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WIND. 



WIND. 



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from north through north-east, east, south-east, to south, and so on 

 round in the same direction through west to north : that it often 

 makes a complete circuit in that direction, or more than one in 

 succession (occupying sometimes many days in so doing); but that 

 it rarely veers, and very rarely or never makes a complete circuit, 

 in the contrary direction." Accordiug to Sir John Herschel, Pro- 

 fesswDove was the first to show that thia tendency is a direct con- 

 sequence of the rotatory motion of the earth ; it lias therefore been 

 denominated " Dove's law of rotation of the wind," to which we shall 

 shortly return. 



The heated equatorial air, while it rises and flows over towards the 

 poles, carries with it the rotatory velocity due to its equatorial situa- 

 tion into a higher latitude, where the earth's surface has less motion. 

 Hence, as it travels northward or southward, it will gain continually 

 more and more on the surface of the earth in its diurnal motion, and 

 assume constantly more and more a westerly relative direction ; and 

 when at length it returns to the surface, in its circulation, which it 

 must do more or less in all the interval between the tropics and the 

 poles, it will act on it by its friction as a powerful south-west wind in 

 the northern hemisphere, and a north-west in the southern, and restore 

 to it the impulse taken up from it at the equator. "We have here 

 the origin of the south-west and westerly gales so prevalent in our 

 latitudes, and of the almost universal westerly winds in the North 

 Atlantic, which are, in fact, nothing else than a part of the general 

 i of the re-action of the trades, and of the process by which the 

 equilibrium of the earth's motion is maintained under their action." 

 The only winds of a regular character which remain to be noticed are 

 the land and sea breezes which occur diurnally on the coasts ami in 

 the islands of the tropical regions, and the periodical winds which are 

 observed to prevail in some parts of Europe. The first are probably 

 caused by the inequality of the sun's action on the land and water ; 

 and both, by the tendency of the atmosphere to preserve a state of 

 nearly uniform density. During the day the land acquires a tempera- 

 ture higher than that of the neighbouring ocean : the atmosphere 

 above it consequently becomes rarefied, and from about 9 A.M. the air 

 from the sea Hows towards the land, to occupy the partial vacuum 

 there produced. In proportion as the heat of the land goes on in- 

 creasing, the force of the sea-breeze also increases, and this continues 

 till 2 or 3 P.M. After that time the temperature over the land di- 

 minishes more rapidly than over the sea, as the heat more readily 

 escapes by radiation from the land than from the water, and about 

 sunset the breeze from the sea ceases. During the night, the land 

 continuing to cool, the air over the sea becomes comparatively warmer 

 and more rarefied, and a breeze from the land takes place : this wind 

 augments in force till near sunrise, when the temperature of the earth 

 begins to increase, and about 9 A.M. the wind blows from the sea as at 

 first. These land-breezes diverge hi every direction towards the coasts 

 of the tropical islands from the high lands in their interior. Mr. Red- 

 field modifies the hypothesis above stated by assuming that when the 

 stratum of air lying on the surface of land which ascends towards the 

 interior of a country becomes rarefied by the sun's heat, it is forced by 

 an excess of pressure at its lowest part to move up the slope ; and 

 during the night the stratum of air on this inclining surface acquiring 

 greater density, its gravity causes it to descend towards the sea. 

 (' Amer. Journal of Science,' vol. xxxiii., No. 1.) 



The Etesian winds (so called from irnattu, "annual ") is a designation 

 formerly given only to those which every summer blow during six 

 weeks over the countries bordering the Mediterranean ; but it has since 

 been applied to other periodical winds, as those which blow on the 

 coast of Holland. They commence in the Levant about the middle of 

 July, rising at 9 A.M., and continuing during the day-time only : the 

 direction of the current of air is from north-east to south-west ; and it 

 is probably caused by the rarefaction of the atmosphere nearly under 

 the tropic of Cancer, in consequence of the heat of the sun at that 

 season. Pliny states that, in Spain and Asia, the etesian winds blow 

 from the east; and he adds that they also take place in winter, when 

 they are called Ornithian winds : these arc, however, said to be more 

 gentle than the others, and to continue during nine days only. (Smyth's 

 ' Mediterranean,' p. 1:70.) 



It may be observed, in addition to what has been said respecting the 

 trade and other regular winds, that those which prevail in the tempe- 

 rate zones are probably the results of currents proceeding about the 

 earth from the tropical regions. Professor Dove suggests (in a paper 

 published in Poggendorff's ' Annalen,' of which a translation appeared 

 in the ' Philosophical Magazine' for September, 1837) that when the 

 Run is on the meridian of any place, as London, situated beyond those 

 regions, the currents of heated air which proceed from the point verti- 

 cally under him must arrive at that place from the south earlier than 

 at any other place eastward or westward of it on the same parallel of 

 latitude. But in proportion as the sun becomes successively vertical 

 at different points westward of the meridian of London, the currents 

 of air, in describing great circles of the sphere, arrive later, and in a 

 direction from the westward of south ; and when, during the summer, 

 he is vertically over a point about 60 degrees west of London (that is, 

 in the evening), they arrive nearly from the west. At midnight, when 

 the sun is on the meridian under the horizon, the current of air passing 

 orer the north pole is felt as a north wind ; and after this time the 

 current* coming from points having less than 180 degrees of longitude 



ABT8 JLSD SCI. DIV. VOL. VIII. 



eastward are felt as easterly winds, which become due east in the 

 morning when the sun is about 60 degrees eastward of the meridian. 

 In this order the movements take place daily, except when the currents 

 are disturbed by accidental circumstances or by the influence of currents 

 which proceed from the pole to the equator in order to supply the place 

 of the heated air which ascends from the surface of the earth between 

 the tropics.- 



The sun is not the sole cause of the currents which are observed in 

 the atmosphere, for they often arise from the condensation of the 

 aqueous vapours which are constantly rising from the surfaces of seas 

 and rivers. Such vapours, being lighter than air, ascend in the atmo- 

 sphere, carrying with them a quantity of heat, which escapes on 

 airiving in a region where there is less heat than at the surface of the 

 earth.; and the vapour being then reduced to a state of water, a partial 

 vacuum is produced, into which the neighbouring air rushes. The 

 heat is at the same time conveyed by the wind with the drops of water, 

 and thus the region in which the rain is falling is sometimes warmer 

 than those which surround it. Rain-winds are produced by the air 

 which descends to the ground with the globules of water ; the particles 

 of air being disengaged from the globules on the latter striking the 

 ground, are then driven off, with considerable force, in every direction 

 from the place where tho rain is falling. 



All mountain districts are subject to sudden and violent gusts of 

 wind from the interruptions which the ridges of high land create to 

 the general currents of the air ; but that which is called the Helm- 

 wind, at Crossfell, in Cumberland, is one of the most remarkable of 

 these phenomena. It occurs at uncertain times between the end of 

 September and the month of May, and occasionally, though rarely, in 

 summer. It is stated that, when not a breath of wind is stirring, and 

 scarcely a cloud is to be seen, there is suddenly formed a line of clouds, 

 called the " Helm," extending nearly north and south along the top 

 ridge of the mountains ; and nearly parallel to this, another line of 

 clouds, called the " Bar," forms itself : the first of these lines of clouds 

 is well defined at its western and the other at its eastern edge ; and the 

 lines unite together at their northern and soiithern extremities so as to 

 contain between them an elliptical space whose length, in the north 

 and south direction, varies from 8 to 30 miles, and its breadth, in an 

 east and west direction, from half a mile to 4 or 5 miles ; the highest 

 point of the ridge of mountains being about the middle of the first line 

 of clouds. In a few minutes after the formation of the Helm a violent 

 wind begins, within the space between the clouds, to blow from some 

 eastern point of the compass, but generally from due east to due west : 

 its force is such as to break trees, disperse the grain in stacks, and 

 overturn a cart with its horse. It continues frequently for nine 

 successive days, and its noise is said to resemble that of the sea 

 in a violent storm ; but it is seldom accompanied by rain. No satis- 

 factory hypothesis has yet been offered to account for the phenome- 

 non ; but that which seems most probable is, that the air from the 

 coast of Northumberland, beiug cooled as it rises to the summit of 

 the mountain, and there condensed, descends from thence with great 

 force, by its gravity, into the district at the foot of the western 

 escarpment. (Rev. J. Watson, in the ' Reports of the British Asso- 

 ciation,' vol. vii.) 



If we contemplate the influence of the winds in the economy of 

 human life, we shall find them highly beneficial. Though storms are 

 often destructive to life and property, both at sea and land, yet they 

 contribute greatly to preserve the health of animated beings by the 

 dissipation of noxious exhalations : the winds impel the clouds from 

 place to place, and thus diffuse over great tracts of country the rains 

 which contribute so much to fertilise the ground. Wind is extensively 

 employed in giving motion to machinery ; and, till the recent appli- 

 cation of steam, it was the only power by which ships were transported 

 across the ocean between different regions of the earth. 



Whirlwind is a violent movement of the atmosphere in a circular or 

 spiral direction apparently about a mathematical axis, the latter having 

 at the same time a progressive motion, rectilinear or curvilinear, on the 

 surface of the land or sea. 



The tornados of North America and the coasts of Africa, as well as 

 the typhoons in the sea of China, have long been known as violent 

 tempests in which the wind has a revolving motion of this kind, but 

 these terms are commonly applied to such storms as are of short dura- 

 tion and comparatively of small extent, the diameters of the vortices 

 varying from a few hundred yards to one or two miles. It is now 

 ascertained, by such evidence as leaves scarcely any doubt of the fact, 

 that in all or most of the great storms which agitate the atmosphere 

 the wind has a rotatory movement, and that the diameter of the 

 circle within which the gyration is performed is sometimes equal 

 in extent to several hundred miles : in great whirlwinds the axis 

 appears to be either vertical or nearly so, but in those of small extent 

 its inclination is often inconsiderable, and it is sometimes parallel to 

 the horizon. 



As early as the middle of the 17th century the revolving motion of 

 the wind, during the great hurricanes which take place in the West 

 Indies, appears to have been noticed ; and in a description of them, 

 which was given at that time in the ' Philosophical Transactions,' it is 

 stated that, after a cessation of the trade-winds, the storm begins from 

 tho north ; that the wind afterwards goes round to the north-west and 

 then to the south, the storm subsiding when the wind comes to the 



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