In this activity Greenwich Observatory practically took no part. Airy, ever mindful of the original purpose of the Observatory, and deeply imbued with views similar to those which we have quoted from Bessel, considered that the new science lay outside the scope of his duties, until in Mr., now Sir William, Huggins's skilful hands the spectroscope showed itself not only as a means for determining the condition and constitution of the stars, but also their movements—until, in short, it had shown itself as an astronomical instrument even within Bessel's narrow definition.

The principle of this inquiry is as follows: If a source of light is approaching us very rapidly, then the waves of light coming from it necessarily appear a little shorter than they really are, or, in other words, that light appears to be slightly more blue—the blue waves being shorter than the red—than it really is. A similar thing with regard to the waves of sound is often noticed in connection with a railway train. If an express train, the whistle of which is blowing the whole time, dashes past us at full speed, there is a perceptible drop in the note of the whistle after it has gone by. The sound waves as it was coming were a little shortened, and the whistle therefore appeared to have a sharper note than it had in reality. And in the same way, when it had gone by, the sound waves were a little lengthened, making the note of the whistle appear a very little flatter.

Such a change of colour in a star could never have been detected without the spectroscope; but since when light passes through a prism the shorter waves are refracted more strongly, that is to say, are more turned out of their course than the longer, the spectroscope affords us the means of detecting and measuring this change. Let us suppose that the lines of hydrogen are recognized in a given star. If we compare the spectrum of this star with the spectrum of a tube containing hydrogen and through which the electric spark is passing, we shall be able to see whether any particular hydrogen line occupies the same place as shown by the two spectra. If the line from the star is a little to the red of the line from the tube, the star must be receding from us; if to the blue, approaching us. The amount of displacement may be measured by a delicate micrometer, and the rate of motion concluded from it.

The principle is clear enough. The actual working out of the observation was one of very great difficulty. The movements of the stars towards us, or away from us, are, in general, extremely slow as compared with the speed of light itself; and hence the apparent shift in the position of a line is only perceptible when a very powerful spectroscope is used. This means that the feeble light of a star has to be spread out into a great length of spectrum, and a very powerful telescope is necessary. The work of observing the motions of stars in the line of sight was started at Greenwich in 1875, the 'Great Equatorial' being devoted to it. This telescope, of 12³/₄ inches aperture, was not powerful enough to do much more than afford a general indication of the direction in which the principal stars were moving, and to confirm in a general way the inference which various astronomers had found, from discussing the proper motions of stars, that the sun and the solar system were moving towards that part of the heavens where the constellations Hercules and Lyra are placed. In 1891, therefore, the work was discontinued, and as already mentioned, the 12³/₄ telescope by Merz was removed to make room for the present much larger instrument by Sir Howard Grubb, upon the same mounting. The new telescope being much larger than the one for which mounting and observing room were originally built, it was not possible to put the spectroscope in the usual position, in the same straight line as the great telescope. It was therefore mounted under it, and parallel to it, and the light of the star was brought into it after two reflections. The observer therefore stood with his back to the object and looked down into the spectroscope. It had, however, become apparent by this time that this most delicate field of work was one for which photography possessed several advantages, and as Sir Henry Thompson had made the munificent gift to the Observatory of a great photographic equatorial, it was resolved to devote the 28-inch telescope chiefly to double-star work, and to transfer the spectroscope to the 'New Building.'

The 'New Observatory' in the south ground is crowned indeed with the dome devoted to the great Thompson photographic refractor, but this is not its chief purpose. Its principal floor contains four fine rooms which are used as 'computing rooms'—for the office work, that is to say, of the Observatory. Of these the principal is in the north wing, where the main entrance is placed, and is occupied by the Astronomer Royal and the two chief assistants. The basement contains the libraries and the workshops of the mechanics and carpenters. The upper floor will eventually be used for the storage of photographs and manuscripts, and the terrace roofs of the four wings will be exceedingly convenient for occasional observations, as, for example, of meteor showers. The central dome, which rises high above the level of the terraces, is the only room in the building devoted to telescopic work. As in the New Altazimuth building, a ring of circular lights just below the coping of the wall recalls the portholes of a ship, and again reminds us of the connection of the Observatory with navigation.

Here the spectroscope is now placed, but not, as it happens, on the Thompson refractor. The equatorial mounting in this new dome is a modification of what is usually called the 'German' form of mounting—that is to say, there is but one pier to support the telescope, and the telescope rides on one side of the pier and a counterpoise balances it on the other The 'Great Equatorial,' on the other hand, is an example of the English mounting, and has two piers, one north and the other south, whilst the telescope swings in a frame between them. In the new dome three telescopes are found rigidly connected with each other on one side of the pier, the telescopes being (1) the great Thompson photographic telescope, double the aperture and double the focal length of the standard astrographic telescope used for the International Photographic Survey; (2) the 12³/₄ telescope by Merz, that used to be in the great South-East dome, but which is now rigidly connected with the Thompson refractor as a guide telescope; and (3) a photographic telescope of 9 inches aperture, already described as the 'Thompson' photo-heliograph, and used for photographing the sun or in eclipse expeditions. The counterpoise to this collection of instruments is not a mere mass of lead, but a powerful reflector of 30 inches' aperture, and it is to this telescope that the spectroscope is now attached. At the present time, however (August, 1900), regular work has not been commenced with it.

Beside this attempt to determine the motions of the stars as they approach us or retreat from us, on rare occasions the spectroscope has been turned on the planets. As these shine by reflected light, their spectra are normally the same as that of the sun. Mars appeared to the writer, as to Huggins and others, to show some slight indication of the presence of water vapour in its atmosphere. Jupiter and Saturn show that their atmospheres contain some absorbing vapour unknown to ours. And Uranus and Neptune, faint and distant as they are, not only show the same dark band given by the two nearer planets, but several others. More attractive has been the examination of the spectra of the brighter comets that have visited us. The years 1881 and 1882 were especially rich in these. The two principal comets of 1881 were called after their respective discoverers, Tebbutt's and Schaeberle's. They were not bright enough to attract popular attention, though they could be seen with the naked eye, and both gave clear indications of the presence of carbon, their spectra closely resembling that of the blue part of a gas or candle flame. There was nothing particularly novel in these observations, since comets usually show this carbon spectrum, though why they should is still a matter for inquiry; but the two comets of the following year were much more interesting. Both comets came very near indeed to the sun. The earlier one, called from its discoverer Comet Wells, as it drew near to the sun, began to grow more and more yellow, until in the first week of June it looked as full an orange as even the so-called red planet, Mars. The spectroscope showed the reason of this at a glance. The comet had been rich in sodium. So long as it was far from the sun the sodium made no sign, but as it came close to it the sodium was turned into glowing vapour under the fierce solar heat. And as the writer saw it in the early dawn of June 7, the comet itself was a disc of much the same colour as Mars, whilst its spectrum resembled that of a spirit lamp that has been plentifully fed with carbonate of soda or common salt. The 'Great Comet' of the autumn of the same year, and which was so brilliant an object in the early morning, came yet nearer to the sun, and the heating process went on further. The sodium lines blazed up as they had done with Comet Wells, but under the fiercer stress of heat to which the Great Comet was subjected, the lines of iron also flashed out, a significant indication of the tremendous temperature to which it was exposed.

There are two other departments of spectroscopic work which it was attempted for a time to carry on as part of the Greenwich routine. These were the daily mapping of the prominences round the sun, and the detailed examination of the spectra of sun-spots. Both are almost necessary complements of the work done in the heliographic department—that is to say, the work of photographing the appearance of the sun day by day, and of measuring the positions and areas of the spots. For the spots afford but one index out of several, of the changes in the sun's activity. The prominences afford another, nor can we at the present moment say authoritatively which is the more significant. Then again, with regard to the spots themselves, it is not certain that either their extent or their changes of appearance are the features which it is most important for us to study. We want, if possible, to get down to the soul of the spot, to find out what makes one spot differ from another; and here the spectroscope can help us. Great sun-spots are often connected with violent agitation of the magnetic needles, and with displays of auroræ. But they are not always so, and the inquiry, 'What makes them to differ?' has been made again and again, without as yet receiving any unmistakable answer. The great spot of November, 1882, which was connected with so remarkable an aurora and so violent a magnetic storm, was as singular in its spectrum as in its earthly effects. The sun was only seen through much fog, and the spectrum was therefore very faint, but shooting up from almost every part of its area, except the very darkest, were great masses of intensely brilliant hydrogen, evidently under great pressure. The sodium lines were extremely broadened, and on November 20 a broad bright flame of hydrogen was seen shooting up at an immense speed from one edge of the nucleus. A similar effect—an outburst of intensely luminous hydrogen—has often been observed in spots which have been accompanied by great magnetic storms; and it may even be that it is this violent eruption of intensely heated gas which has the directest connection with the magnetic and auroral disturbances here upon earth.

This sun-spot work was not carried on for very long, as only one assistant could be spared for the entire solar work of whatever character. Yet in that time an interesting discovery was made by the writer—namely, that in the green part of the spectrum of certain spots a number of broad diffused lines or narrow bands made their appearance from time to time, and especially when sun-spots were increasing in number, or were at their greatest development.

The prominence work had also to be dropped, partly for the same reason, but chiefly because the atmospheric conditions at Greenwich are not suitable for these delicate astrophysical researches. When the Observatory was founded 'in the golden days' of Charles II., Greenwich was a little country town far enough removed from the great capital, and no interference from its smoke and dust had to be feared or was dreamt of. Now the 'great wen,' as Cobbett called it, has spread far around and beyond it, and the days when the sky is sufficiently pure round the sun for successful spectrum work on the spots or prominences are few indeed.

Whether in the future it will be thought advisable for the Royal Observatory to enter into serious competition in inquiries of this description with the great 'astrophysical' observatories of the Continent and of America—Potsdam, Meudon, the Lick, and the Yerkes—we cannot say. That would involve a very considerable departure from its original programme, and probably also a departure from its original site. For the conditions at Greenwich tend to become steadily less favourable for such work, and it would most probably be found that full efficiency could only be secured by setting up a branch or branches far from the monster town.

With the older work it is otherwise. So long as Greenwich Park and Blackheath are kept—as it is to be hoped they always will be—sacred from the invasion of the builder; so long as no new railways burrow their tunnels in the neighbourhood of the Observatory, so long the fundamental duties laid upon Flamsteed, 'of Rectifying the Tables of the Motions of the Heavens and the Places of the Fixed Stars,' will be carried out by his successors on Flamsteed Hill.

CHAPTER XII

THE ASTROGRAPHIC DEPARTMENT

The two last departments mentioned, the heliographic and spectroscopic, lie clearly and unmistakably outside the terms of the original warrant of the Observatory, though the progress of science has led naturally and inevitably to their being included in the Greenwich programme. But the Astrographic Department, though it could no more have been conceived in the days of Charles II. than the spectroscopic, does come within the terms of the warrant, and is but an expansion of that work of 'Rectifying the Places of the Fixed Stars,' which formed part of the programme enjoined upon Flamsteed, the first Astronomer Royal, at the first foundation of the Observatory, and which was so diligently carried out by him, the first Greenwich catalogue, containing about 3000 stars, being due to his labours.

His immediate successors did much less in this field, though Bradley's observations were published, long after his death, as a catalogue of 3222 stars, in some aspects the most important ever issued. Pond, the sixth Astronomer Royal, restored catalogue-making to a prominent place in the Greenwich routine, and his precedent is sedulously followed to-day. But each of these was confined to about 3000 stars. The necessity has long been felt for a much ampler census, and Argelander, at the Bonn Observatory, brought out a catalogue of 324,000 stars north of South declination 2°, a work which has been completed by Schönfeld, who carried the census down to South declination 23°, and by the two great astronomers of Cordoba, South America, Dr. Gould and Dr. Thome, by whom it was extended to the South Pole.

These last three catalogues embrace stars of all magnitudes down to the 9th or 10th; but certain astronomers had endeavoured to go much lower, and to make charts of limited portions of the sky down to even the 14th magnitude.

From the very earliest days that men observed the stars, they could not help noticing that 'one star differeth from another star in glory,' and consequently they divided them into six classes, according to their brightness—classes which are commonly spoken of now as magnitudes. The ordinary 6th magnitude star is one which can be clearly seen by average sight on a good night, and it gives us about one-hundredth the light of an average 1st magnitude star. Sirius, the brightest of all the fixed stars, is called a 1st magnitude star, but is really some six or seven times as bright as the average. It would take, therefore, more than two and a half million stars of the 14th magnitude to give as much light as Sirius.

It is evident that so searching a census as to embrace stars of the 14th magnitude would involve a most gigantic chart. But the work went on in more than one Observatory for a considerable time, until at last the observers entered on to the region of the Milky Way. Here the numbers of the stars presented to them were so great as to baffle all ordinary means of observation. What could be done?

Just at this time immense interest was caused in the astronomical world by the appearance of the great comet of 1882. It was watched and observed and sketched by countless admirers, but more important still, it was photographed, and some of its photographs, taken at the Royal Observatory, Cape of Good Hope, showed not only the comet with marvellous beauty of detail, but also thousands of stars, and the success of these photographs suggested to her Majesty's Astronomer at the Cape, Dr. Gill, that in photography we possessed the means for making a complete sky census even to the 14th magnitude.

The project was thought over in all its bearings, and in 1887 a great conference of astronomers at Paris resolved upon an international scheme for photographing the entire heavens. The work was to be divided between eighteen Observatories of different nationalities. It was to result in a photographic chart extending to the 14th magnitude, and probably embracing some forty million stars, and a catalogue made from measures of the photographs down to the 11th magnitude, which would probably include between two and three million stars.

The eighteen Observatories all undertook to use instruments of the same capacity. This was to be a photographic refractor, with an object-glass of 13 inches aperture and 11 feet focus. At Greenwich this telescope is mounted equatorially—that is, so as to follow the stars in their courses—and is mounted on the top of the pier that once supported Halley's quadrant. The telescope is driven by a most efficient clock, whose motive power is a heavy weight. The rate of the weight in falling is regulated by an ingenious governor, which brings its speed very nearly indeed to that of the star, and any little irregularities in its motion are corrected by the following device. A seconds pendulum is mounted in a glass case on the wall of the Observatory, and a needle at the lower end of the pendulum passes at each swing through a globule of mercury. On one of the wheels of the clock are arranged a number of little brass points, at such intervals apart that the wheel, when going at the proper rate, takes exactly one second to move through the distance between any pair. A little spring is arranged above the wheel, so that these points touch it as they pass. If this occurs exactly as the pendulum point passes through the mercury nothing happens, but if the clock is ever so little late or early, the electric current from the pendulum brings into action a second wheel, which accelerates or retards the driving of the clock, as the case may be. The total motion, therefore, is most beautifully even.

But even this is not quite sufficient, especially as the plates for the great chart have to be exposed for at least forty minutes. Rigidly united with the 13-inch refractor, so that the two look like the two barrels of a huge double-barrelled gun, is a second telescope for the use of the observer. In its eyepiece are fixed two pairs of cross spider lines, commonly called wires, and a bright star, as near as possible to the centre of the field to be photographed, is brought to the junction of two wires. Should the star appear to move away from the wire, the observer has but to press one of two buttons on a little plate which he carries in his hand, and which is connected by an electric wire with the driving clock, to bring it back to its position.

The photographs taken with this instrument are of two kinds. Those for the great chart have but a single exposure, but this lasts for forty minutes. Those for the great catalogue have three exposures on them, the three images of a star being some 20 seconds of arc apart. These exposures are of six minutes', three minutes', and twenty seconds' duration, and the last exposure is given as a test, since, if stars of the 9th magnitude are visible with an exposure of twenty seconds, stars of the 11th magnitude should be visible with three minutes' exposure.

Thus it will be seen that in three minutes an impression is got of many scores of stars, whose places it would require many hours to determine at the transit instrument. But the positions of these stars on the plate still remain to be measured. For this purpose a net-work of lines, at right angles to each other, is printed on the photograph before its development, and, after it has been developed, washed and dried, the distances of the stars from their nearest cross-lines are measured in the measuring machine.

The measuring machine is constructed to hold two plates, one half its breadth higher than the other. In fact, in each of the two series of photographs the whole sky is taken twice, but the two photographs of any region are not simply duplicates of each other. The centre of each plate is at a corner of four other plates, and in the micrometer the stars on the quarter common to two plates are measured simultaneously.

In this way will be carried out a great census of the sky that will exceed Flamsteed's ten thousand fold. And just as Flamsteed's was but the first of many similar catalogues, so, no doubt, will this be followed by others—not superseded, for its value will increase with its age and the number of those that follow it, by comparison with which it will prove an inexhaustible mine of information concerning the motions of the stars and the structure of the universe.

There is a great difference between the work of the observer with the 'Astrographic Telescope,' as this great twin photographic instrument is called, and the work of the transit observer. The latter sees the star gliding past him, and telegraphs the instant that the star threads itself on each of the ten vertical wires in succession. The astrographic observer, on the other hand, sees his star shining almost immovably in the centre of his field, threaded on the two cross wires placed there, for the driving-clock moves the telescope so as to almost exactly compensate for the rotation movement of the earth. The observer's duty in this case is to telegraph to his driving-clock, when it has in the least come short of or exceeded its duty, and so to bring back the 'guiding star' to its exact proper place on the cross wires.

So far, the work of the Astrographic Department has been, as mentioned above, a development on an extraordinary scale, but a development still, of the original programme of the Observatory. But the munificent gift of Sir Henry Thompson has put it within the power of the Astronomer Royal to push this work of sidereal photography a stage further. Sir Henry Thompson gave to the Observatory, not merely the photographic refractor of 9 inches' aperture, now used for solar photography, and known as the 'Thompson photo-heliograph,' but also one of 26 inches' aperture and 22¹/₂ feet focal length. This instrument was specially designed of exactly double the dimensions of the standard astrographic telescope used for the International Photographic Survey, the idea being that, in the case of a field of special interest and importance, a photograph could be obtained with the larger instrument on exactly double the scale given by the smaller. It has rather, however, found its usefulness in a slightly different field. The observation of the satellites of Jupiter was suggested by Galileo as a means of determining the longitude at sea. As already pointed out, the suggestion did not prove to be a practical one for that purpose, but observations of the satellites have been made none the less with a view simply to improving our knowledge of their movements, and of the mass of Jupiter. The utilitarian motive for the work having fallen through, it has been carried on as a matter of pure science.

And the work has not stopped with the satellites of Jupiter; eight satellites were in due time discovered to Saturn, four to Uranus, and two to Mars; and though these could give not the remotest assistance to navigation, they too have been made the subjects of observation for precisely the same reason as those of Jupiter have been.

In just the same way, when the discovery of Neptune was followed by that of a solitary companion to it, this also had to be followed. The difficulties in the way of observing the fainter of all these satellites were considerable, and the work has been mostly confined to two or three observatories possessing very large telescopes. As the largest telescope at Greenwich was only 7 inches in aperture up to 1859, and only 12³/₄ inches up to 1893, it is only very recently that it has been able to take any very substantial part in satellite measures. But since the Thompson photographic telescope was set up, it has been found that a photograph of Neptune and its satellite can be taken in considerably less time than a complete set of direct measures can be made, whilst the photograph, which can be measured at leisure during the day, gives distinctly the more accurate results.

So, too, the places of the minor planets can be got more accurately and quickly by means of photographs with this great telescope than by direct observation, and photographs of the most interesting of them all, the little planet Eros, have been very successfully obtained. So that, though doing nothing directly to improve the art of navigation, or to find the longitude at sea, the great photographic refractor takes its share in the work of 'Rectifying the Tables of the Planets.'

The reflector of 30 inches' aperture, which acts as a counterpoise to the sheaf of telescopes of the Thompson, is intended for use with the spectroscope, the quality which mirrors possess of bringing all rays, whatever their colour, to the same focus being of great importance for spectroscopic work. But the experiments which have been made with it in celestial photography have proved so extremely successful as to cause the postponement of the recommencement of the spectroscopic researches. Chief amongst these photographs are some good ones of the moon, and more recently some exceedingly fine photographs of the principal nebulæ.

In no department of astronomy has photography brought us such striking results as in regard to the nebulæ. Dr. Roberts' photograph of the great nebula in Andromeda converted the two or three meaningless rifts—which some of the best drawings had shown—into the divisions between concentric rings; and what had appeared a mere shapeless cloud was seen to be a vast symmetrical structure, a great sidereal system in the making. The great nebula in Orion has grown in successive photographs in detail and extent, until we have a large part of the constellation bound together in the convolutions of a single nebula of the most exquisite detail and most amazing complexity. The group of the Pleiades has had a more wonderful record still. Manifestly a single system even to the naked eye, and showing some faint indications of nebulosity in the telescope, the photographs have revealed its principal stars shining out from nebulous masses, in appearance like carded wool, and have shown smaller stars threaded on nebulous lines like pearls upon a string.

Such photographs are, of course, of no utilitarian value, and at present they lead us to no definite scientific conclusions. They lie, therefore, doubly outside the limits of the purely practical, but they attract us by their extreme beauty, and by the amazing difficulty of the problems they suggest. How are these weird masses of gas retained in such complex form over distances which must be reckoned by millions of millions of miles? By what agency are they made to glow so as to be visible to us here? What conceivable condition threads together suns on a line of nebula? What universes are here in the making, or perhaps it may be falling into ruin and decay?

CHAPTER XIII

THE DOUBLE-STAR DEPARTMENT

The foregoing chapters will have shown that though the original purpose of the Observatory has always been kept in view, yet the progress of science has caused many researches to be undertaken which overstep its boundaries. Thus in the present transit room, beside the successive transit instruments we find upon the wall two long thin tubes, labelled respectively Alpha Aquilæ and Alpha Cygni. These were two telescopes set up by Pond for a special purpose. Dr. Brinkley, Royal Astronomer for Ireland, had announced that he had found that several stars shifted their apparent place in the sky in the course of a year, due to the change in the position of the earth from which we view them, by an amount which would show that they were only about six to nine billions of miles distant from us; or, in other words, they showed a parallax of from two to three seconds of arc. Pond was not able to confirm these parallaxes from his observations, and to decide the point he set up these two telescopes, the Alpha Aquilæ telescope being rigidly fixed on the west side of the pier of Troughton's mural circles; the Alpha Cygni telescope on another pier, the one which now forms the base of the pier of the astrographic telescope. Pond's method was to compare the position of these two stars with that of a star almost exactly the same distance from the pole, but at a great distance from it in time of crossing the meridian; in other words, of almost the same declination, but widely different right ascension. The result proved that Brinkley was wrong, and vindicated the delicacy and accuracy of Pond's observations.

These two telescopes, therefore, had their day and ceased to be. Others have followed them. An ingenious telescope was set up by Sir George Airy in order to ascertain if the speed of light were different when passing through water than when passing through air. Or, in other words, if the aberration of light would give the same value as at present if we observed through water. The water telescope, as it was called, is kept on the ground floor of the central octagon of the new observatory. The observations obtained with it were hardly quite satisfactory, but gave on the whole a negative result.

Turning back to the transit room, and leaving it by the south-west door, we come into the little passage which leads at the back of Bradley's transit room into the lower computing room. Just inside this passage, on the left-hand side, there is a little room of a most curious shape, the 'reflex zenith room.' Here is fixed a telescope pointing straight upwards, the eye-piece being fixed by the side of the object-glass. The light from a star—the star Gamma Draconis—which passes exactly over the zenith of Greenwich, enters the object-glass, passes downwards to a basin of mercury, and is reflected upwards from the surface of the mercury to a little prism placed over the centre of the object-glass, from which it is reflected again into the eye-piece. By means of this telescope the distance of the star Gamma Draconis from the zenith could be measured very exactly, and, consequently, the changes in the apparent position of the star due to aberration, parallax, and other causes could be very exactly followed, and the corrections to be applied on account of these causes precisely determined.

This particular telescope was devised by Airy, and the observations with it were continued to the end of his reign. The germ of the idea may be traced back, however, to the time of Flamsteed, who would seem to have occasionally observed Gamma Draconis from the bottom of a deep well; the precise position of the well is not, however, now known. Later, Bradley set up his celebrated 12¹/₂-foot zenith sector, still preserved in the transit room, first at Wanstead and then at Greenwich, for the determination of the amount of aberration. Later, a zenith tube by Troughton, of 25 feet focus, was used by Pond in conjunction with the mural circle for observations of Gamma Draconis in order to determine the zenith point of the latter instrument.

These telescopes for special purposes have passed out of use. Observations with the spectroscope have been suspended for some years. The work of the Astrographic Department will come to an end, in the ordinary course of events, when the programme assigned to Greenwich in the International Scheme is completed.

Within the last few years a new department has come into being at Greenwich—a department which has been steadily worked at many foreign public observatories, but only recently here.

This is the Department of Double-Star Observation. The first double star, Zeta Ursæ Majoris, was discovered 250 years ago. Bradley discovered two exceedingly famous double stars whilst still a young man observing with his uncle at Wanstead—Gamma Virginis and Castor. Bradley made also other discoveries of double stars after his appointment to Greenwich, and Maskelyne succeeded him in the same line, but the great foundation of double-star astronomy was laid by Sir William Herschel.

At first it was supposed that double stars were double only in appearance; one star comparatively near us 'happened' to lie in almost exactly the same direction as another star much further off. It was, indeed, in the very expectation that this would prove to be the case, that the elder Herschel first took up their study. But he was soon convinced that many of the objects were true double stars—members of the same system of which the smaller revolved round the larger—not merely apparently double, one star appearing by chance to be close to another with which it had no connection—but real double stars. The discovery of these has led to the establishment of a new department of astronomy, again scientific rather than utilitarian.

As mentioned above, it is only recently that Greenwich has taken any appreciable part in this work. Under Airy, the largest equatorial of the time had been furnished with a good micrometer, and observations of one or two double stars been made now and again; but Airy's programme of work was far too rigid, and kept the staff too closely engaged for such observations to be anything but extremely rare. And, indeed, when the micrometers of the equatorials were brought into use, they were far more generally devoted to the satellites of Saturn than to the companions of stars. In the main, double-star astronomy has been in the hands of amateurs, at least in England. But the discovery in recent years of many pairs so close that a telescope of the largest size is required for their successful observation, has put an important section of double stars beyond the reach of most private observers, and therefore the great telescope at Greenwich is now mainly devoted to their study. The Astronomer Royal, therefore, soon after the completion of the great equatorial of 28-inches aperture placed in the south-east dome, added this work to the Observatory programme.

The 28-inch equatorial is a remarkable-looking instrument, its mounting being of an entirely different kind to that of the other equatorials in the Observatory, with the solitary exception of the Shuckburgh, which is set up in a little dome over the chronograph room. The Shuckburgh was presented to the Observatory in the year 1811, by Sir G. Shuckburgh. It was first intended to be mounted as an altazimuth, but proved to be unsteady in that position, and was then converted into an equatorial without clockwork, and mounted in its present position. The position is about as hopelessly bad a one as a telescope could well have, completely overshadowed as it is by the trees and buildings close at hand. The dome is a small one, and the arrangements for the shutters and for turning the dome are as bad as they could possibly be. It has practically been useless for the last forty years.

Its only interest is that the method of mounting employed is a small scale model of that of the great telescope in the S.-E. dome. In the German or Fraunhofer form of mounting for an equatorial there is but a single pillar, which carries a comparatively short polar axis. At the upper end of the polar axis we find the declination axis, and at one end of the declination axis is the telescope, whilst at the other end is a heavy weight to counterpoise it. The German mounting has the advantage that the telescope can easily point to the pole of the heavens; its drawbacks are that, except in certain special forms, the telescope cannot travel very far when it is on the same side of the meridian as the star to which it is pointed, the end of the telescope coming into contact under such circumstances with the central pier, whilst the introduction of mere deadweight as the necessary counterpoise, is not economical. It has been already pointed out that the present Astronomer Royal has not only considerably modified the German mounting in the great collection of telescopes in the Thompson dome, but has used a powerful reflector as a counterpoise to the sheaf of refractors at the other end of the declination axis.

The English equatorial requires two piers. Between these two piers is a long polar axis. Both in the little Shuckburgh and in the great 28-inch equatorial the frame of the polar axis consists of six parallel rods disposed in two equilateral triangles, with their bases parallel to each other, the telescope swinging in the space between the two bases. The construction of this form of equatorial, therefore, is expensive, as it requires two piers. It takes much more room than the German form, and the telescope cannot be directed precisely to the pole. But the instrument is symmetrical, there is no deadweight, and the telescope can follow a star from rising to setting without having to be reversed on crossing the meridian.

The great stability of the English form of mounting, therefore, commended it very highly to Airy, and he designed the great Northumberland equatorial of the Cambridge Observatory on that plan, as well as one for the Liverpool Observatory at Bidston, and in 1858 the S.-E. equatorial at Greenwich.

The telescope at first mounted upon it had an object-glass of 12³/₄ inches' aperture, and 18 feet focal length. That was dismounted in 1891, and is now used as the guiding telescope of the Thompson 26-inch photographic refractor. Its place was taken by an immensely heavier instrument, the present refractor of 28 inches' aperture, and 28 feet focal length; and that this change was effected safely was an eloquent testimony to the solidity of the original mounting.

The clock that drives this great instrument, so that it can follow a star or other celestial object in its apparent daily motion across the sky, is in the basement of the S.-E. tower. It is a very simple looking instrument, a conical pendulum in a glass case. The pendulum makes a complete revolution once in two seconds. Below it in a closed case is a water turbine. A cistern on the roof of the staircase supplies this turbine with water, having a fall of about thirty feet. The water rushing out of the arms of the turbine forces it backward, and the turbine spins rapidly round, driving a spindle which runs up into the dome, and gears through one or two intermediate wheels with the great circle of the telescope; the extremely rapid rotation of the spindle, four times in a second, being converted by these intermediate wheels into the exceedingly slow one of once in twenty-four hours. Just above the centre of motion of the turbine is a set of three small wheels, all of exactly the same size, and of the same number of teeth. Of these the bottom wheel is horizontal, and is turned by the turbine. The top wheel is also horizontal, and is turned by the pendulum. The third wheel gears into both these, and is vertical. If the top and bottom wheels are moving exactly at the same rate, the intermediate wheel simply turns on its axis, but does not travel; but if the turbine and pendulum are moving at different rates, then the vertical wheel is forced to run in one direction or the other, and, doing so, it opens or closes a throttle valve, which controls the supply of water to the turbine, and so speedily brings the turbine into accord with the pendulum. The control of the motion of the great telescope is therefore almost as perfect as that of the astrographic and Thompson equatorials, though the principle employed is very different. And the control needs to be perfect, for, as said above, the great telescope is mostly devoted to the observation of double stars, and there can be no greater hindrance to this work than a telescope which does not move accurately with the star.

There is a striking contrast between the great telescope and all the massive machinery for its direction and movement, and the objects on which it is directed—two little points of light separated by a delicate hair of darkness.

The observation is very unlike those of which we have hitherto spoken. The object is not to ascertain the actual position in the sky of the two stars, but their relative position to each other. A spider's thread of the finest strands is moved from one star to the other by turning an exquisitely fine screw; this enables us to measure their distance apart. Another spider thread at right angles to the first is laid through the centres of both stars, and a divided circle enables us to read the angle which this line makes to the true east and west direction. Such observations repeated year after year on many stars have enabled the orbits of not a few to be laid down with remarkable precision; and we find that their movements are completely consistent with the law of gravitation. Further, just as Neptune was pre-recognized and discovered from noting the irregularities in the motion of Uranus, so the discordances in the place of Sirius led to the belief that it was attracted by a then unseen companion, whose position with respect to the brighter star was predicted and afterwards seen.