says the proverb, and the saying is not without a shrewd amount of truth. For perhaps nowhere can we find a more striking combination of imperfect observation and inconsequent deduction than in the saws which form the stock-in-trade of the ordinary would-be weather prophet. How common it is to find men full of the conviction that the weather must change at the co-called 'changes of the moon,' forgetful that

They will say, truly enough, no doubt, that they have known the weather to change at 'new' or 'full,' as the case may be, and they argue that it, therefore, must always do so. But, in fact, they have only noted a few chance coincidences, and have let the great number of discordances pass by unnoticed.

But observations of this kind seem scientific and respectable compared with those numerous weather proverbs which are based upon the mere jingle of a rhyme, as

a proverb which is deftly inverted in some districts by making 'oak' rhyme to 'choke.'

Others, again, are based upon a mere childish fancy, as, for example, when the young moon 'lying on her back' is supposed to bode a spell of dry weather, because it looks like a cup, and so might be thought of as able to hold the water.

During the present reign, however, a very different method of weather study has come into action, and the foundations of a true weather wisdom have been laid. These have been based, not on fancied analogies or old wives' rhymes, or a few forechosen coincidences, but upon observations carried on for long periods of time and over wide areas of country, and discussed in their entirety without selection and bias. Above all, mathematical analysis has been applied to the motions of the air, and ideas, ever gaining in precision and exactness, have been formulated of the general circulation of the atmosphere.

As compared with its sister science, astronomy, meteorology appears to be still in a very undeveloped state. There is such a difference between the power of the astronomer to foretell the precise position of sun, moon, and planets for years, even for centuries, beforehand, and the failure of the meteorologist to predict the weather for a single season ahead, that the impression has been widely spread that there is yet no true meteorological science at all. It is forgotten that astronomy offered us, in the movements of the heavenly bodies, the very simplest and easiest problem of related motion. Yet for how many thousands of years did men watch the planets, and speculate concerning their motions, before the labours of Tycho, Kepler, and Newton culminated in the revelation of their meaning? For countless generations it was supposed that their movements regulated the lives, characters, and private fortunes of individual men; just as quite recently it was fancied that a new moon falling on a Saturday, or two full moons coming within the same calendar month, brought bad weather!

It is still impossible to foresee the course of weather change for long ahead; but the difference between the modern navigator, surely and confidently making a 'bee-line' across thousands of miles of ocean to his destination, and the timid sailor of old, creeping from point to point of land, is hardly greater than the contrast between the same two men, the one watching his barometer, the other trusting in the old wives' rhymes which afforded him his only indication as to coming storms.

It is still impossible to foresee the weather change for long ahead, but in our own and in many other countries, especially the United States, it has been found possible to predict the weather of the coming four-and-twenty hours with very considerable exactness, and often to forecast the coming of a great storm several days ahead. This is the chief purpose of the two great observatories of the storm-swept Indian and Chinese seas, Hong Kong and Mauritius; and the value of the work which they have done in preventing the loss of ships, and the consequent loss of lives and property, has been beyond all estimate.

The Royal Observatory, Greenwich, is a meteorological as well as an astronomical observatory, but, as remarked above, it does not itself issue any weather forecasts. Just as the Greenwich observations of the places of the moon and stars are sent to the Nautical Almanac Office, for use in the preparation of that ephemeris; just as the Greenwich determinations of time are used for the issue of signals to the Post Office, whence they are distributed over the kingdom, so the Greenwich observations of weather are sent to the Meteorological Office, there to be combined with similar records from every part of the British Isles, to form the basis of the daily forecasts which the latter office publishes. To each of these three offices, therefore, the Royal Observatory, Greenwich, stands in the relation of purveyor. It supplies them with the original observations more or less in reduced and corrected form, without which they could not carry on most important portions of their work.

Let it be noted how closely these three several departments, the Nautical Almanac Office, the Time Department, and the Meteorological Office, are related to practical navigation. Whatever questions of pure science—of knowledge, that is, apart from its useful applications—may arise out of the following up of these several inquiries, yet the first thought, the first principle of each, is to render navigation more sure and more safe.

The first of all meteorological instruments is the barometer, which, under its two chief forms of mercurial and aneroid, is simply a means of measuring the pressure exerted by the atmosphere.

There are two important corrections to which its readings are subject. The first is for the height of the station above the level of the sea; the second is for the effect of temperature upon the mercury in the barometer itself, lengthening the column. To overcome these, the height of the standard barometer at Greenwich above sea-level has been most carefully ascertained, and the heights relative to it of the other barometers of the Observatory, particularly those in rooms occupied by fundamental telescopes, have also been determined, whilst the self-recording barometer is mounted in a basement, where it is almost completely protected from changes of temperature.

Next in importance to the barometer as a meteorological instrument comes the thermometer. The great difficulty in the Observatory use of the thermometer is to secure a perfectly unexceptionable exposure, so that the thermometer may be in free and perfect contact with the air, and yet completely sheltered from any direct ray from the sun. This is secured in the great thermometer shed at Greenwich by a double series of 'louvre' boards, on the east, south, and west sides of the shed, the north side being open. The shed itself is made a very roomy one, in order to give access to a greater body of air.

A most important use of the thermometer is in the measurement of the amount of moisture in the air. To obtain this, a pair of thermometers are mounted close together, the bulb of one being covered by damp muslin, and the other being freely exposed. If the air is completely saturated with moisture, no evaporation can take place from the damp muslin, and consequently the two thermometers will read the same. But if the air be comparatively dry, more or less evaporation will take place from the wet bulb, and its temperature will sink to that at which the air would be fully saturated with the moisture which it already contained. For the higher the temperature, the greater is its power of containing moisture. The difference of the reading of the two thermometers is, therefore, an index of humidity. The greater the difference, the greater the power of absorbing moisture, or, in other words, the dryness of the air. The great shed already alluded to is devoted to these companion thermometers.

If, further, we had information from these various observing stations of the direction in which the wind was blowing, we should soon perceive other relationships. For instance, if we found that the barometer read about the same in a line across the country from east to west, but that it was higher in the north of the islands than in the south, we should then have a general set of winds from the east, and a similar relation would hold good if the barometer were highest in some other quarter; that is, the prevailing wind will come from a quarter at right angles to the region of highest barometer, or, as it is expressed in what is known as 'Buys Ballot's law,' 'stand with your back to the wind, and the barometer will be lower on your left hand than on your right.' This law holds good for the northern hemisphere generally, except near to the equator; in the southern hemisphere the right hand is the side of low barometer.

The instruments for wind observation are of two classes: vanes to show its direction, and anemometers to show its speed and its pressure. These may be regarded as two different modes in which the strength of the wind manifests itself. Pressure anemometers are usually of two forms: one in which a heavy plate is allowed to swing by its upper edge in a position fronting the wind, the amount of its deviation from the vertical being measured; and the other in which the plate is supported by springs, the degree of compression of the springs being the quantity registered in that case. Of the speed anemometers, the best known form is the 'Robinson,' in which four hemispherical cups are carried at the extremities of a couple of cross bars.

For the mounting of these wind instruments the old original Observatory, known as the Octagon Room, has proved an excellent site, with its flat roof surmounted by two turrets in the north-east and north-west corners, and raised some two hundred feet above high-water mark.

The two chief remaining instruments are those for measuring the amount of rainfall and of full sunshine. The rain gauge consists essentially of a funnel to collect the rain, and a graduated glass to measure it. The sunshine recorder usually consists of a large glass globe arranged to throw an image of the sun on a piece of specially prepared paper. This image, as the sun moves in the sky, moves along the paper, charring it as it moves, and at the end of the day it is easy to see, from the broken, burnt trace, at what hours the sun was shining clear, and when it was hidden by cloud.

An amusing difficulty was encountered in an attempt to set on foot another inquiry. The Superintendent of the Meteorological Department at the time wished to have a measure of the rate at which evaporation took place, and therefore exposed carefully measured quantities of water in the open air in a shallow vessel. For a few days the record seemed quite satisfactory. Then the evaporation showed a sudden increase, and developed in the most erratic and inexplicable manner, until it was found that some sparrows had come to the conclusion that the saucer full of water was a kindly provision for their morning 'tub,' and had made use of it accordingly.

A large proportion of the meteorological instruments at Greenwich and other first-class observatories are arranged to be self-recording. It was early felt that it was necessary that the records of the barometer and thermometer should be as nearly as possible continuous; and at one time, within the memory of members of the staff still living, it was the duty of the observer to read a certain set of instruments at regular two-hour intervals during the whole of the day and night—a work probably the most monotonous, trying, and distasteful of any that the Observatory had to show.

The two-hour record was no doubt practically equivalent to a continuous one, but it entailed a heavy amount of labour. Automatic registers were, therefore, introduced whenever they were available. The earliest of these were mechanical, and several still make their records in this manner.

On the roof of the Octagon Room we find, beside the two turrets already referred to, a small wooden cabin built on a platform several feet above the roof level. This cabin and the north-western turret contain the wind-registering instruments. Opening the turret door, we find ourselves in a tiny room which is nearly filled by a small table. Upon this table lies a graduated sheet of paper in a metal frame, and as we look at it, we see that a clock set up close to the table is slowly drawing the paper across it. Three little pencils rest lightly on the face of the paper at different points. One of these, and usually the most restless, is connected with a spindle which comes down into the turret from the roof, and which is, in fact, the spindle of the wind vane. The gearing is so contrived that the motion on a pivot of the vane is turned into motion in a straight line at right angles to the direction in which the paper is drawn by the clock. A second pencil is connected with the wind-pressure anemometer. The third pencil indicates the amount of rain that has fallen since the last setting, the pencil being moved by a float in the receiver of the rain gauge.

An objection to all the mechanical methods of continuous registration is that, however carefully the gearing between the instrument itself and the pencil is contrived, however lightly the pencil moves over the paper, yet some friction enters in and affects the record: this is of no great moment in wind registration, when we are dealing with so powerful an agent as the wind, but it becomes a serious matter when the barometer is considered, since its variations require to be registered with the greatest minuteness. When photography, therefore, was invented, meteorologists were very prompt to take advantage of this new ally. A beam of light passing over the head of the column of mercury in a thermometer or barometer could easily be made to fall upon a drum revolving once in the twenty-four hours, and covered with a sheet of photographic paper. In this case, when the sensitive paper is developed, we find its upper half blackened, the lower edge of the blackened part showing an irregular curve according as the mercury in the thermometer or barometer rose or fell, and admitted less or more light through the space above it.

Here we have a very perfect means of registration: the passage of the light exercises no friction or check on the free motion of the mercury in the tube, or on the turning of the cylinder covered by the sensitive paper, whilst it is easy to obtain a time scale on the register by cutting off the light for an instant—say at each hour. In this way the wet and dry bulb thermometers in the great shed make their registers.

The supply of material to the Meteorological Office is not the only use of the Greenwich meteorological observations. Two elements of meteorology, the temperature and the pressure of the atmosphere, have the very directest bearing upon astronomical work. And this in two ways. An instrument is sensible to heat and cold, and undergoes changes of form, size, or scale, which, however absolutely minute, yet become, with the increased delicacy of modern work, not merely appreciable, but important. So too with the density of the atmosphere: the light from a distant star, entering our atmosphere, suffers refraction; and being thus bent out of its path, the star appears higher in the heavens than it really is. The amount of this bending varies with the density of the layers of air through which the light has to pass. The two great meteorological instruments, the thermometer and barometer, are therefore astronomical instruments as well.

In the arrangements at Greenwich the Magnetic Department is closely connected with the Meteorological, and it is because the two departments have been associated together that the building devoted to both is constructed of wood, not brick, since ordinary bricks are made of clay which is apt to be more or less ferruginous. Copper nails have alone been employed in the construction of the buildings. The fire-grates, coal-scuttles, and fire-irons are all of the same metal.

The growth of the Observatory has, however, made it necessary to set up some of the new telescopes, into the mounting of which much iron enters, very close to the magnetic building. The present Astronomer-Royal has therefore erected a Magnetic Pavilion right out in the park at an ample distance from these disturbing causes.

The double department is, therefore, the most widely scattered in the whole Observatory. It is located for computing purposes in the west wing of the New Observatory; many of its magnetic instruments are in the old Magnet House, others are across the park in the new Magnetic Pavilion; the anemometers are on the roof of the Octagon Room, Flamsteed's original observatory, and the self-registering thermometers are in the south ground between the old Magnet House and the New Observatory.

'To make a magnet, you take a needle, and rub it on a lodestone. If it refuses or declines to become a magnet, that is magnetic declination; if it is easily made a magnet, or is inclined to become one, that is magnetic inclination.'

One greatly regretted that it was necessary to mark the reply according to its ignorance, and not, as one would have wished, in proportion to its ingenuity. Magnetic declination, however, as everybody knows, measures the deviation of the 'needle' from the true geographical north and south direction; the inclination or dip is the angle which a 'needle' makes with the horizon.

At one time the only method of watching the movements of the magnetic needles was by direct observation, just precisely as it was wont to be in the case of the barometer and thermometer. But the same agent that has been called in to help in their case has enabled the magnets also to give us a direct and continuous record of their movements. In principle the arrangement is as follows: A small light mirror is attached to the magnetic needle, and a beam of light is arranged to fall upon the mirror, and is reflected away from it to a drum covered with sensitive paper. If, then, the needle is perfectly at rest, a spot of light falls on the drum and blackens the paper at one particular point. The drum is made to revolve by clockwork once in twenty-four hours, and the black dot is therefore lengthened out into a straight line encircling the drum. If, however, the needle moves, then the spot of light travels up or down, as the case may be.

Now, if we look at one of these sheets of photographic paper after it has been taken from the drum, we shall see that the north pole of the magnet has moved a little, a very little, towards the west in the early part of the day, say from sunrise to 2 p.m., and has swung backwards from that hour till about 10 p.m., remaining fairly quiet during the night. The extent of this daily swing is but small, but it is greater in summer than in winter, and it varies also from year to year.

Besides this daily swing, there occasionally happen what are called 'magnetic storms;' great convulsive twitchings of the needle, as if some unseen operator were endeavouring, whilst in a state of intense excitement, to telegraph a message of vast importance, so rapid and so sharp are the movements of the needle to and fro. These great storms are felt, so far as we know, simultaneously over the whole earth, and the more characteristic begin with a single sharp twitch of the needle towards the east.

Besides the movements of the magnetic needle, the intensity of the currents of electricity which are always passing through the crust of the earth are also determined at Greenwich; but this work has been rendered practically useless for the last few years by the construction of the electric railway from Stockwell to the City. Since it was opened, the photographic register of earth currents has shown a broad blurring from the moment of the starting of the first train in the morning to the stopping of the last train at night. As an indication of the delicacy of modern instruments, it may be mentioned that distinct indications of the current from this railway have been detected as far off as North Walsham, in Norfolk, a distance of more than a hundred miles. A further illustration of the delicacy of the magnetic needles was afforded shortly after the opening of the railway referred to. On one occasion the then Superintendent of the Magnetic Department visited the Generating Station at Stockwell, and on his return it was noticed day after day that the traces from the magnets showed a curious deflection from 9 a.m. to 3 p.m., the hours of his attendance. This gave rise to some speculation, as it did not seem possible that the gentleman could himself have become magnetized. Eventually, the happy accident of a fine day solved the mystery. That morning the Superintendent left his umbrella at home, and the magnets were undisturbed. The secret was out. The umbrella had become a permanent magnet, and its presence in the lobby of the magnetic house had been sufficient to influence the needles.

CHAPTER X

THE HELIOGRAPHIC DEPARTMENT

So far the development of the Observatory had been along the central line of assistance to navigation. But the Magnetic Department led on to one which had but a very secondary connection with it.

A greatly enhanced interest was given to the observations of earth magnetism, when it was found that the intensity and frequency of its disturbances were in close accord with changes that were in progress many millions of miles away. That the surface of the sun was occasionally diversified by the presence of dark spots, had been known almost from the first invention of the telescope; but it was not until the middle of the present century that any connection was established between these solar changes and the changes which took place in the magnetism of the earth. Then two observers, the one interesting himself entirely with the spots on the sun, the other as wholly devoted to the study of the movements of the magnetic needle, independently found that the particular phenomenon which each was watching was one which varied in a more or less regular cycle. And further, when the cycles were compared, they proved to be the same. Whatever the secret of the connection, it is now beyond dispute that as the spots on the sun become more and more numerous, so the daily swing of the magnetic needle becomes stronger; and, on the other hand, as the spots diminish, so the magnetic needle moves more and more feebly.

This discovery has given a greatly increased significance to the study of the earth's magnetism. The daily swing, the occasional 'storms,' are seen to be something more than matters of merely local interest; they have the closest connection with changes going on in the vast universe beyond; they have an astronomical importance.

And it was soon felt to be necessary to supplement the Magnetic Observatory at Greenwich by one devoted to the direct study of the solar surface; and here again that invaluable servant of modern science, photography, was ready to lend its help. Just as, by the means of photography, the magnets recorded their own movements, so even more directly the sun himself makes register of his changes by the same agency, and gives us at once his portrait and his autograph.

This new department was again due to Airy, and in 1873 the 'Kew' photo-heliograph, which had been designed by De la Rue for this work, was installed at Greenwich.

In order to photograph so bright a body as the sun, it is not in the least necessary to have a very large telescope. The one in common use at Greenwich from 1875 to 1897, is only four inches in aperture and even that is usually diminished by a cap to three inches, and its focal length is but five feet. This is not very much larger than what is commonly called a 'student's telescope,' but it is amply sufficient for its work.

This 'Dallmeyer' telescope, so called from the name of its maker, is one of five identical instruments which were made for use in the observation of the transit of Venus of 1874, and which, since they are designed for photographing the sun, are called 'photo-heliographs.'

The image of the sun in the principal focus of this telescope is about six-tenths of an inch in diameter; but a magnifying lens is used, so that the photograph actually obtained is about eight inches. Even with this great enlargement, the light of the sun is so intense that with the slowest photographic plates that are made the exposure has to be for only a very small fraction of a second. This is managed by arranging a very narrow slit in a strip of brass. The strip is made to run in a groove across the principal focus. Before the exposure, it is fastened up so as to cut off all light from entering the camera part of the telescope. When all is ready, it is released and drawn down very rapidly by a powerful spring, and the slit, flying across the image of the sun, gives exposure to the plate for a very minute fraction of a second—in midsummer for less than a thousandth of a second.

Two of these photographs are taken every fine day at Greenwich; occasionally more, if anything specially interesting appears to be going on. But in our cloudy climate at least one day in three gives no good opportunity for taking photographs of the sun, and in the winter time long weeks may pass without a chance. The present Astronomer-Royal, Mr. Christie, has therefore arranged that photographs with precisely similar instruments should be taken in India and in the Mauritius, and these are sent over to Greenwich as they are required, to fill up the gaps in the Greenwich series. We have therefore at Greenwich, from one source or another, practically a daily record of the state of the sun's surface.

More recently the 'Dallmeyer' photo-heliograph, though still retained for occasional use, has been superseded generally by the 'Thompson'; a photographic refractor of nine inches aperture, and nearly nine feet focal length, presented to the Observatory by Sir Henry Thompson. The image of the sun obtained after enlargement in the telescope, with this instrument, is seven and a half inches in diameter. The 'Thompson' is mounted below the great twenty-six-inch photographic refractor,—also presented to the Observatory by Sir Henry Thompson,—in the dome which crowns the centre of the New Observatory.

A photograph of the sun taken, it has next to be measured, the four following particulars being determined for each spot: First, its distance from the centre of the image of the sun; next, the angle between it and the north point; thirdly, the size of the spot; and fourthly, the size of the umbra of the spot, that is to say, of its dark central portion. The size or area of the spot is measured by placing a thin piece of glass, on which a number of cross-lines have been ruled one-hundredth of an inch apart, in contact with the photograph. These cross-lines make up a number of small squares, each the ten-thousandth (¹/₁₀₀₀₀ in.) part of a square inch in area. When the photograph and the little engraved glass plate are nearly in contact, the photograph is examined with a magnifying glass, and the number of little squares covered by a given spot are counted. It will give some idea of the vast scale of the sun when it is stated that a tiny spot, so small that it only just covers one of these little squares, and which is only one-millionth of the visible hemisphere of the sun in area, yet covers in actual extent considerably more than one million of square miles.

The dark spots are not the only objects on the sun's surface. Here and there, and especially near the edge of the sun, are bright marks, generally in long branching lines, so bright as to appear bright even against the dazzling background of the sun itself. These are called 'faculæ,' and they, like the spots, have their times of great abundance and of scarcity, changing on the whole at the same time as the spots.

After the solar photographs have been measured, the measures must be 'reduced,' and the positions of the spots as expressed in longitude and latitude on the sun computed. There is no difficulty in doing this, for the position of the sun's equator and poles have long been known approximately, the sun revolving on its axis in a little more than twenty-five days, and carrying of course the spots and faculæ round with him.

There are few studies in astronomy more engrossing than the watch on the growth and changes of the solar spots. Their strange shapes, their rapid movements, and striking alterations afford an unfailing interest. For example, the amazing spectacle is continually being afforded of a spot, some two, three, or four hundred millions of square miles in area, moving over the solar surface at a speed of three hundred miles an hour, whilst other spots in the same group are remaining stationary. But a higher interest attaches to the behaviour of the sun as a whole than to the changes of any particular single spot; and the curious fact has been brought to light, that not only do the spots increase and diminish in a regular cycle of about eleven years in length, but they also affect different regions of the sun at different points of the cycle. At the time when spots are most numerous and largest, they are found occupying two broad belts, the one with its centre about 15° north of the equator, the other about as far south, the equator itself being very nearly free from them. But as the spots begin to diminish, so they appear continually in lower and lower latitudes, until instead of having two zones of spots there is only one, and this one lies along the equator. By this time the spots have become both few and small. The next stage is that a very few small spots are seen from time to time in one hemisphere or the other at a great distance from the equator, much farther than any were seen at the time of greatest activity. There are then for a little time three sun-spot belts, but the equatorial one soon dies out. The two belts in high latitude, on the other hand, continually increase; but as they increase, so do they move downwards in latitude, until at length they are again found in about latitude 15° north or south, when the spots have attained their greatest development.

This great disturbance, evidently something of the nature of a storm in the solar atmosphere, stretched over one hundred thousand miles on the surface of the sun. The disturbance extended farther still, even to nearly one hundred millions of miles. For simultaneously with the appearance of the spot the magnetic needles at Greenwich began to suffer from a strange excitement, an excitement which grew from day to day until it had passed half-way across the sun's disc. As the twitchings of the magnetic needle increased in frequency and violence, other symptoms were noticed throughout the length of the British Isles. Telegraphic communication was greatly interfered with. The telegraph lines had other messages to carry more urgent than those of men. The needles in the telegraph instruments twitched to and fro. The signal bells on many of the railway lines were rung, and some of the operators received shocks from their instruments. Lastly, on November 17, a superb aurora was witnessed, the culminating feature of which was the appearance, at about six o'clock in the evening, of a mysterious beam of greenish light, in shape something like a cigar, and many degrees in length, which rose in the east and crossed the sky at a pace much quicker than but nearly as even as that of sun, moon, or stars, till it set in the west two minutes after its rising.

So far we have been dealing only with effects. Their causes still rest hidden from us. There is clearly a connection between the solar activity as shown by the spots and the agitation of the magnetic needles. But many great spots find no answer in any magnetic vibration, and not a few considerable magnetic storms occur when we can detect no great solar changes to correspond.

Thus even in the simplest case before us we have still very much to explain. Two far more difficult problems are still offered us for solution. What is the cause of these mysterious solar spots? and have they any traceable connection with the fitful vagaries of earthly weather? It was early suggested that probably the first problem might find an answer in the ever-varying combinations and configurations of the various planets, and that the sun-spots in their turn might hold the key of our meteorology. Both ideas were eagerly followed up—not that there was much to support either, but because they seemed to offer the only possible hope of our being able to foretell the general current of weather change for any long period in advance. So far, however, the first idea may be considered as completely discredited. As to the second, there would appear to be, in the case of certain great tropical and continental countries like India, some slight but by no means conclusive evidence of a connection between the changes in the annual rainfall and the changes in the spotted surface of the sun. Dr. Meldrum, the late veteran Director of the great Meteorological Observatory in Mauritius, has expressed himself as confident that the years of most spots are the years of most violent cyclones in the Indian Ocean. But this is about as far as real progress has been made, and it may be taken as certain that many years more of observation will be required, and the labours of many skilful investigators, before we can hope to carry much farther our knowledge as to any connection between storm and sun.

A further relation of great interest has come to light within the last few years. The year 1868 opened a new epoch in the study of eclipses of the sun. These, perhaps, scarcely lie within the scope of a book on the Royal Observatory, since Greenwich has seen but one in all its history. That fell in the year 1715; for the next it must wait many centuries. Yet, as the late Astronomer Royal conducted three expeditions to see total eclipses, and as the present Astronomer Royal has undertaken a like number, and members of the staff have been sent on other occasions, it may not be deemed quite a digression to refer to one feature which they have brought to light.

When the dark body of the moon has entirely hidden the sun, we have revealed to us, there and then only, that strange and beautiful surrounding of the sun which we call the corona. The earlier observations of the corona seem to reveal it as a body of the most weird and intricate form, a form which seemed to change quite lawlessly from one eclipse to another. But latterly it has been abundantly clear that the forms which it assumes may be grouped under a few well-defined types. In 1878 the corona was of a particularly simple and striking character. Two great wings shot out east and west in the direction of the sun's equator; round either pole was a cluster of beautiful radiating 'plumes.' It was then recollected that the corona of 1867 had been of precisely the same character, both years being years when sun-spots were at their fewest. The coronæ, on the other hand, seen at times when sun-spots are more abundant, were of an altogether different character, the streamers being irregularly distributed all round the sun. Other types also have been recognized, and it is perfectly apparent that the corona changes its shape in close accordance with the eleven-year period. The eclipses of 1889 and 1900, for example, showed coronæ that bore the very closest resemblance to those of 1878 and 1866, the interval of eleven years bringing a return to the same form.

The further problem, therefore, now confronts us: Does the corona produce the sun-spots, or do the sun-spots produce the corona, or are both the result of some mysterious magnetic action of the sun, an action powerful enough on occasion to thrill through and through this world of ours, ninety-three millions of miles away?

CHAPTER XI

THE SPECTROSCOPIC DEPARTMENT

Another department was set on foot by Airy at the same time as the Heliographic Department, and in connection with it; and it is the department which has the greatest of interest for the general public. This deals with astronomical physics, or astrophysics, as it is sometimes more shortly called; the astronomy, that is, which treats of the constitution and condition of the heavenly bodies, not with their movements.

The older astronomy, on the other hand, confined itself to the movements of the heavens so entirely that Bessel, the man whose practical genius revolutionized the science of observation, and whose influence may be traced throughout in Airy's great reconstitution of Greenwich Observatory, denied that anything but the study of the celestial movements had a right to the title of astronomy at all. Hardly more than sixty years ago he wrote:

'What astronomy is expected to accomplish is evidently at all times the same. It may lay down rules by which the movements of the celestial bodies, as they appear to us upon the earth, can be computed. All else which we may learn respecting these bodies, as, for example, their appearance, and the character of their surfaces, is, indeed, not undeserving of attention, but possesses no proper astronomical interest. Whether the mountains of the moon are arranged in this way or in that is no further an object of interest to astronomers than is a knowledge of the mountains of the earth to others. Whether Jupiter appears with dark stripes upon its surface, or is uniformly illuminated, pertains as little to the inquiries of the astronomer; and its four moons are interesting to him only for the motions they have. To learn so perfectly the motions of the celestial bodies that for any specified time an accurate computation of these can be given—that was, and is, the problem which astronomy has to solve.'

There is a curious irony of progress which seems to delight in falsifying the predictions of even master minds as to the limits beyond which it cannot advance. Bessel laid down his dictum as to the true subjects of astronomical inquiry, Comte declared that we could never learn what were the elements of which the stars were composed, at the very time that the first steps were being taken towards the creation of a research which should begin by demonstrating the existence in the heavenly bodies of the elements with which we are familiar on the earth, and should go on to prove itself a true astronomy, even in Bessel's restricted sense, by supplying the means for determining motion in a direction which he would have thought impossible—that is to say, directly to or from us.

The years that followed Kirchhoff's application of the spectroscope to the study of the sun, and his demonstration that sodium and iron existed in the solar atmosphere, were crowded with a succession of brilliant discoveries in the same field. Kirchhoff, Bunsen, Angström, Thal·n, added element after element to the list of those recognized in the sun. Huggins and Miller carried the same research into a far more difficult field, and showed us the same elements in the stars. Rutherfurd and Secchi grouped the stars according to the types of their spectra, and so laid the foundations of what may be termed stellar comparative anatomy. Huggins discovered true gaseous nebulæ, and so revived the nebular theory, which had been supposed crushed when the great telescope of Lord Rosse appeared to have resolved several portions of the Orion nebula into separate stars. The great riddle of 'new stars'—which still remains a riddle—was at least attacked, and glowing hydrogen was seen to be a feature in their constitution. Glowing hydrogen, again, was, in the observation of total eclipses, seen to be a principal constituent of those surroundings of our own sun which we now call prominences and chromosphere. Then the method was discovered of observing the prominences without an eclipse, and they were found to wax and wane in more or less sympathy with the solar spots. Sun-spots, planets, comets, meteors, variable stars, all were studied with the new instrument, and all yielded to it fresh and valuable, and often unexpected, information.