Familiar as the practical use of weather-glasses is, at sea as well as
on land, only those who have long watched their indications, and
compared them carefully, are really able to conclude more than that the
rising glass[3] USUALLY foretells less wind or rain, a falling barometer
more rain or wind, or both; a high one fine weather, and a low, the
contrary. But useful as these general conclusions are _in most cases_,
they are _sometimes_ erroneous, and then remarks may be rather hastily
made, tending to discourage the inexperienced.
By attention to the following observations (the results of many years'
practice and many persons' experience) any one not accustomed to use a
barometer may do so without difficulty.
The barometer shows whether the air[4] is getting lighter or heavier, or
is remaining in the same state. The quicksilver falls as the air becomes
lighter, rises as it becomes heavier, and remains at rest in the glass
tube while the air is unchanged in weight. Air presses on everything
within about forty miles of the world's surface, like a _much_ lighter
ocean, at the bottom of which we live--not feeling its weight, because
our bodies are full of air, but feeling its currents, the winds. Towards
any place from which the air has been drawn by suction,[5] air presses
with a force or weight of nearly fifteen pounds on a square inch of
surface. Such a pressure holds the limpet to the rock when, by
contracting itself, the fish has made a place without air[6] under its
shell. Another familiar instance is that of the fly which walks on the
ceiling with feet that stick. The barometer tube, emptied of air, and
filled with pure mercury, is turned down into a cup or cistern
containing the same fluid, which, feeling the weight of air, is so
pressed by it as to balance a column of about thirty inches (more or
less) in the tube, where no air presses on the top of the column.
If a long pipe, closed at one end only, were emptied of air, filled with
water, the open end kept in water and the pipe held upright, the water
would rise in it more than thirty feet. In this way water barometers
have been made. A proof of this effect is shown by any well with a
sucking pump--up which, as is commonly known, the water will rise nearly
thirty feet, by what is called suction, which is, in fact, the pressure
of air towards an empty place.
The words on scales of barometers should not be so much regarded for
weather indications, as the rising or falling of the mercury; for, if it
stand at _Changeable_, and then rise towards _Fair_, it presages a
change of wind or weather, though not so great, as if the mercury had
risen higher; and, on the contrary, if the mercury stand above _fair_
and then fall, it presages a change, though not to so great a degree as
if it had stood lower: besides which, the direction, and force of wind,
are not in any way noticed. It is not from the point at which the
mercury may stand that we are alone to form a judgment of the state of
the weather, but from its _rising_ or _falling_; and from the movements
of immediately preceding days as well as hours, keeping in mind effects
of change of _direction_, and dryness, or moisture, as well as
alteration of force or strength of wind.
* * *
In this part of the world, towards the higher latitudes, the quicksilver
ranges, or rises and falls, nearly three inches--namely, between about
thirty inches and nine-tenths (30·9), and less than twenty-eight inches
(28·0) on extraordinary occasions; but the usual range is from about
thirty inches and a half (30·5), to about twenty-nine inches. Near the
Line, or in equatorial places, the range is but a few tenths, except in
storms, when it sometimes falls to twenty-seven inches.
The sliding-scale (vernier) divides the tenths into ten parts each, or
hundredths of an inch. The number of divisions on the vernier exceeds
that in an equal space of the fixed scale by one.[7]
* * *
By a thermometer the _weight_ of air is _not_ shown. No air is within
the tube. None can get in. But the bulb of the tube is full of mercury,
which contracts by cold, and swells by heat--according to which effect
the thread of metal in the small tube is drawn down or pushed up so many
degrees: and thus shows the temperature.[8]
If a thermometer have a piece of linen tied round the bulb, wetted
enough to keep it damp by a thread or wick dipping into a cup of water,
it will show less heat than a dry one, in proportion to the dryness of
the air, and quickness of drying.[9] In very damp weather, with or
_before_ rain, fog, or dew, two such thermometers will be nearly alike.
For ascertaining the dryness or moisture of air, the readiest, and
surest method is the comparison of two thermometers; one dry, the other
_just_ moistened, and _kept so_. Cooled by evaporation as much as the
state of the air admits--the moist (or wet) bulb thermometer shows a
temperature nearly equal to that of the other one, when the atmosphere
is extremely damp, or moist; but lower at other times,--in proportion to
the dryness of air, and consequent evaporation,--as far as twelve or
fifteen degrees in this climate; twenty or even more elsewhere. From
four to eight degrees of difference is usual in England; and about seven
is considered healthy for living rooms.
The thermometer fixed to a barometer intended to be used only as a
weather-glass shows the temperature of air about it nearly--but does not
show the temperature of mercury within exactly. It does so however near
enough for ordinary practical purposes--provided that no sun, nor fire,
nor lamp heat is allowed to act on the instrument partially.
The mercury in the cistern and tube being affected by cold or heat,
makes it advisable to consider this when endeavouring to foretell coming
weather by the length of the column.
* * *
Briefly, the barometer shows weight or pressure of the air; the
thermometer--heat and cold, or temperature; and the wet thermometer,
compared with a dry one, the degree of moisture or dampness.[10]
It should be remembered that the state of the air _foretells_, rather
than shows present weather (an invaluable fact too often overlooked);
that the longer the time between the signs and the change foretold by
them, the longer such altered weather will last; and, on the contrary,
the less the time between a warning and a change, the shorter will be
the continuance of such foretold weather.
* * *
If the barometer has been about its ordinary height, say near thirty
inches, at the sea level,[11] and is steady, or rising--while the
thermometer falls, and dampness becomes less--North-westerly, Northerly,
or North-easterly wind--or less wind--may be expected.
On the contrary--if a fall takes place, with a rising thermometer and
increased dampness, wind and rain may be expected from the
South-eastward, Southward, or South-westward.
A fall, with a low thermometer, foretells snow.
Exceptions to these rules occur when a North-easterly wind, with wet
(rain or snow) is impending, before which the barometer often rises (on
account of the _direction_ of the coming wind alone), and deceives
persons who, from that sign only, expect fair weather.
When the barometer is rather below its ordinary height, say, below
twenty-nine inches and nine-tenths (at the sea level _only_), a rise
foretells less wind, or a change in its direction towards the
Northward,--or less wet; but when the mercury[12] has been low, say near
29 inches--the first rising usually precedes, and foretells, strong
wind--(at times heavy squalls)--from the North-westward--Northward--or
North-eastward--_after_ which violence a rising glass foretells
improving weather--if the thermometer falls. But, if the warmth
continue, probably the wind will back (shift against the sun's course),
and more Southerly, or South-westerly wind will follow. "Backing" is a
bad sign, with any wind.
The most dangerous shifts of wind, and the heaviest Northerly[13] gales
happen after the mercury first rises from a very low point.
Indications of approaching changes of weather, and the direction and
force of winds are shown less by the height of mercury in the tube, than
by its falling or rising. Nevertheless, a height of about 30 inches (at
the level of the sea) with a continuance of it, is indicative of fine
weather and moderate winds.
The barometer is said to be _falling_ when the mercury in the tube is
sinking, at which time its upper surface is _sometimes_ concave or
hollow. The barometer is _rising_ when the mercurial column is
lengthening; its upper surface being then, as in _general_, convex or
rounded.[14]
A rapid rise of the barometer indicates unsettled weather. A slow rise,
or steadiness, with dryness, shows fair weather.
A considerable and rapid fall is a sign of stormy weather and rain.
Alternate rising and sinking show very unsettled weather.
The greatest depressions of the barometer are with gales from the S.E.,
Southward, or S.W.; the greatest elevations, with winds from the N.W.,
Northward, or N.E., or when calm.
Although the barometer generally falls with a Southerly, and rises with
a Northerly wind, the contrary _sometimes_ occurs; in which cases the
Southerly wind is dry and the weather fine; or the Northerly wind is wet
and violent.[15]
When the barometer sinks considerably, high wind, rain, or snow will
follow: the wind will be from the Northward if the thermometer is low
(for the season)--from the Southward if the thermometer is high.
Sudden falls of the barometer, with a Westerly wind, are sometimes
followed by violent storms from N.W. or North.
If a gale sets in from the Eastward or S.E., and the wind veers by the
South, the barometer will continue falling until the wind becomes S.W.,
when a comparative lull may occur; after which the gale will be renewed;
and the shifting of the wind towards the N.W. will be indicated by a
fall of the thermometer as well as a rise of the barometer.