CHAPTER XXXI.
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.]
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.]