691. _What is a barometer?_
A barometer is an instrument which _indicates the pressure of the
atmosphere_, and which takes its name from two Greek words signifying
_measurer of weight_.
692. _Why does a barometer indicate the pressure of the atmosphere?_
Because it consists of a tube containing _quicksilver_, closed at one
end and open at the other, so that the pressure of the air upon the
open end balances the weight of the column of mercury (quicksilver),
and when the pressure of the air upon the open surface of the mercury
increases or decreases, the mercury _rises or falls_ in response
thereto.
693. _Why is a barometer called also a "weather-glass"?_
Because changes in the weather are generally preceded by _alterations
in the atmospheric pressure_. But we cannot perceive those changes
as they gradually occur; the alteration in the height of the column
of mercury, therefore, enables us to know that atmospheric changes
are taking place, and, by observation, we are enabled to determine
certain rules by which _the state of the weather may be foretold_ with
considerable probability.
694. _Why are barometers constructed with circular dials, and an index
to denote changes?_
Because that is a convenient mechanical arrangement, by which the
alterations of the relative pressures of the air and the mercury are
_more clearly denoted than by an inspection of the mercury itself._
[Verse: "Fair weather cometh out of the north: with God is terrible
majesty."--JOB XXXVII.]
[Illustration: Fig. 20.--BAROMETER.]
[Illustration: Fig. 21.--TUBE OF BAROMETER, WHEEL, AND PULLEY.]
695. _Why does the hand of the weather dial change its position when
the column of mercury rises or falls?_
Because a weight, which _floats upon the open surface of the mercury_,
is attached to a string, having a nearly equal weight at the other
extremity; the string is laid over a revolving pivot to which the
hand is fixed, and _the friction of the string turns the hand, as the
mercury rises or falls_.
[Verse: "Thou visitest the earth, and waterest it: thou greatly
enrichest it with the river of God, which is full of water: thou
preparest them corn, when thou hast so provided for it."--PSALM LXV.]
696. _Why does tapping the face of the barometer sometimes cause the
hand to move?_
Because the weight on the surface of the mercury frequently _leans
against the sides of the tube_, and does not move freely. And, also,
the mercury clings to the sides of the tube by _capillary attraction_;
therefore, tapping on the face of the barometer _sets the weight free_,
and overcomes the attraction which _impedes the rise or fall of the
mercury_.
Fig. 21 illustrates the mechanism at the back of the barometer. A
is a glass tube; between A and E there exists a _vacuum_, caused
by the weight of the mercury pressing downwards. This space
being a vacuum, makes the barometrical column more sensitive, as
there is no internal force to resist or modify the effects of
the external pressure. E represents the height of the column of
mercury; C the open end of the tube; F the weight resting on the
surface of the mercury; P the pivot over which the string passes,
and upon which the hand turns; W the weight which forms the pulley
with the weight F.
697. _Which is the heavier, dry or vaporised air?_
Dry air is _heavier_ than air impregnated with vapours.
698. _Why is dry air heavier than moist air?_
Because of the _extreme tenuity of watery vapours_, the density of
which is _less than that of atmospheric air_.
699. _Why does the fall of the barometer denote the approach of rain?_
Because it shows that as the air _cannot support the full weight of the
column of mercury_, the atmosphere must be thin with watery vapours.
The fall of the mercury in the long arm of the tube would cause
the weight F to be pressed upwards. This would release the string
to which the weight W is attached; it would, therefore, fall, and
turn the hand down to Rain or Much Rain.
700. _Why does the rise of the barometer denote the approach of fine
weather?_
Because the external air becoming dense, and free from highly elastic
vapours, presses with increased force upon the mercury upon which the
weight F floats; that weight, therefore, sinks in the short tube as the
mercury rises in the long one, and in sinking turns the hand to Change,
Fair, &c.
[Verse: "He caused an east wind to blow in the heaven; and by his
power he brought in the south wind."--PSALM LXXVIII.]
701. _Why does the barometer enable us to calculate the height of
mountains?_
Because, as the barometer is carried up a mountain, _there is a less
depth of atmosphere above to press upon the mercury_; it therefore
falls, and by comparing various observations, it has been found
practicable to _calculate the height of mountains by the fall of the
mercury in a barometer_.
702. _To what extent of variation is the weight of the atmosphere
liable?_
It may vary as much as _a pound and a half to the square inch_ at the
level of the sea.
703. _When does the barometer stand highest?_
When there is a _duration of frost_, or when _north-easterly winds_
prevail.
704. _Why does the barometer stand highest at these times?_
Because the atmosphere is exceedingly _dry and dense_, and fully
balances the _weight of the column of mercury_.
705. _When does the barometer stand lowest?_
When _a thaw follows a long frost_; or when _south-west winds_ prevail.
706. _Why does the barometer stand lowest at those times?_
Because _much moisture exists in the air_, by which it is rendered less
dense and heavy.
707. _What effect has heat upon the barometer?_
It causes the mercury to fall, _by evaporating moisture into the air_.
708. _What effect has cold upon the barometer?_
It causes the mercury to rise, by _checking evaporation_, and
_increasing the density of the air_.
[Verse: "For so the Lord said unto me, I will take my rest, and I
will consider in my dwelling place like a clear heat upon herbs, and
like a cloud of dew in the heat of harvest."--ISAIAH XVIII.]
In noting barometrical indications, more attention should be paid
to the _tendency_ of the mercury at the time of the observation,
than to the _actual state of the column_, whether it stands _high_
or _low_. The following rules of barometric reading are given as
generally accurate, but liable to exceptions:--
_Fair weather_ indicated by the _rise_ of the mercury.
_Foul_ weather by the _fall_ of the mercury.
_Thunder_, indicated by the _fall_ of the mercury in _sultry
weather_.
_Cold_, indicated by the _rise_ of the mercury in spring, autumn,
and winter.
_Heat_, by the _fall_ of the mercury in summer and autumn.
_Frost_, indicated by the _rise_ of the mercury in winter.
_Thaw_, by the _fall_ of the mercury during a frost.
_Continued bad weather_, when the _fall_ of the mercury has been
_gradual_ through several fine days.
_Continued fine weather_, when the _rise_ of the mercury has been
_gradual_ through several foul days.
_Bad weather of short duration_, when it sets in quickly.
_Fine weather of short duration_, when it sets in quickly.
_Changeable weather_, when an _extreme_ change has _suddenly_ set
in.
_Wind_, indicated by a rapid _rise_ or _fall unattended by a
change of temperature_.
The mercury _rising_, and the air becoming _cooler_, promises
_fine weather_; but the mercury _rising_, and the air becoming
_warmer_, the weather will _be changeable_.
If the top of the column of mercury appears _convex_, or curved
upwards, it is an additional proof that the mercury is _rising_.
Expect _fine_ weather.
If the top of the column is _concave_, or curved downwards, it is
an additional proof that the mercury is _falling_. Expect _bad_
weather.