666. _What is wind?_
Wind is air _in motion_. (_See_ 234.)
667. _What are the velocities of winds?_
A _breeze_ travels ten feet in a second; a _light gale_, sixteen feet
in a second; a _stiff gale_, twenty-four feet in a second; a _violent
squall_, thirty-five feet in a second; _storm wind_, from forty-three
to fifty-four in a second; _hurricane_ of the temperate zone, sixty
feet in a second; _hurricane_ of the torrid zone, one hundred and
twenty to three hundred feet in a second. When wind flies at one mile
an hour, it is scarcely perceptible. When its velocity is one hundred
miles an hour, it tears up trees, and devastates its track.
668. _What are trade winds?_
Trade winds are vast currents of air, which _sweep round the globe_
over a belt of some 12,000 miles in width.
[Verse: "They shall be as the morning cloud, and as the early dew
that passeth away, as the chaff that is driven with the whirlwind out
of the floor, and as the smoke out of the chimney."--HOSEA XIII.]
669. _What is the cause of trade winds?_
The air over the tropical regions becomes heated and ascends; it then
diverges in two high currents, one towards the north, and the other
towards the south pole, where, being cooled, it again descends, and
returns towards the equator to replace the air as it ascends therefrom.
There is, therefore, a constant revolution of vast currents of air
between the tropics and the poles, producing _north and south winds_.
670. _Why do the trade winds blow from east to west, though, in their
origin, their direction is from north to south and from south to north?_
Because, as the north and south winds blow towards the equator, they
are affected by the revolution of the earth from _west to east_. As
the two winds from the poles approach the equator, they are gradually
diverted from their northerly and southerly course, to an easterly
direction, by the revolution of the earth.
671. _Why is there a prevalence of calms at the equator?_
Because, as the north and the south winds move towards the equator,
they drive before them volumes of atmosphere, which, meeting in
opposite directions, resist and counterpoise each other, and abide in a
state of stillness between the north and south-easterly winds, one on
the north and the other on the south of the equator.
672. _What are monsoons?_
Monsoons are _periodical winds_ which blow at a given period of the
year from one quarter of the compass, and in another period of the year
from the opposite quarter of the compass.
673. _What is the cause of monsoons?_
Monsoons are caused by changes in the position of the sun. When the
sun is in the southern hemisphere, it produces a _north-east wind_,
and when it is in the northern hemisphere, a _north-west wind_. The
north-east monsoon blows from November to March, and the south-west
monsoon from the end of April to the middle of October. The region of
monsoons lies a little to the north of the northern border of the trade
wind, and they blow with the greatest force, and with most regularity,
between the eastern coast of Africa and Hindustan.
[Verse: "He shall blow upon them and they shall wither, and the
whirlwind shall take them away as stubble."--ISAIAH XL.]
674. _What determines the character of winds?_
The character of winds is influenced by the condition of _the surfaces
over which they blow_. Winds blowing over dry and arid plains and
deserts are _dry and hot_. Winds blowing across snow-capped mountains
and regions of ice are _cold_. Winds that cross oceans are _wet_; and
those that cross extensive continents are _dry_.
675. _What winds are most prevalent in England?_
In England out of a _thousand days_, north winds prevail in 82;
north-east, 111; east, 99; south-east, 81; south, 111; south-west, 225;
west, 171; north-west, 120.
676. _What is the cause of storms?_
Storms result from violent commotions of the atmosphere, and are
chiefly the result of extreme _changes of temperature_.
The _magnetic_ state of the earth, and the _electrical_ state of the
atmosphere, also materially influence the phenomena of storms.
By some persons the theory is entertained that storms result from
various winds _rushing into a centre_ in which the atmosphere has
become extremely condensed. According to this theory, a storm is a
mighty whirlwind.
A most violent hurricane occurred in 1780, which destroyed Lord
Rodney's fleet, and a vast number of merchant ships. It is said to
have killed 9,000 persons in Martinique alone, and 6,000 in St.
Lucia. The town of St. Pierre in Martinique was totally destroyed;
and only fourteen houses in the town of Kingston, in St. Vincent,
were left uninjured.
677. _Why do the most violent storms occur in and near the tropics?_
Because there the temperature is very high, and the cold currents
of air rushing towards the equator from the poles, causes great
_atmospheric disturbance_.
678. _What are whirlwinds?_
Whirlwinds are produced by violent and contrary currents meeting and
striking upon each other, producing _a circular motion_. They generally
occur after long calms, attended by much heat.
Whirlwinds occurring at sea, or over the surface of water, sometimes
put the water in motion, and as the wind rises upwards it lifts with it
a whirling mass of water, producing a _water spout_.
[Verse: "Out of the south cometh the whirlwind; and cold out of the
north."--JOB XXXVII.]
[Illustration: Fig. 19.--A WATER SPOUT.]
679. _Why does the chimney smoke when the fire is first lighted?_
Because the air in the chimney is of the same temperature as that in
the room, and therefore _will not ascend_.
680. _Why does the smoking (into the room) cease, after the fire has
been lighted a little while?_
Because the air in the chimney, being warmed by the fire beneath,
becomes lighter and ascends rapidly.
681. _Why does a long chimney create a greater draught than a short
one?_
Because the short chimney contains _less air_ than the long one; there
is, consequently, less difference of weight between the warm air of
the short chimney and the external air; it therefore has not so great
an _ascensive power._
[Verse: "And, lo, the smoke of the country went up as the smoke of a
furnace."--GEN. XIX.]
682. _Why does smoke issue in folds and curls?_
Because it is _pressed upon_ by the _cold air_ which always _rushes
towards a rarer atmosphere_. It thus illustrates the development of
_storms_.
683. _Why do some chimneys smoke when the doors and windows are closed?_
Because the draught of air is not sufficient to supply the wants of the
fire, and enable it to create an _upward current_.
684. _What is the best method of conveying air to fires?_
Tubes built in the walls, communicating with the outer air, and
terminating _underneath the grates_.
685. _Why is this the best method of ventilation?_
Because doors and windows may then be made air-tight, and _draughts
across rooms be prevented_.
686. _Why do chimneys that stand under elevated objects, such as hills,
trees, and high buildings, smoke?_
Because the wind, striking against the elevated object, _flies back_,
and a part of it _rushes downward_.
687. _Why do sooty chimneys smoke?_
Because the accumulation of the soot _diminishes the size of the
flue_, and lessens the ascensive power of the draught, by reducing the
quantity of _warm air_. It also obstructs the motion of the air, by the
_roughness of its surface_.
688. _Why do chimneys smoke in damp and gusty weather?_
Because the ascending air is _suddenly chilled_ by gusts of damp and
cold air, and driven down the chimney.
[Verse: "Remember that thou magnify his work, which men behold.
Every man may see it; man may behold it afar off."--JOB XXXVI.]
689. _Why does smoke ascend in a straight line in mild and fine
weather?_
Because the air is still, and being dry and warm it _does not chill the
smoke_, nor drive it out of its course.
690. _Why do the wings of wind-mills turn round?_
Because the wind, striking _at an angle_ upon the wings, forces them
aside; and as there are four wings all upon the same angle, and fixed
upon the same centre, the _oblique pressure_ of the wind causes the
centre to rotate.
There is a world of _miniature phenomena_ which has never been
fully recognised, in which we may see the mightier works of nature
pleasingly and truthfully illustrated.
When the wind blows into the corner of a street, and whirling
around, catches straw, dust, and feathers in its arms, and then
wheels away, flinging the troubled atoms in all directions,--it
is a miniature of the mightier _whirlwind_, which wrecks ships,
uproots trees, and levels houses with the earth.
When a cloud of dust, on a hot summer's day, rises and flies
along the thirsty road, making the passenger close his eyelids,
and dusting the leaves of wayside vegetation,--it is a miniature
of the terrible _simoom_, which blows from the desert sands,
scattering death and devastation in its track.
When steam issues from the tea-urn, and becomes condensed in
minute drops upon the window-pane,--the miniature is of the
_earth's heat_, evaporating the waters, and the cold air of night
condensing the vapours into _dew_.
When grass and corn bend before the wind, and are beaten down by
its force; when the pond forgets its calm, and rises in troubled
waves, casting the flotilla of natural boats that move upon its
surface, in rude disorder upon its windward shore,--the little
storm is but a miniature of those great _hurricanes_ which wrecked
a fleet in the Black Sea, and levelled the encampments of a mighty
army.
When the snow that has gathered upon the house-top, warming
beneath the smiles of the sun, slips from its bed, and drops in
accumulated heaps from the roof,--it is a miniature of those
terrible _avalanches_ which in the Pyrenees bury villages in their
icy pall, and doom man and beast to death.
When the rivulet hurries on its course, and meeting with
obstructions, leaps over them in mimic wrath, overturning some
little raft upon which, perchance, a weary fly has alighted,--it
is a miniature of those _rapids_ on whose banks the hippopotamus
and the alligator yet live; and where, though rarely, man may be
seen directing his raft over the troubled current, amid the rush
of _debris_ from forests unexplored.
And when, in a basin of the rivulet, two opposing currents meet,
and form a little vortex into which insect life and vegetable
fragments coming within the sphere of its influence are drawn,--it
is a miniature of the roaring _whirlpool_, or the wilder
_maelstrom_ of the Norwegian seas.
Nature rehearses all her parts in mild whispers; and for every
picture that she paints, she places a first study upon the canvas.
Man need not go into the heart of her terrors to understand their
laws. Many an unknown Humboldt, sitting by the river's side,
may rejoice in the "aspects of nature," and share the bliss of
knowledge with the great philosopher.
[Verse: "Can any understand the spreadings of the clouds, or the
noise of his tabernacle?"--JOB XXXVI.]