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.
[Illustration: 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.
[Illustration: 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.
[Illustration: 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.
[Illustration: 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.
[Illustration: 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.