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WIND. 



WIND. 



926 



who had passed some time at the Canaries, to which the trade- winds 

 extend in summer, seems to have conceived a just idea of their extent. 

 On his first voyage, after leaving the Canaries, his crew were greatly 

 alarmed at finding that the wind always blew from the north-east and 

 east, and feared that they would be prevented by it from returning to 

 their native country. Columbus did not participate in their fears ; and 

 on his return from the newly-discovered islands his track was north of 

 thu trade-winds, in the region of the variable winds. After the time 

 of Columbus, European navigation extended rapidly in the Atlantic 

 and Indian oceans, and the trade-winds became generally known. It 

 does not, however, appear that any attempt to explain this phenomenon 

 was made before the time of Galileo, who, in adopting the astronomical 

 system of Copernicus and the revolution of the earth round its axis, 

 thought he found some confirmation of this opinion in the trade-winds, 

 which, as he conjectured, owed their origin to the revolution of the 

 earth and to the circumstance that the atmosphere, though it partici- 

 pated in that motion, could not follow with equal speed the motion of 

 the dense parts of our planet, and that a motion in the air was thus 

 produced which was contrary to that of the earth round its axis, or 

 from east to west. The strongest fact in favour of this hypothesis 

 was the circumstance that the trade-winds occur only in the lower lati- 

 tudes, where the surface of the earth, in its revolution round its axis, 

 > make a large circle in twenty-four hours, and consequently 

 must move with a greater degree of rapidity than in the higher 

 latitudes. Galileo's theory was relinquished about the end of the 17th 

 century, in favour of a not less fallacious one proposed by Dr. Edmund 

 Hilluy, who, however, had collected extensive information respecting 

 these winds, and had indeed discovered several facts which were incom- 

 patible with the opinion of Galileo. The two most decisive were, that 

 there are no trade-winds near the equator, where the diurnal motion of 

 the earth is greatest, and that the trade-winds change their position 

 according to the seasons, which could not take place if they were only 

 the effect of the rotation of the earth. Galileo, however, appears to 

 iirvd a true, though obscure, perception that the rotatory motion 

 "i the earth must be somehow concerned in the production of the 

 . though he singularly omitted to consider the {operation 

 of the sun's heat. 



The trade-winds are met with on both sides of the equator. The 



mean boundary-line of the region in which they blow, and beyond 



which variable winds prevail, is about 28 Lit. in the eastern parts of 



the ocean, but in the western parts this line is generally two or three 



degrees farther north and south. To the north of the equator they 



blow iu the eastern parts of the ocean from the north-east, seldom 



from the eastward of east-north-east, or from the northward of north- 



north east. In proceeding farther west they become more easterly, 



ami often they blow from due east, and sometimes from the south of 



east, but generally they are one or two points north of east. To the 



south of the equator the trade-winds in the eastern parts of the 



'ulow from south-east, and usually between south-east and east, 



but they also decline more to due cast in reaching the western portion 



of tha ocean. They do not occur in the vicinity of the continents, 



but are chiefly separated from them by a tract of sea in which either 



periodical or variable winds prevail. The trade-winds therefore are 



xperienced when we are well out from the land in the open sea. 



The wind blows with less force and steadiness in the eastern than in 



'stern portion of the ocean. It is also stronger and more con- 



iti the hemisphere where the sun is net, than in that which is 



il to its perpendicular rays; in the latter, however, it is more 



!y than in the former. The region in which the trade-winds 



is distinguished by an almost continual serenity and fair weather. 



Though the trade-winds of the northern and southern hemisphere 



blow in an oblique direction towards one another, they do not meet in 



general, but are ilividtd by a tract of sea in which calms frequently 



!. and also variable light winds, mostly from the west, are met 



with. This region of the calms is distinguished by a thick foggy air, 



and frequent rains of short duration attended by thunder and light- 



i egion of calms which separates the north-east trade-winds 



; ' i ,-mith eastern, and which usually occupies a width of four or 



livi- *'> free* of latitude, is not always found at the same part of the 



ocean, but advancrs fartln-r north when the sun has a northern decli- 



i, .ind farther south when it is in the southern hemisphere. 



;<.] The same is observed respecting the winds themselves. 



;!i th<i mean boundary-line of the trade-winds is 28 of lat. in 



parts of the ocean, it extends two, three, and even four 



< farther north when the sun approaches the northern tropic, 



and about the same distance farther southward when the sun is near 



its greatest southern declination. It sometimes happens that a north- 



eastern wind occurs as far north as 40 in the Atlantic, along the 



j:n coasts of Spain and Portugal, but as this is seldom the case, 



it is supposed that such a wind cannot be considered as a trade- wind, but 



only ad one of the variable winds which prevail to the north and south 



of tliu trade-winds. There are also a few instances on record in which 



the north-east and south-east trade-winds have not been found 



separated by a region of calms, but in which a vessel, with the inter- 



vention of a calm of short duration, has passed from one trade-wind 



into the other. 



The true explanation of thee "magnificent phenomena," as they 

 hare justly been called, according to Dove and Herschel, was first 



delivered in the 'Philosophical Transactions' for 1735, by George 

 Hadley. It has often been erroneously attributed to Dr. Edmund 

 Halley, in part, doubtless, from the similarity of his name, and partly 

 on account of a theory of these winds, now long proved to bo fallacious, 

 having been actually proposed by him, as already noticed. The true 

 theory was also divined by the sagacity of John Daltou, who, not 

 knowing what Hadley had done, printed it in his ' Meteorological Obser- 

 vations and Essays' in the year 1793, but discovered Hadley's priority 

 in time to acknowledge it in his preface. 



The astronomical point of view from which the objects of meteoro- 

 logy have always been viewed by Sir John F. W. Herschel, has given 

 to his treatment of atmospheric phenomena, a breadth and perspicuity 

 scarcely to be found in the writings of other meteorologists. His 

 view of the theory of the trade-winds presents a remarkable example 

 of this, and, as a whole, is, we think, unequalled. In what follows we 

 give an epitome of it, condensed from two works his ' Outlines of 

 Astronomy,' and ' Treatise on Meteorology,' with some adaptations of 

 our own. 



It is a matter of observed fact that the sun is constantly vertical 

 over some one or other part of the earth between the tropics, and that 

 the whole of the zone or belt so included between the tropics, and 

 equally divided by the equator, is, in consequence of the great altitude 

 attained by the sun in its diurnal course, maintained at a much higher 

 temperature than those regions to the north and south which lie nearer 

 the poles. The heat thus acquired by the earth's surface, agreeably to 

 the principles explained above, is communicated to the incumbent air, 

 and becomes the universal primary cause of the phenomena of the 

 winds, iu conjunction with the earth's rotation. The colder and 

 heavier air glides in on both sides, along the surface, from -the regions 

 beyond the tropics ; while the displaced air, thus raised above its due 

 level, and unsustained by any lateral pressure, flows over, as it were, 

 and forms an upper current in the contrary direction, or towards the 

 poles ; which being cooled in its course, and also pressed down by the 

 mass of the atmosphere above to supply the deficiency in the extra- 

 tropical regions, thus keeps up a continual circulation. " That this is 

 a real cause (vera causa) is placed in complete evidence by the general 

 fact that the atmospheric pressure at the surface of the sea diminishes 

 regularly from either tropic to the equator, where the barometer stands 

 habitually about 0'2 in. lower than in the temperate zones." 



The principle whose action was made known and applied by Hadley 

 now comes into play. The equatorial portion of the earth's surface 

 has the greatest velocity of rotation, and all other parts less in the 

 proportion of the radii of the circles of latitude to which they corre- 

 spond. But as the air, when relatively and apparently at rest on any 

 part of the earth's surface, is only so, because in reality it participates 

 in the motion of rotation proper to that' part, as indicated at the 

 beginning of this article, it follows that when a mass of air near the 

 poles is transferred to the region near the equator by any iiu|>n!,-i! 

 urfjiti;,' it directly towards that circle, in every point of its progress 

 towards its new situation it must be found deficient iu rotatory 

 velocity, and therefore unable to keep up with the speed of the new 

 surface over which it is brought. Hence, the currents of air which set 

 in towards the equator from the north and south, must, as they glide 

 along the surface, at the same time lag, or hang back, and drag upon 

 it in the direction opposite to the earth's rotation, that is, from east to 

 west. Thus these currents, which but for the rotation would be 

 simply northerly and southerly winds, acquire, from this cause, a 

 direction towards the west, and assume the character of 

 permanent north-easterly and south-easterly winds. 



It follows from this, then, that as the winds from both sides 

 approach the equator, their easterly tendency must diminish ; a fact 

 which though inevitably resulting from the principle maintained by 

 Hadley, was reserved for the late Captain Basil Hall to reason out, 

 and, though thus for the first time, in a very distinct manner, iu his 

 ' Fragments of Voyages and Travels.' The lengths of the diurnal 

 circles increase very slowly in the immediate vicinity of the equator, 

 and for several degrees on either side of it hardly change at all. Thus 

 the friction of the surface has more time to act in accelerating the 

 velocity of the air, bringing it towards a state of retatirc rest, ;im] 

 diminishing thereby the relative set of the currents from east to west, 

 which, on the other hand, is feebly, and, at length, not at all, rein- 

 forced by the cause which originally produced it. Arrived, then, at 

 the equator, thy trade winds must be expected to lose their easterly 

 character altogether. And not only this, but the northern and 

 southern currents here meeting and opposing, will mutually destroy 

 each other, leaving only such preponderancy as may be due to a 

 difference of local causes acting in the two hemispheres which in 

 some regions around the equator may lie one way, in some another 

 as will presently be seen. " The result, then," says Sir John Herschel, 

 " must be the production of two great tropical belts, in the northern 

 of which a constant north-easterly, and in the southern a south- 

 easterly, wind must prevail, while the winds in the equatorial belt, 

 which separates the two former, should be comparatively calm, and free 

 from any steady prevalence of easterly character. All these con- 

 sequences are agreeable to observed fact, and the system of aerial 

 currents above described constitutes in reality what is understood by 

 the regular trade irinds," 



On the subject of the region of calms, intervening between the trade- 



