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.
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.
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.
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.
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.
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.
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.
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.
'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.'