THE MAGNETIC AND METEOROLOGICAL DEPARTMENTS
Passing out of the south door of the new altazimuth building, we come to a white cruciform erection, constructed entirely of wood. This is the Magnet House or Magnetic Observatory, the home of a double Department, the Magnetic and Meteorological.
This department does not, indeed, lie within the original purpose of the Observatory as that was defined in the warrant given to Flamsteed, and yet is so intimately connected with it, through its bearing on navigation, that there can be no question as to its suitability at Greenwich. Indeed, its creation is a striking example of the thorough grasp which Airy had upon the essential principles which should govern the great national observatory of an essentially naval race, and of the keen insight with which he watched the new development of science. The Magnetic Observatory, therefore, the purpose of which was to deal with the observation of the changes in the force and direction of the earth's magnetism—an inquiry which the greater delicacy of modern compasses, and, in more recent times, the use of iron instead of wood in the construction of ships has rendered imperative—was suggested by Airy on the first possible occasion after he entered on his office, and was sanctioned in 1837. The Meteorological Department has a double bearing on the purpose of the Observatory. On the one side, a knowledge of the temperature and pressure of the atmosphere is, as we have already seen, necessary in order to correct astronomical observations for the effect of refraction. On the other hand, meteorology proper, the study of the movements of the atmosphere, the elucidation of the laws which regulate those movements, leading to accurate forecasts of storms, are of the very first necessity for the safety of our shipping. It is true that weather forecasts are not issued from Greenwich Observatory, any more than the Nautical Almanac is now issued from it; but just as the Observatory furnishes the astronomical data upon which the Almanac is based, so also it takes its part in furnishing observations to be used by the Meteorological Office at Westminster for its daily predictions.
Those predictions are often made the subject of much cheap ridicule; but, however far short they may fall of the exact and accurate predictions which we would like to have, yet they mark an enormous advance upon the weather-lore of our immediate forefathers.
'He that is weather wise
Is seldom other wise,'
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
'If we'd no moon at all—
And that may seem strange—
We still should have weather
That's subject to change.'
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
'If the ash is out before the oak,
You may expect a thorough soak'—
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.
THE SELF-REGISTERING THERMOMETERS.
Very closely connected with atmospheric pressure, as shown us by the barometer, is the study of the direction of winds. If we take a map of the British Isles and the neighbouring countries, and put down upon them the barometer readings from a great number of observing stations, and then join together the different places which show the same barometric pressure, we shall find that these lines of equal pressure—technically called 'isobars'—are apt to run much nearer together in some places than in others. Clearly, where the isobars are close together it means that in a very short distance of country we have a great difference of atmospheric pressure. In this case we are likely to get a very strong wind blowing from the region of high pressure to the region of low pressure, in order to restore the balance.
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 ANEMOMETER ROOM, NORTH-WEST TURRET.
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.
THE ANEMOMETER TRACE.
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.
MAGNETIC PAVILION—EXTERIOR.
(From a photograph by Mr. Lacey.)
The object of the Magnetic Observatory is to study the movements of the magnetic needle. The quaintest answer that I ever received in an examination was in reply to the question, 'What is meant by magnetic inclination and declination?' The examinee replied:
'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.
MAGNETIC PAVILION—INTERIOR.
(From a photograph by Mr. Lacey.)
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.