant will quietly walk round the room calling out the error
of each chronometer as he passes it, as fast as a second assistant
seated at the table can enter it at his dictation in the chronometer
ledgers. The seconds beat of a clock sympathetic with the solar
standard, rings out loud and clear above the insect-like chatter
of the ticking of the hundreds of chronometers, and wherever the
assistant stands, he has but to lift his eyes to see straight before
him, if not a complete clock-face, at least a seconds dial moving in
exact accordance with the solar standard.
The test to which chronometers are subjected is not merely one of
rate, but one of rate under carefully altered conditions. Thus
they may be tried with the XII pointing in succession to the four
points of the compass, or, in the case of chronometer watches,
they may be laid flat down on the table or hung from the ring or
pendant, or with the ring right or left, as it would be likely to
be when carried in the waistcoat pocket. But the chief test is the
performance of a chronometer when subjected to considerable heat
for a long period. This is a matter of great consequence, since
a chronometer travelling from England to India, Australia, or the
Cape, would necessarily be subjected to very different conditions
of temperature from those to which it would be exposed in England.
They are therefore kept for eight weeks in a closed stove at a
temperature of about 85° or 90°. At one time a cold test was also
applied, and Sir George Airy, the late Astronomer Royal, in one of
his popular lectures, drew a humorous comparison between the unhappy
chronometers thus doomed to trial, now in heat and now in frost,
and the lost spirits whom Dante describes as alternately plunged
in flame and ice. The cold test has, however, been done away with.
It is perfectly easy on the modern ship to keep the chronometer
comfortably warm even on an Arctic expedition. The elaborate cold
testing applied to Sir George Nares' chronometers before he started
on his polar journey was found to have been practically quite
superfluous; the chronometers were, if anything, kept rather too
warm. The exposure of the chronometer in the cooling box, moreover,
was found to be attended with a risk of rusting its springs.
[Illustration: THE CHRONOMETER OVEN.]
Once the determination of the longitude at sea became possible,
it was clearly the next duty to fix with precision the position
of the principal places, cities, ports, capes, islands, the world
over. Of all the work done in this department none has ever been
done better, in proportion to the means at command, than that
accomplished by Captain Cook in his celebrated three voyages. As has
already been pointed out, it is the extent and thoroughness of the
hydrographic surveys of the British Admiralty which have largely
contributed to the honour done to England by the international
selection of the English meridian, and of English standard time, as
in principle those for the whole civilized world. The generosity and
public spirit therefore which led the second Astronomer Royal to
help forward and support his rival, has almost directly led to this
great distinction accruing to the Observatory of which he was the
head.
Three different methods have successively been used in the
determination of longitudes of distant places. In each case the
problem required was to ascertain the time at the standard place,
say Greenwich, at the same time that it was being determined in
the ordinary way at the given station. One method of ascertaining
Greenwich time when at a distance from it was, as stated in Chapter
I., to use the moon, as it were, as the hand of a vast clock, of
which the sky was the face and the stars the dial figures. This is
the method of 'lunar distances,' the distances of the moon from a
certain number of bright stars being given in the _Nautical Almanac_
for every three hours of Greenwich time.
As chronometers were brought to a greater point of perfection, it
was found easier and better in many cases to use 'chronometer runs,'
that is, to carry backwards and forwards between the two stations
a number of good chronometers, and by constant comparison and
re-comparison to get over the errors which might attach to any one
of them.
[Illustration: THE TRANSIT PAVILION.
(_From a photograph by Mr. Lacey._)]
But of late years another method has proved available. Distant
nations are now woven together across thousands of miles of ocean
by the submarine telegraph. The American reads in his morning
paper a summary of the debates of the previous night in the House
of Commons at Westminster. The Londoner watches with interest
the scores of the English cricket team in Australia. It is now
therefore possible for an astronomer in England to record, should
he so desire, the time of the transit of a star across the wires
of his instrument, not only on his own chronograph, but upon that
of another observatory, it may be 2000 miles away. Or, much more
conveniently, each observer may independently determine the error of
his own clock, and then bring his clock into the current, so that
it may send a signal to the chronograph of the other station.
In one way or another this work of the determination of geographical
longitudes has been an important part of the extra-routine work
at Greenwich, part of the work which has built up and sustained
its claim to define 'longitude nought'; and many distinguished
astronomers, especially from the leading observatories of the
Continent, have come here from time to time to obtain more
accurately the longitude of their own cities. The traces of their
visits may be seen here and there about the Observatory grounds in
flat stones which lie level with the surface, and bear a name and
date like the gravestones in some old country churchyard. These are
not, as one might suppose, to mark the burial-places of deceased
astronomers, but record the sites where, on their visits for
longitude purposes, different foreign astronomers have set up their
transit instruments. Now, however, a permanent pier has been erected
in the courtyard, and a neat house--the Transit Pavilion--built over
it, so that in all probability no fresh additions will be made to
these sepulchral-looking little monuments.
It might be asked, What reason is there for a foreign observer to
come over to England for such a purpose? Would it not be sufficient
for the clock signals to be exchanged? But a curious little fact
has come out with the increase of accuracy of transit observation,
and that is, that each observer has his own particular habit or
method of observation. A hundred years ago, Maskelyne, the fifth
Astronomer Royal, was greatly disturbed to find that his assistant,
David Kinnebrook, constantly and regularly observed a star-transit
a little later than he did himself. The offender was scolded,
warned, exhorted, and finally, when all proved useless to bring
his observations into exact agreement with the Astronomer Royal's,
dismissed as an incompetent observer. As a matter of fact, poor
Kinnebrook has a right to be regarded as one of the martyrs of
science, and Maskelyne, by this most natural but mistaken judgment,
missed the chance of making an important discovery, which was not
made until some thirty years later. Astronomers now would be more
cautious of concluding that observations were bad simply because
they differed from what had been expected. They have learnt by
experience that these unexpected differences are the most likely
hunting-ground in which to look for new discoveries.
In a modern transit observation with the use of the chronograph
it will be seen at once that before the observer can register a
star-transit on the chronograph, he has to perceive with his eye
that the star has reached the wire, he has to mentally recognize
the fact, and consciously or unconsciously to exert the effort of
will necessary to bring his finger down on the button. A very slight
knowledge of character will show that this will require different
periods of time for different people. It will be but a fraction
of a second in any case, but there will be a distinct difference,
a constant difference, between the eager, quick, impulsive man
who habitually anticipates, as it were, the instant when he sees
star and wire together, and the phlegmatic, slow-and-sure man
who carefully waits till he is quite sure that the contact has
taken place, and then deliberately and firmly records it. These
differences are so truly personal to the observer that it is quite
possible to correct for them, and after a given observer's habit has
become known, to reduce his transit times to those of some standard
observer. It must, of course, be remembered that this 'personal
equation' is an exceedingly minute quantity, and in most cases is
rather a question of hundredths of seconds than of tenths.
It will be seen from the foregoing description how little of what
may be termed the picturesque or sensational side of astronomy
enters into the routine of the Time Department, the most important
of all the departments of the Observatory. The daily observation of
sun and of many stars--selected from a carefully chosen list of some
hundreds, and known as 'clock stars'--the determination of the error
of the standard clock to the hundredth of a second if possible, and
its correction twice a day, the sending out of time signals to the
General Post Office and other places, whence they are distributed
all over the country; the care, winding, and rating of hundreds of
chronometers and chronometer watches, and from time to time the
determination of the longitude of foreign or colonial cities, make
up a heavy, ceaseless routine in which there is little opportunity
for the realization of an astronomer's life as it is apt to be
popularly conceived.
Yet there is interest enough in the work. There is the charm
which always attaches to work of precision, the delight of using
delicate and exact instruments, and of obtaining results of steadily
increasing perfection. It may be akin to the sporting passion for
record-breaking, but surely it is a noble form of it which has led
the assistants, in recent years, to steadily increase the number of
observations in a normal night's work up to the very limit, taking
care the while that their accuracy has in no degree suffered. In
longitude work also 'the better is the enemy of the good,' and there
is the ambition that each fresh determination shall be markedly
more precise than all that have preceded it. The constant care of
chronometers soon reveals a kind of individuality in them which
forms a fresh source of interest, whilst if a man has but a spark
of imagination, how easily he will wrap them round with a halo of
romance!
Glance through the ledgers, and you will see how some of them have
heard the guns at the siege of Alexandria, others have been carried
far into the frozen north, others have wandered with Livingstone or
Cameron in the trackless forests of equatorial Africa.
More striking still are those pages across which the closing line
has been drawn; never again will the time-keeper there scheduled
return to the kindly inquisition of Flamsteed Hill. This sailed away
in the Wasp, and was swallowed up in the eastern typhoon; that went
down in the sudden squall that smote the Eurydice off the Isle of
Wight; these foundered with the Captain. The last fatal journey of
Sir John Franklin to find the North-West Passage leaves its record
here; the chronometers of the Erebus and Terror will never again
appear on the Greenwich muster roll. Land exploration claims its
victims too. Sturt's ill-fated expedition across Australia, and
Livingstone's last wandering, are represented.
[Illustration: 'LOST IN THE BIRKENHEAD.']
Sometimes an amusing entry interrupts the silent pathos of these
closed pages. 'Lost by Mr. Smith on the coast of Africa,' reads at
first sight like a rather thin attempt of some one to shift the
responsibility of his own carelessness on to the broad shoulders of
Mr. Nobody. In reality it probably gives a hint of the necessary,
dangerous, and exciting work of slave-dhow chasing which gives
employment to our ships on the African coast. 'Mr. Smith' was no
doubt a petty officer who was told off to carry the chronometer for
a boat's crew sent to search for a slave-dhow up some equatorial
estuary. Probably the dhow was found, and the Arabs who manned it
gave so stout a resistance that 'Mr. Smith' and his men had other
things to do than take care of chronometers before they could
overcome them. We may take it that the real story outlined here was
one of courage and hard fighting, not of carelessness and shirking.
Stories of higher valour and nobler courage yet are also hinted:
the calm discipline of the crew of the Victoria as she sank from
the ram of the Camperdown, the yet nobler devotion of the men of
the Birkenhead, as they formed up in line on deck and cheered the
boats that bore away the women and children to safety, whilst they
themselves went down with the ship into the shark-crowded sea.
'There rose no murmur from the ranks, no thought
By shameful strength, unhonoured life to seek;
Our post to quit we were not trained, nor taught
To trample down the weak.
'What followed, why recall? The brave who died
Died without flinching in that bloody surf.
They sleep as well beneath that purple tide
As others under turf.'