The barometer was invented by Torricelli, an Italian philosopher of
the seventeenth century. It consists essentially of a long tube open
at one end and closed at the other, and partly filled with mercury;
but instead of being filled like ordinary vessels, with the open
end or mouth upwards and the closed end or bottom downwards, the
barometer-tube is inverted, and has its open mouth downwards. This open
mouth is either dipped into a little cup of mercury or bent a little
upwards.
Why does not the mercury run out of this lower open end and overflow
the little cup when it is inverted after being filled?
The answer to this question includes the whole mystery and principle
of the barometer. The mercury does not fall down because something
pushes it up and supports it with a certain degree of pressure, and
that something is the atmosphere which extends all round the world,
and presses downwards and sideways and upwards—in every direction, in
fact—with a force equal to its weight, _i.e._, with a pressure equal
to about 15 lbs. on every square inch. A column or perpendicular square
stick of air one inch thick each way, and extending from the surface
of the sea up to the top of the atmosphere, weighs about 15 lbs.;
other columns or sticks next to it on all sides weigh the same, and so
on with every portion; and all these are for ever squeezing down and
against each other, and, being fluid, transmit their pressure in every
direction, and against the earth and everything upon it, and therefore
upon the mercury of the barometer-tube.
We have supposed the air to be made up of columns or sticks of air one
inch each way, but might have taken any other size, and the weight
and pressure would be proportionate. Now mercury, bulk for bulk, is
so much heavier than air, that a stick or column of this liquid metal
about 30 inches high weighs as much as a stick or column of air of
same thickness reaching from the surface of the earth to the top of
the atmosphere; therefore, the 30-inch stick of mercury balances the
pressure of the many miles of atmosphere, and is supported by it. Thus
the column of mercury may be used to counterbalance the atmosphere and
show us its weight; and such a column of mercury is a barometer, or
“weight measure.” The word _barometer_ is compounded of the two Greek
words—_baros_, weight, and _metron_, a measure.
If you take a glass tube a yard long, stopped at one end and open at
the other, fill it with mercury, stop the open end with your thumb,
then invert the tube and just dip the open end in a little cup of
mercury, some of the mercury in the tube will fall into the cup, but
not all; only six inches will fall, the other 30 inches will remain,
with an empty space between it and the stopped end of the tube. When
you have done this you will have made a rude barometer. If you prop up
the tube, and watch it carefully from day to day, you will find that
the height of the column of mercury will continually vary. If you live
at the sea-level, or thereabouts, it will sometimes rise more than 30
inches above the level of the mercury in the cup, and frequently fall
below that height. If you live on the top of a high mountain, or on any
high ground, it will never reach 30 inches, will still be variable, its
average height less than if you lived on lower ground; and the higher
you go the less will be this average height of the mercury.
The reason of this is easily understood. When we ascend a mountain
we leave some portion of the atmosphere below us, and of course less
remains above; this smaller quantity must have less weight and press
the mercury less forcibly. If the barometer tells the truth, it must
show this difference; and it does so with such accuracy that by means
of a barometer, or rather of two barometers—one at the foot of the
mountain and one at its summit—we may, by their difference, measure
the height of the mountain provided we know the rules for making the
requisite calculations.
The old-fashioned barometer, with a large dial-face and hands like a
clock, is called the “wheel barometer,” because the mercury, in rising
and falling, moves a little glass float resting upon the mercury of
the open bent end of the tube; to this float and its counterpoise a
fine cord is attached; and this cord goes round a little grooved wheel
to which the hands are attached. Thus the rising and falling of the
mercury moves the float, the float-cord turns the wheel, and the wheel
moves the hand that points to the words and figures on the dial. When
this hand moves towards the right, or in the direction of an advancing
clock-hand, the barometer is rising; when it goes backwards, or
opposite to the clock-hand movement, the mercury is falling. By opening
the little door at the back of such a barometer, the above-described
mechanism is seen. In doing this, or otherwise moving your barometer,
be careful always to keep it upright.
It sometimes happens to these wheel barometers that they, suddenly
cease to act; and in most cases the owner of the barometer may save the
trouble and expense of sending it to the optician by observing whether
the cord has slipped from the little wheel, and if so, simply replacing
it in the groove upon its edge. If, however, the mischief is caused by
the tube being broken, which is seen at once by the mercury having run
out, the case is serious, and demands professional aid.
The upright barometer, which shows the surface of the mercury itself,
is the most accurate instrument, provided it is carefully read. This
form of instrument is always used in meteorological observatories,
where minute corrections are made for the expansion and contraction
which variations of temperature produce upon the length of the mercury
without altering its weight, and for the small fluctuations in the
level of the mercury cistern. With such instruments, fitted with an
apparatus called a “vernier” the height of the mercury may be read to
hundredths of an inch.
The necessity for the 30 inches of mercury renders the mercurial
barometer a rather cumbrous instrument: it must be more than 30 inches
long, and is liable to derangement from the spilling of the mercury.
On this account portable barometers of totally different construction
have been invented. The “aneroid” barometer is one of these—the only
one that is practically used to any extent. It contains a metal box
partly filled with air; one face of the box is corrugated, and so thin
that it can rise and fall like a stretched covering of india-rubber.
As the pressure of the outside air varies it does rise and fall,
and by a beautifully-delicate apparatus this rising and falling is
magnified and represented upon the dial. Such barometers are made small
enough to be carried in the pocket, and are very useful for measuring
the heights of mountains; but they are not quite so accurate as the
mercurial barometer, and are therefore not used for rigidly scientific
measurements; but for all ordinary purposes they are accurate enough,
provided they are occasionally compared with a standard mercurial
barometer, and adjusted by means of a watch-key axis provided for that
purpose, and seen on the back of the instrument. They are sufficiently
delicate to tell the traveller in a railway whether he is ascending
or descending an incline, and will indicate the difference of height
between the upper and lower rooms of a three-story house. With due
allowance for variations of level, the traveler may use them as weather
indicators; especially as it is the direction in which the barometer
is moving (whether rising or falling) rather than its absolute height
that indicates changes of weather. Thus by placing the aneroid in his
room on reaching his hotel at night, carefully marking its height then
and there, and comparing this with another observation made on the
following morning, he may use it as a weather-glass in spite of hill
and dale.
Water barometers have been made on the same principle as the mercury
barometer; but as water is 13½ times lighter, bulk for bulk, than
mercury, the height of the column must be 13½ times 30 inches, or,
allowing for variations, not less than 34 feet. This, of course,
is very cumbrous; the evaporation of the water presents another
considerable difficulty,[21] still such a barometer is a very
interesting instrument, as it shows the atmospheric fluctuations on
13½ times the scale of the ordinary barometer. A range of about five
feet is thus obtained; and not only the great waves, but even the
comparatively small ripples of the atmospheric ocean are displayed by
it. In stormy weather it may be seen to rise and fall and pulsate like
a living creature, so sensitively does it respond to every atmospheric
fluctuation.
But why should the height of the barometer vary while it remains in the
same place?
If the quantity of air surrounding the earth remains the same, and if
the barometer measures its weight correctly, why should the barometer
vary?
Does the atmosphere grow bigger and smaller, lighter and heavier, from
time to time?
These are fair questions, and they bring us at once to some of the
chief uses of the barometer. The atmosphere is a great gaseous ocean
surrounding the earth, and we are creeping about on the bottom of this
ocean. It has its tides and billows and whirling eddies, but all these
are vastly greater than those of the watery ocean. At one time we are
under the crest or rounded portion of a mighty atmospheric wave, at
another the hollow between two such waves is over our heads, and thus
the depth of atmosphere, or quantity of air, above us is variable.
This variation is the combined result of many co-operating causes. In
the first place, there are great atmospheric tides, caused, like those
of the sea, by the attraction of the sun and moon; but these do not
_directly_ affect the barometer, because the attracting body supports
whatever it lifts. Variations of temperature also produce important
fluctuations in the height and density of the atmosphere, some of which
are indicated by the barometer—others are not. Thus a mere expansion
or contraction of _dry_ air, increasing the depth or the density of the
atmospheric ocean, would not affect the barometer, as mere expansion
and contraction only alter the _bulk_ without affecting the _weight_ of
the air. But our atmosphere consists not only of the permanent gases,
nitrogen and oxygen; it contains besides these and carbonic acid, a
considerable quantity of gaseous matter, which is not permanent, but
which may be a gas at one moment—contributing its whole weight to
that of the general atmosphere—and at another moment some of it may
be condensed into liquid particles that fall through it more or less
rapidly, and thus contribute nothing to its weight.
What, then, is this variable constituent that sometimes adds to the
weight of the atmosphere and the consequent height of the barometer,
and at others may suddenly cease to afford its full contribution to
atmospheric pressure?
It is simply water, which, as we all know, exists as solid, liquid, or
gas, according to the temperature and pressure to which it is exposed.
We all know that steam when it first issues from the spout of a
tea-kettle is a transparent gas, or true vapor, but that presently, by
contact with the cool air, it becomes white, cloudy matter, or minute
particles of water; and that, if these are still further cooled, they
will become hoar-frost or snow, or solid ice. Artificial hoar-frost and
snow may be formed by throwing a jet of steam into very cold, frosty
air. If you take a tin canister or other metal vessel, fill it with a
mixture of salt with pounded ice or snow, and then hold the outside of
the canister against a jet of steam, such as issues from the spout of a
tea-kettle, a snowy deposit of hoar-frost will coat the outside of the
tin. Now let us consider what takes place when a warm south-westerly
wind, that has swept over the tropical regions of the Atlantic ocean,
reaches the comparatively cold shores of Britain. It is cooled thereby,
and some of its gaseous water is condensed—forming mists, clouds,
rain, hoar-frost or snow. The greater part of this forms and falls on
the western coasts, on Cornwall, Ireland, the Western Highlands of
Scotland. Ireland gets the lion’s share of this humidity, and hence her
“emerald” verdure. The western slope of a mountain, in like manner,
receives more rain than the side facing the east.
How does this condensation affect the barometer?
It must evidently cause it to fall, inasmuch as the air must be
lightened to the exact extent of all that is taken out of it and
precipitated. But the precipitation is not completed immediately the
condensation occurs. It takes some time for the minute cloudy particles
to gather into rain drops and fall to the earth, while the effect of
the condensation upon the barometer is instantaneous; the air begins
to grow lighter immediately the gas is converted into cloud or mist,
and the barometer falls just at the same time and same rate as this is
produced; but the rain comes some time afterwards. Hence the use of the
barometer as a “weather glass.” When intelligently and properly used it
is very valuable in this capacity; but, like most things, it may easily
be misunderstood and misused.
The most common error in the use of the barometer is that to which
people are naturally led by the words engraved upon it, “Stormy, Much
Rain, Rain, Change, Fair, Set Fair,” etc. A direct and absolute blunder
or falsehood is usually short-lived, and deceives but few people;
but a false statement, with a certain amount of superficial truth,
may survive for ages, and deceive whole generations. Now this latter
is just the character of the weather signs that are engraved on our
popular barometers; they are unsound and deceptive, but not utterly
baseless.
_Stormy_, _Much Rain_, and _Rain_ are marked against the low readings
of the barometer, and _Very Dry_, _Set Fair_, and _Fair_ against the
higher readings. A low barometer is not a reliable sign of wet or
stormy weather, neither is a high barometer to be depended upon for
expecting fine weather; and yet it is true that we are more likely to
have fine weather with a high than with a low barometer, and also the
liability to rain and storms is greater with a low than with a high
barometer.
The best indications of the weather are those derived from the
direction in which the barometer is moving—whether rising or
falling—rather than its mere absolute height.
A sudden and considerable fall is an almost certain indication of
strong winds and stormy weather. This is the most reliable of the
prophetic warnings of the barometer, and the most useful, inasmuch as
it affords the mariner just the warning he requires when lying off a
dangerous coast, or otherwise in peril by a coming gale. Many a good
ship has been saved by intelligent attention to the barometer, and by
running into haven, or away from a rocky shore when the barometer has
fallen with unusual rapidity.
The next in order of reliability is the indication afforded by a
steady and continuous fall after a long period of fine weather. This is
usually followed by a decided change of weather, and the greater the
fall the more violent the change. If the fall is slow, and continues
steadily for a long time, the change is likely to be less sudden but
more permanent, _i.e._, the rain will probably arrive after some time,
and then continue steadily for a long period.
In like manner, a steady, regular rise, going on for some days in the
midst of wet weather, may be regarded as a hopeful indication of coming
continuous fine weather—the more gradual and steady the rise, the
longer is the fine weather likely to last.
The least reliable of all the barometric changes is a sudden rise. In
winter it may be followed by hard and sudden frost, in summer by sultry
weather and thunder-storms. All that may be safely said of such sudden
rise is, that it indicates a change of some sort.
The barometer is usually high with N.E. winds, and low with S.W. winds.
The preceding explanations show the reason of this. In a given place
the extreme range of variation is from 2 to 2½ inches.
It has been proposed that the following rules should be engraved on
barometer-plates instead of the usual words:—
1st. Generally, the rising of the mercury indicates the approach of
fair weather; the falling of it shows the approach of foul weather.
2d. In sultry weather, the fall of the barometer indicates coming
thunder. In winter, the rise of the mercury indicates frost. In frost,
its fall indicates thaw, and its rise indicates snow.
3d. Whatever change in the weather suddenly follows a change in the
barometer, may be expected to last but a short time.
4th. If fair weather continues for several days, during which the
mercury continually falls, a long succession of foul weather will
probably ensue; and again, if foul weather continues for several days,
while the mercury continually rises, a long succession of fair weather
will probably follow.
5th. A fluctuating and unsettled state of the mercurial column
indicates changeable weather.
As the barometer is subject to slight diurnal variations, irrespective
of those atmospheric changes which affect the weather, it is desirable
in making comparative observations to do so at fixed hours of the day.
Nine or ten in the morning and same hour in the evening are good times
for observations that are to be recorded. These are about the hours of
daily maxima or highest readings due to regular diurnal variation.
The true reading of the barometer is the height at which it would stand
if placed at the level of the sea at high tide; but, as barometers are
always placed more or less above this level, a correction for elevation
is necessary. When the height of the place is known this correction may
be made by adding one tenth of an inch to the actual reading for every
85 feet of elevation up to 510 feet; the same for every 90 feet between
510 and 1140 feet, for every 95 feet between 1140 and 1900 feet, and
for every 100 feet above this and within our mountain limits. This
simple and easy rule is sufficiently accurate for practical purposes.
Thus, a barometer on Bray Head, or any place 800 feet above the sea,
would require a correction of six-tenths for the first 510 feet, and
a little more than three-tenths more for the remaining 290 feet.
Therefore, if such a barometer registered the pressure at 29-1/10, the
proper sea-level reading would be a little above 30 inches.
The most important prognostications of the barometer are those afforded
by what is called the “barometric gradient or incline,” showing the
up-hill and down-hill direction of the atmospheric inequalities; but
this can only be ascertained by comparing the state of the barometer
at different stations at the same time. Thus, if the barometer is
one-fourth of an inch higher at Dublin than at Galway, and the
intermediate stations show intermediate heights, there must be an
atmospheric down-hill gradient from Dublin to Galway; Dublin must
be under the upper and Galway under the lower portion of a great
atmospheric wave or current. It is evident that when there is thus
more air over Dublin than over Galway, there must follow (if nothing
else interferes) a flow of air from Dublin towards Galway. It is also
evident that, in order to tell what else may interfere, we must know
the atmospheric gradients beyond and around both Dublin and Galway, and
for considerable distances.
We are now beginning to obtain such information by organizing
meteorological stations and observatories, and transmitting the results
of simultaneous observations by means of the electric telegraph to
certain head-quarters.
The subject is occupying much attention, and the managers of those
splendid monuments of British energy—our daily newspapers—are
publishing daily weather charts, and therefore a few simple
explanations of the origin, nature, and significance of such charts
will doubtless be appreciated by our readers.
The grand modern improvement of the barometer, the thermometer,
the anemometer, the pluviometer, etc., is that of making them
“self-registering.” We are told that Cadmus invented the art of
writing, and we honor his memory accordingly. But he ventured no
further than teaching human beings to write. Modern meteorologists
have gone much further; they have taught the winds and the rains and
the subtle heavings of the invisible air to keep their own diaries,
to write their own histories on paper that is laid before them, with
pencils that are placed in their fleshless, boneless, and shapeless
fingers. This achievement is wrought by comparatively simple means. The
paper is wound upon an upright drum or cylinder, and this cylinder is
made to revolve by clock-work, in such a manner that a certain breadth
travels on during the twenty-four hours. This breadth of paper is
divided by vertical lines into twenty-four parts, each of which passes
onward in one hour. Connected with the barometer is a pencil which,
by means of a spring, presses lightly upon the revolving sheet, and
this pencil, while thus pressing, rises and falls with the mercury.
It is obvious that, in this manner, a line will be drawn as the paper
moves. If the mercury is stationary, the line will be horizontal—only
indicating the movement of the drum; if the mercury falls, the line
will slope downwards; if it rises, it will incline upwards. By ruling
horizontal lines upon the paper, representing inches, tenths, and
smaller fractions, if desired, the whole history of the barometrical
movements will be graphically recorded by the waving or zigzag lines
thus drawn by the atmosphere itself.
The subjoined copy of the _Daily Telegraph_ Barometer Chart represents,
on a small scale, a four days’ history of barometrical movements:
The large figures at the side (29 and 30) represent inches; the smaller
figures tenths of inches.
[Illustration]
The pressure of the wind is similarly pictured by means of a large vane
which turns with the wind, and to the windward face of which a flat
board or plate of metal, one foot square, is attached perpendicularly.
As the wind strikes this it presses against it with a force
corresponding to a certain number of pounds, ounces, and fractions of
an ounce. A spring like that of an ordinary spring letter-balance is
compressed in proportion to this pressure. This movement of the spring
is transmitted mechanically to another pencil like the above described,
working against the same drum; thus another history is written on the
same paper—the horizontal lines now representing fractions of pounds
of pressure, instead of fractions of inches of mercury.
It has been found that if a semi-globular cup of thin metal is exposed
to the wind, the pressure upon the round or convex side of the
hemisphere is equal to two thirds of that upon the hollow or concave
side. By placing four such cups upon cross-arms, and the arms on a
pivot, the wind, from whatever quarter it may come, will always blow
them round with their convex faces foremost; and they will move with
one third of the actual velocity of the wind. By a simple clock-work
arrangement, these arms move another pencil, in such a manner that it
strikes the paper hammer-fashion every time the wind has completed a
journey of one mile, or other given distance; and thus a series of dots
upon the revolving paper records the velocity of the wind according
to their distances apart. As the pressure of the wind is governed by
two factors, viz., the density and velocity of the moving air, the
relations between the barometer curve, the pressure curve, and the
velocity dots, are very interesting.
The direction of the wind is written by a pencil fixed to a quick
worm—a screw-thread upon the axis of the vane. As the vane turns
round—N., E., S., or W.—it screws the pencil up or down, and thus the
horizontal lines first described as registering tenths of inches of
barometric pressure do duty as showing the points of the compass from
which the wind is blowing; and, by reference to the zigzag line drawn
by this pencil of the wind, its direction at any particular time of day
may be ascertained as certified by its own sign-manual.
The wind-gauge is called an anemometer. Connected with this is the
pluviometer, or rain-gauge—an upright vessel with an open mouth of
measured area—say 100 square inches. This receives the rain that
falls. By means of a pipe the water is conveyed to a vessel having a
surface of—say one square inch. By this arrangement, when sufficient
rain has fallen to cover the surface of the earth to the depth of one
hundredth of an inch, the little vessel below will contain water one
inch in depth. By balancing this vessel at the end of a long arm, it
is made to preponderate gradually as the weight of water it receives
increases, and finally, when filled, it tips over altogether, empties
itself, and then rises to its starting place in equilibrium. To the
other end of this arm a pencil is attached, which inscribes all these
movements on the revolving paper, and thus tells the history of the
rainfall. The line is zigzag while the rain is falling, and horizontal
while the weather is fair. The amount of inclination of the zigzag
line measures the depth of rain by means of the same ruled lines on
the paper as measure the height of the barometer, etc. Every time
the measuring vessel tips over a perpendicular line is drawn, and
the pencil resumes its starting level. The papers containing these
autographs of the elements may, of course, be kept as permanent records
for reference whenever needed, or the results may be tabulated in other
forms.
There are many modifications in the details of these self-registering
instruments. In some of them photography is made to do a part of the
work. The above description indicates the main principles of their
construction, without attempting to enter upon minute details.
Meteorological observatories are provided with these instruments, and
all nations worthy of the name of civilized co-operate with more or
less efficiency in providing and endowing such establishments. They
are placed in suitable localities, and communicate with each other,
and with certain head-quarters, by means of the electric telegraph.
One of these head-quarters is the Meteorological Office, at No. 116
Victoria Street, Westminster, S.W., which daily receives the results of
the observations taken at about fifty stations on the British Islands
and the Continent. The chief observations are made simultaneously—at
8 A.M.—and telegraphed in cypher to London, where they usually
arrive before 10 A.M. As they come in they are marked down in their
proper places upon a large chart, and when this chart is sufficiently
completed, a condensed or abstract copy is made containing as much
information as may be included in the small newspaper charts. This is
copied mechanically on a reduced scale on a slab on which the outline
chart has been already engraved. This engraving completed, casts are
made in fusible metal with the black lines in relief, for printing with
ordinary type, and the casts are set up with the ordinary newspaper
types, and printed with the letterpress matter.
The engravings overleaf are taken from two of the newspaper weather
charts for the dates of October 5th and 6th. They are enlarged and
printed more clearly than the originals, with an explanation of signs
at foot of the charts.
It will be observed that, in the chart for October 5th, an isobar of
29.2 runs up in a N.E. direction from between the Orkney and Shetland
islands, crosses the North Sea, strikes the coast of Norway near
Bergen, and then proceeds onwards towards Throndhjem. An isobar of 29.5
crosses Scotland, following very nearly the line of the Grampians,
enters the North Sea about Aberdeen, and crosses to Christiansund; then
runs up the Skager Rack and Christiania Fjord towards Christiania.
Another isobar of 29.8 crosses Ireland through Connaught to Dublin,
onward across England by Liverpool and the Humber, over the North Sea,
and through Sleswig to the Baltic. These three are nearly parallel;
but now we find another isobar—that of 30.2—taking quite a different
course, by starting from the Bay of Biscay about Nantes; running on
towards Paris and Strasbourg, and then bending sharp round, as though
frightened by the Germans, and retreating to the Gulf of Lyons by an
opposite course to that on which it started. On the following day all
has changed; the northern isobars are running down south-eastwards
instead of north-east, and are remarkably parallel. In the left-hand
upper corner of this chart is a note that “_our west, north, and
eastern coasts were warned yesterday_.” Why was this? It was mainly
because the barometric _gradient_ or incline was so steep. On the 5th
there was one inch of difference between the Orkneys and the Bay of
Biscay, or between Bergen and Paris, while the barometer was still
falling in Norway and at the same moment rising in Ireland and France.
On the following day these movements culminated in a gradient of
1.4—nearly one and a half inches—between Cornwall and the ancient
capital of Norway.
[Illustration: WEATHER CHART, OCTOBER 5, 1875.]
[Illustration: WEATHER CHART, OCTOBER 6, 1875.]