1. The direction of the wind--the North-east wind tending to raise it
most--the South-west to lower it the most, and wind from points of the
compass between them proportionally as they are nearer one or the other
extreme point.
N.E. and S.W. may therefore be called the wind's extreme bearings
(rather than _poles_?)
The range, or difference of height, of the mercury, due to change of
direction _only_, from one of these bearings to the other (supposing
strength or force, and moisture, to remain the same) amounts in these
latitudes to about half an inch (shown by the barometer as read off).
2. The amount, taken by itself, of vapour, moisture, wet, rain, hail, or
snow, in the wind or current of air (direction and strength remaining
the same) seems to cause a change amounting, in an extreme case, to
about half an inch.
3. The strength or force alone of wind from any quarter (moisture and
direction being unchanged) is preceded, or foretold, by a fall or rise,
according as the strength will be greater or less, ranging, in an
extreme case, to more than two inches.
Hence, supposing the three causes to act together--in extreme cases--the
mercury might range from about 31 (30·9) inches to near 27 inches, which
has happened _occasionally_.
Generally, however, as the three act much less strongly, and are less in
accord--ordinary varieties of weather (the wind varying as usual--with
more or less cloudiness, or rain) occur much more frequently than
extreme changes.
Another general rule requires attention; which is, that the wind usually
veers, shifts, or goes round, _with the sun_, (right-handed in northern
places, left-handed in the southern parts of the world,) and that, when
it does not do so, or backs, more wind or bad weather may be expected
instead of improvement.
In a barometer the mercury begins to rise occasionally before the
conclusion of gale, sometimes even at its commencement, as the
equilibrium of the atmosphere begins to be restored. Although the
mercury falls lowest before high winds, it frequently sinks considerably
before heavy rain only. The barometer falls, but _not always_, on the
approach of thunder and lightning, or when the atmosphere is highly
charged with electricity.[16] Before and during the earlier part of
serene and settled weather, the mercury commonly stands high, and is
stationary.[17]
Instances of fine weather, with a low glass, occur exceptionally, but
they are always preludes to a duration of wind or rain, _if not both_.
After very warm and calm weather, rain or a storm is likely to occur;
or at any time when the atmosphere has been _heated_ much above the
usual temperature of the season.
Allowance should invariably be made for the previous state of the
instrument during some days as well as hours, because its indications
may be affected by remote causes, or by changes close at hand. Some of
these changes may occur at a greater or less distance, influencing
neighbouring regions, but not visible to each observer whose barometer,
nevertheless, feels their effect.
There may be heavy rains or violent winds beyond the horizon, out of
view of an observer, by which his instruments may be affected
considerably, though no particular change of weather occurs in his
immediate locality.
It may be repeated, that the longer a change of wind or weather is
foretold by the barometer before it takes place, the longer the presaged
weather will last; and, conversely, the shorter the warning, the less
time whatever causes the warning; whether wind or a fall of rain, hail,
or snow, will continue.
Sometimes severe weather from an equatorial[18] direction, not lasting
long, may cause no great fall of the barometer, because followed by a
_duration_ of wind from polar regions:--and at times it may fall
considerably with polar winds and fine weather, apparently against these
rules, because a _continuance_ of equatorial wind is about to follow. By
such changes as these one may be misled, and calamity may be the
consequence if not thus forewarned.
The veering of the winds is a direct consequence of the earth's
rotation, while currents of air from the polar regions are alternating
or contending with others from the equator.
The polar currents are cold, dry, and heavy. Those from the equatorial
parts of the world are warm, moist, and comparatively light. Their
alternate or combined action, with the agencies of solar heat and
electricity, cause the varieties of weather that we experience.
It is not intended to discourage attention to what is usually called
"weather wisdom." On the contrary, every prudent person will combine
observation of the elements with such indications as he may obtain from
instruments.
The more carefully and accurately these two sources of foreknowledge are
compared and combined, the more satisfactory will the results prove.
A few of the more marked signs of weather--useful alike to seaman,
farmer, and gardener, are the following:
Whether clear or cloudy, a rosy sky at sunset presages fine weather; a
red sky in the morning, bad weather, or much wind (if not rain):--a grey
sky in the morning fine weather; a high dawn, wind; a low dawn; fair
weather.[19]
Soft-looking or delicate clouds foretell fine weather, with moderate or
light breezes;--hard edged oily-looking clouds, wind. A dark, gloomy,
blue sky is windy;--but a light, bright blue sky indicates fine weather.
Generally, the _softer_ clouds look, the less wind (but perhaps more
rain) may be expected;--and the harder, more "greasy," rolled, tufted,
or ragged, the stronger the coming wind will prove. Also, a bright
yellow sky at sunset presages wind; a pale yellow, wet:--and thus by the
prevalence of red, yellow, or grey tints, the coming weather may be
foretold very nearly: indeed, if aided by instruments, almost
exactly.[20]
Small inky-looking clouds foretell rain; a light scud, driving across
heavy clouds, wind and rain; but if alone, wind only.
High upper clouds crossing the sun, moon, or stars, in a direction
different from that of the lower clouds, or wind then blowing, foretell
a change of wind (beyond tropical latitudes).[21]
After fine clear weather the first signs (in the sky) of change are
usually small, curled, streaked, or spotty clouds, followed by an
overcasting of vapour, that grows into cloudiness. This murky
appearance, more or less oily or watery, as wind or rain will prevail,
is a sure sign. The higher and more distant the clouds seem to be, the
more gradual, but extensive, the coming change of weather will prove.
Generally speaking, natural, quiet, delicate tints or colours, with soft
undefined forms of clouds, foretell fine weather: but gaudy or unusual
hues, with hard, definite outlines, presage rain and wind.
Misty clouds forming, or hanging on heights, show wind and rain
coming--if they remain, or descend. If they rise, or disperse, the
weather will improve, or become fine.
When sea birds fly out early, and far to seaward, moderate wind and fair
weather may be expected. When they hang about the land, or over it,
sometimes flying inland, expect a strong wind, with stormy weather. As
many creatures, besides birds, are affected by the approach of rain or
wind, such indications should not be slighted by the observer of
weather.
There are other signs of a coming change in the weather known less
generally than may be desirable; and, therefore worth notice here.
When birds of long flight, such as swallows and others, hang about home
and fly low--rain or wind may be expected. Also when animals seek
sheltered places, instead of spreading over their usual range: when pigs
carry straw to their sties; and when smoke from chimneys does not ascend
readily, (straight upwards during a calm,) an unfavourable change may be
looked for.
Dew is an indication of fine weather. So is fog. Neither of these two
formations occurs under an overcast sky, or when there is much wind. One
sees the fog occasionally rolled away, as it were, by wind--but not
formed while it is blowing.
Remarkable clearness of atmosphere, near the horizon; distant objects,
such as hills, unusually visible; or raised (by refraction); and what is
called "a good _hearing_ day" may be mentioned among signs of wet, if
not wind, to be expected.[22]
More than usual twinkling of the stars; indistinctness or apparent
multiplication of the moon's horns; haloes; "wind-dogs;" and the
rainbow; are more or less significant of increasing wind, if not
approaching rain.[23]
Near land, in sheltered harbours, in valleys, or over low ground, there
is usually a marked diminution of wind during part of the night--and a
dispersion of clouds. At such times an eye on an overlooking height may
see an extended body of vapour below; which the cooling of night has
rendered visible.
* * *
Although the preceding remarks are probably sufficient for their
principal purpose--these pages may fall into the hands of persons
familiar with the subject, to whom the following observations may be
addressed, as some of the _reasons_ for what has been so briefly, if not
too positively outlined.
As the mercurial column rises with increase of pressure by the
atmosphere, and descends when the pressure diminishes, it indicates a
greater or less accumulation of air, which, like other fluid, such as
water (when heaped above its average level or reduced below it, from
whatever cause),--will have a tendency to fall or rise till the general
equilibrium is restored. An observer may be under the centre of such
accumulation or depression, he may be more or less distant from it,
though within the influence of whatever horizontal movement of air may
be caused by such temporary increase or diminution of pressure. Hence
the barometer shows, and generally foretells, changes of wind; but as
complications always occur, and as changes are of greater or less
extent, affecting or extending through a wider or more limited area,
accompanied by hygrometric and electrical alterations, it is extremely
difficult at times to say beforehand what particular change of weather
is to be expected, and at what interval of time; although after the
event the correspondence of barometric changes with those of the weather
can be readily traced. However, notwithstanding occasional perplexity,
the general character of weather during the next few days may be
predicted by an observer who understands the nature and use of this
instrument and the thermometer, and has watched them in the few
immediately preceding days.
In endeavouring to foretell weather, the general peculiarity should
always be remembered, that the barometric column usually stands higher
with easterly than it does with westerly winds; and with winds from the
polar regions higher than with those from the direction of the equator.
Hence the highest columns are observed with north-east winds in northern
latitudes, and with south-east in the southern hemisphere.
In middle latitudes there is an average difference (unreduced or
observed height as read off) of about half an inch, other things being
similar, between the heights of the mercury with North-easterly, and
with South-westerly winds.
The steadier the column, or the more gradually it moves, the more
settled in character will the weather be, and conversely: because it
shows a quiet settled state of the atmosphere; or, if otherwise, the
reverse. In the tropics, when the barometric column moves contrary to
its usual daily motion, inferior weather may be expected (temporarily),
because the usual air currents are disturbed.
This regular movement, whether tidal, or otherwise connected with the
sun's influence--sensible in tropical latitudes, but more or less masked
elsewhere--amounts to nearly two-tenths of an inch near the equator, the
highest being at about nine, and the lowest near three o'clock.
* * *
Some movements of the atmosphere may be illustrated by reference to the
motion of water drawn off from a reservoir by a small opening below; or
by similar _upward_ draught through a syphon; or by a gradual pouring in
at the upper surface.
From a slight motion at the commencement, affecting only that portion of
the fluid adjoining either of those places of diminution or repletion,
gradually all the water becomes influenced and acquires more or less
rapid movement. But suppose a long reservoir or canal of fluid which has
two such points of exhaustion or two of such repletion (as imagined
above), and that one of either is near each end of the vessel. If each
aperture be opened at the same moment, equal effects will be caused in
each half of the fluid towards either end of the vessel, but in the
middle there must be a neutral point at which the water falls, yet has
no horizontal motion. The converse takes place in raising the level. And
in the case of fluid drawn off or diminished in weight at one end while
increased by repletion at the other, the _whole_ body of water will move
similarly to that in the former vessel, but unequally. Hence it is
evident, that before horizontal motion occurs, an augmentation or a
diminution of pressure must take place somewhere more or less remote;
and so it is with the lighter fluid atmosphere,--which has centres,
lines, or areas of depression towards which currents flow.
Such considerations show in some degree why the barometric changes
usually precede, but sometimes only accompany, changes of weather: and,
though very rarely, occur without any sensible alteration in the wind
current of the atmosphere. An observer may be near a central point
towards which the surrounding fluid tends,--or from which it diverges.
He may be at the very farthest limit of the portion of fluid that is so
influenced. He may be at an intermediate point--or he may be between
bodies of atmosphere tending towards opposite directions.
It has been said, that "a whirlwind which sets an extended portion of
the atmosphere into a state of rapid revolution diminishes the pressure
of the atmosphere over that portion of the earth's surface, and most of
all at the centre of the whirl. The depth of the compressing column of
air will, at the centre, be least, and its weight will be diminished in
proportion to the violence of the wind." Yet this has been controverted
with respect to the _general_ effect of air in horizontal motion, and
the depth of the column in question.
Certainly there are two kinds of whirlwinds--one caused by rarefaction,
tending to lighten vertical pressure under the vortex, though not,
perhaps, under all the current drawn towards it; and the other, a
consequence of opposing winds, which occasion huge eddies or whirlwinds
of compression.
Some whirlwinds are accompanied by rushes from the upper atmosphere,
from the colder regions, which, mingling with warmer and moister air
near the sea, cause dense clouds. About their centre it sometimes
happens that the barometer falls as much as two or three inches, showing
a diminution of atmospheric pressure by nearly a tenth part; when it
should be expected, from physical considerations alone, that very dense
clouds would be formed.[24]
* * *
The column of mercury falls about one tenth of an inch for each of the
first few hundred feet above the sea level, but varying when it becomes
much more elevated.[25] Due allowance, therefore, should be made in
observing, when on high land.
The tides are affected by atmospheric pressure, so much that a rise of
one inch in the barometer will have a corresponding fall in the tides of
nine to sixteen inches, or about one foot for each inch.[26]
* * *
Vessels sometimes enter docks, or even harbours, where they have
scarcely a foot of water more than their draught; and as docking, as
well as launching large ships, requires a close calculation of height of
water, the state of the barometer becomes of additional importance on
such occasions.
* * * * *
To render these pages rather more useful at sea, in _any_ part of the
world, a few words about squalls and hurricanes are here offered to the
young seaman.
Generally, squalls are preceded, or accompanied, or followed by clouds;
but the very dangerous "white squall" (of the West Indies and other
regions), is indicated only by a rushing sound, and by white wave
crests.
"Descending squalls" come slanting downwards, off high land,[27] or from
upper regions of atmosphere. They are dangerous, being sometimes
violently strong.
A squall cloud that can be seen through or under is not likely to bring,
or be accompanied by, so much wind as a dark continued cloud extending
beyond the horizon. How the comparative hardness or softness of clouds
foretells more or less wind or rain, was stated in pages 13 and 14.
The expressions "hardening up," "softening," or looking "greasy," are
familiar to seamen: and such very sure indications are the appearances
so designated, that they can hardly be mistaken.
The rapid or slow rise of a squall cloud--its more or less disturbed
look--that is, whether its body is much agitated, and changing form
continually, with broken clouds, or scud, flying about--or whether the
mass of cloud is shapeless and nearly quiet, though floating onwards
across the sky--foretells more or less wind accordingly.
An officer of a watch, with a good eye for clouds and signs of changing
weather, may save his men a great deal of unnecessary exposure, as well
as work, besides economising sails, spars, and rigging.
In some of the "saws" about wind and weather, there is so much truth
that, though trite and simple, their insertion here can do no harm.
Adverting to the barometer:--
When rise begins, after low,
Squalls expect and clear blow.
Or:--_First_ rise, after very low,
Indicates a stronger blow.
Also:--Long foretold, long last:
Short notice, soon past.
To which may be added:--In squalls--
When rain comes before wind,
Halyards, sheets, and braces mind.
And:--When wind comes before rain,
Soon you may make sail again.
* * *
Also, generally speaking:--
When the glass falls low,
Prepare for a blow;
When it rises high,
Let all your kites fly.[28]
* * *
To these short expressions--well known, in practice, to the experienced;
a very concise but sure rule may be added, for avoiding the central or
strongest part of a hurricane, cyclone, typhoon, tornado, or circling
storm.
With your face towards the wind, in North latitude, the centre of the
circling, or rotatory storm, will be square to your right. In South
latitude, square to your left.
The apparent veering of the wind, and the approach or retreat of the
dangerous central circle, depend on your position in the curvilinear
whirl or sweep.
Draw a circle;--mark the direction of the rotation or circulation, by an
arrow with the head towards the left hand (against the movement of a
watch's hands) in North latitude; but towards the right (or with the
hands of a watch) if in South latitude. The direction of the wind, and
the bearing of the centre, show your position in the meteor, for such it
is, though perhaps hundreds of miles in diameter; and the veering of the
wind, or the contrary, and its change in strength, will show how the
meteor is moving bodily--over an extensive region, revolving
horizontally--or inclined at a certain angle to the horizontal plane.
If the observer be stationary, in North latitude, and the centre pass on
his polar side, he will experience a change of wind from Southward by
the West towards North; but if it pass between him and the Equator, the
change will be from Southward by the East towards North; but otherwise
in South latitude, as his place in circles sketched will show more
clearly than words. The roughest sketch or diagram, indicating the
various directions of wind, and the course of the meteor's centre, will
show more plainly than descriptions--which must necessarily vary with
each case, and are tedious.
Cyclonology, or really meteorology, is simple enough in these great
characteristic effects; but their causes must be the philosopher's
study, rather than that of the young practical seaman.
Were it not for this reflection, one might endeavour to show how all the
great Easterly trade winds--the no less important anti-trades,[29] or
nearly constant Westerly winds,--and their complicated eddying offsets,
are all (on greater or smaller scales) breadths, or zones of atmosphere,
alternating, or circulating, or crossing (superposed or
laterally)--between which, at distant intervals, occur those strong
eddies, or storms, called hurricanes--typhoons--tornadoes--or cyclones.
The great easterly and westerly movements--so clearly shown by
philosophers to be the consequences of cold polar currents of air--warm
equatorial currents--and diurnal rotation of the earth;[30] are grand
ruling phenomena of meteorology--to which storms, and all local changes,
occurring but occasionally, are subordinate and exceptional. Further
investigations into electrical and chemical peculiarities will probably
throw additional light, perhaps the strongest, on meteorological
science.
* * * * *
In the previous observations, general reference has been made to
mercurial barometers of the ordinary kind; but, excepting the
construction of the instruments themselves, those observations apply to
all barometers, wheel--aneroid--or metallic--and likewise, of course, to
the sympiesometer, which is a modified barometer. But as these four
last-mentioned instruments are scarcely so familiar as the simplest form
of barometer, it may be useful to add a few words about each of them.
* * *
The WHEEL barometer has a syphon tube, partly filled with mercury, on
which, at the short or open end of the tube, a float moves, to which a
line is attached that moves a wheel, carrying an index.[31]
* * *
ANEROID barometers, if often compared with good mercurial columns, are
similar in their indications, and valuable; but it must be remembered
that they are not independent instruments; that they are set originally
by a barometer,[32] require adjustment occasionally, and may deteriorate
in time, though slowly.
The aneroid is quick in showing the variation of atmospheric pressure,
and to the navigator who knows the difficulty, at times, of using
barometers, this instrument is a great boon, for it can be placed
anywhere, quite out of harm's way, and is not affected by the ship's
motion, although faithfully giving indication of increased or diminished
pressure of air.[33] In ascending or descending elevations, the hand of
the aneroid may be seen to move (like the hand of a watch), showing the
height above the level of the sea, or the difference of level between
places of comparison.[34]
The principle on which it is constructed may be explained in a few
words, without going into a scientific or too minute detail of its
various parts. The weight of a column of air, which in a common
barometer acts on the mercury, in the aneroid presses on a small
circular metal box, from which nearly all air is extracted; and to this
box is connected, by nice mechanical arrangement, the hand visible over
the face of the instrument. When the atmospheric pressure is lessened on
the vacuum box, a spring acting on levers, turns the hand to the left,
and when the pressure increases, the spring is affected differently, the
hand being turned to the right. It acts in any position, but as it
_often varies several hundredths with such a change_, it should be held
uniformly, while read off.
The known expansion and contraction of metals under varying
temperatures, caused doubts as to the accuracy of the aneroid under such
changes; but they were partly removed by introducing into the vacuum box
a small portion of gas, as a compensation for the effects of heat or
cold. The gas in the box, changing it bulk on a change of temperature,
was intended to compensate for the effect on the metals of which the
aneroid is made. Besides which, a further and more, reliable
compensation has lately been effected by a combination of brass and
steel bars.[35]
METALLIC barometers (in _outer_ shape and size like aneroids) have not
yet been tested adequately in very moist, hot, or cold air for a
sufficient time. They, as well as sympiesometers, are likewise dependent
or secondary instruments, and liable to deterioration. For limited
employment, when sufficiently compared, they may be very useful,
especially in a few cases of electrical changes not foretold or shown by
mercury.
The SYMPIESOMETER is considered to be more sensitive than the marine
barometer, falling sooner, and rising earlier: but this is partly in
consequence of the marine barometer tube being contracted, to prevent
oscillation or "pumping." In the sympiesometer a gas is used, which
presses on the confined surface of the liquid with an uniform pressure
at an equal state of temperature. The liquid is raised or depressed by
an increase or diminution in the density of the atmosphere, and change
of temperature is allowed for, by the sliding scale of the instrument
being always set to agree with the height of the mercury in the attached
thermometer, bringing the _pointer_ on the sliding scale of the
sympiesometer to the same degree on the inverted scale (over which it
slides) as is indicated by the thermometer. The height of the fluid, as
then shown by the sliding scale, indicates the pressure of the
atmosphere.
As the instrument is delicate, great care should be taken, in carrying
or handling, to keep the top always upwards, and to exclude casual rays
of the sun, or a fire, or lamp.
Oil sympiesometers seem to be affected more than mercurial, or others,
and much more than the barometer, by lightning or electricity. That
they, and the hermetically sealed "STORM GLASSES," are influenced by
causes besides pressure and temperature, appears now to be certain.
* * *
The daily movement of the barometer may be noted (in a form or table of
double entry) at the time of each observation, by a dot at the place
corresponding to its altitude, and the time of observing; which dot
should be connected with the previous one by a line. The resulting free
curve (or zig-zag) will show at a glance what have been the movements
during the days immediately previous, by which, and not merely by the
last observation, a judgment may be formed of the weather to be
expected.
Such a diagram may be filled up by _uncorrected_ observations, its
object being to serve as a weather guide for immediate use, rather than
for future investigation. If closely kept up, it will prove to be of
utility, and will in some degree reward the trouble of keeping a regular
record. For purely scientific objects much more nicety and detail are
required.
* * * * *
HESITATION is sometimes felt by young seamen, at first using the vernier
of a barometer, for want of some such familiar explanation as the
following:--
The general principle of this moveable dividing scale is, that the total
number of the smallest spaces or subdivisions of the vernier are made
equal, taken altogether, to one less than that number of the smallest
spaces in an equal length of the fixed scale.
For example: ten spaces on the vernier being made equal to nine on the
scale, each vernier space is one tenth less than a scale space; and if
the first line or division of the vernier agree exactly with any line of
the scale, the next line of the vernier must be one tenth of a tenth (or
one hundredth) of an inch from agreement with the next _scale_ division;
the following vernier line must be two hundredths out, and so on:
therefore, the number of such differences (from the next tenth on the
scale) at which a vernier line agrees with a scale line, when set, is
the number of hundredths to be added to the said tenth; (in a common
barometer, reading only to hundredths of an inch).
The vernier of a barometer reading to thousandths of an inch, is on a
similar principle, though differently divided. In this application of
it, generally, twenty-five vernier spaces equal twenty-four of the scale
spaces, which are each half a tenth, or five hundredths of an inch;
therefore, the difference between one of the vernier and one of the
scale is two-tenths of a hundredth, or two thousandths of an inch
[25)·050(·002].
This is the usual graduation of scientific barometers; but for ordinary
purposes, as weather-glasses, a division, or reading, to the hundredth
of an inch is sufficient.
When set properly, the vernier straight edge, the top of the mercury,
and the observer's eye, should be on the same level; the edge (or
pointer) just _touching_[36] the middle and uppermost point of the
column.
Great care should be taken to look thus square, or at right angles to
the scale.
Light, or something white, at the _back_ of the tube, assists in
accurately setting the vernier, and may be shifted about to aid in
reading off.
* * *
THE ANEROID has been recommended, in these pages, as a weather-glass;
but it may increase its usefulness to append a table for measuring
heights (approximately) by this, or any barometer, which can be compared
with another, or itself, at a higher or lower station.
If the measure of a height rather greater than the aneroid will commonly
show, be required, it may be _re-set_ thus--When at the upper station
(_within its range_), and having noted the reading carefully, touch the
screw behind so as to bring back the hand a few inches (if the
instrument will admit), then read off and start again. _Reverse the
operation when descending._ This may add some inches of measure
_approximately_.
* * * * *
In the following Table, the difference between the number of feet
opposite the height of a barometer, at one station, and that at another
station, is their approximate difference of height.