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<h2>CHAPTER XXXI.</h2> 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? [Pg 152] Trade winds are vast currents of air, which sweep round the globe over a belt of some 12,000 miles in width. <hr class="bible-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. <hr class="bible-verse" /> 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, [Pg 153] 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. <hr class="bible-verse" /> "He shall blow upon them and they shall wither, and the whirlwind shall take them away as stubble."—Isaiah xl. <hr class="bible-verse" /> 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? [Pg 154] 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. <hr class="bible-verse" /> "Out of the south cometh the whirlwind; and cold out of the north."—Job xxxvii. <hr class="bible-verse" /> <div class="figcenter"> <ANTIMG src="images/i-154.jpg" width-obs="400" height-obs="368" alt="" /> <div class="caption">Fig. 19.—A WATER SPOUT.</div> </div> 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 [Pg 155] air of the short chimney and the external air; it therefore has not so great an ascensive power. <hr class="bible-verse" /> "And, lo, the smoke of the country went up as the smoke of a furnace."—Gen. xix. <hr class="bible-verse" /> 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. [Pg 156] <hr class="bible-verse" /> "Remember that thou magnify his work, which men behold. Every man may see it; man may behold it afar off."—Job xxxvi. <hr class="bible-verse" /> 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 [Pg 157] 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. <hr class="bible-verse" /> "Can any understand the spreadings of the clouds, or the noise of his tabernacle?"—Job xxxvi. <hr class="bible-verse" /> <hr class="chap" /> <div style="break-after:column;"></div>

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