TRADE WINDS.] 



METEOKOLOGY. 



1135 



tion it is rendered specifically heavier, falls to the surface 

 of the earth, and floats along towards the equator, thus 

 generating two principal currents one north, the other 

 south. Whilst yet in the frigid and the temperate zones, 

 these primary currents encounter so many interferences, 

 that their primary or fundamental direction is masked or 

 veiled : but still proceeding north and south, the persis- 

 tent directive tendency of the trade winds is at length 

 developed. But the currents no longer flow directly 

 north and south. By the operation of a cause which will 

 presently be rendered evident, the directive tendency of 

 either current has acquired a certain impulse towards the 

 west ; or, in other words, both currents come more or 

 less from the east. But at length they blow almost 

 from due east, and finally cease altogether ; so that the 

 equator, and a certain space north and south of the equa- 

 tor, are comprehended within what is termed the region 

 of calms. The north trade wind meeting the south trade 

 wind, they are mutually destructive of each other. Two 

 conflicting aerial forces by mutual impact come to rest, 

 just as two billiard balls, each rolling gently from an op- 

 posite point, become quiescent. 



No part of the earth's surface, however, is subject to 

 such capricious and such violent tempests as the so-called 

 region of calms. This is a result which theory would 

 lead us to suspect. The cause has now to be explained 

 why the trade winds do not blow directly north and south. 

 If our globe were at rest, or if its only motion were mo- 

 tion iu its orbit, such would be the result ; but it re- 

 volves on its axis also from west to east ; and this circum- 

 stance fully explains the deviation from northness and 

 soiithness of the trade winds. If the lower or returning 

 aerial current which constitutes the trade winds ceased to 

 exist altogether, then our globe's diurnal rotation would 

 generate a current in the apparent direction of east to 

 west. We say apparent direction, because, in point of 

 fact, we may regard the atmosphere under these circum- 

 stances as being tranquil or passive, and our globe re- 

 volving in the midst of it, in the direction of west to 

 east. Inasmuch as the force representing this apparent 

 east wind, and the force representing the north and south 

 atmospheric currents, flowing towards the equator from 

 either pole, are simultaneously operating, there occurs a 

 resultant, which is the trade wind. The following dia- 

 gram (Fig. 3C) roughly illustrates the points which have 



been described. Towards the extreme north and ex- 

 treme south, the great aerial currents, ultimately destined 

 to become the trade winds, are represented fluctuating 

 and variable ; gradually, however, they acquire a northern 

 and a southern directive tendency respectively ; lastly, 

 they come from the point of almost due east, and then 

 MO altogether. 



A consideration of the influences which determine 

 iple winds, leads to a facile explanation of many aerial 

 currents of less extent. The heating influence referable 

 to the geographical position of the tropics is not the only 

 influence of this kind A peculiar condition of the earth's 

 surface, taken conjointly with a favourable condition of 

 the sun's rays, may bring about similar results. In this 

 manner the Mediterranean etesian winds, or aerial cur- 

 rents from the north, may bo accounted for. Looking at 

 the geographical condition of localities south of the Medi- 



terranean, we find the Sahara, or Great Desert, a region 

 whose surface is strewed with masses of pebbles and sand, 

 and almost totally devoid of vegetation. Such a surface 

 must necessarily become elevated by the solar rays to a 

 high temperature, an atmospheric column must ascend, 

 travel to the north, then fall, and at length return from 

 the north to the Sahara, from whence it came. 



LAND AND SEA BREEZES. Many countries near the 

 sea are subjected to winds of diurnal periodicity, known 

 as land and sea-breezes. About eight or nine A. M. , an 

 aerial current begins to flow from the sea towards the 

 laud, and persists until about three P.M., when a current 

 in the reverse direction, or from the land towards the 

 sea, takes its place, and continues throughout the night 

 until sunrise next morning, when it ceases, and a calm 

 ensues until the completion of a period of twenty-four 

 hours from the occurrence of the preceding land-breeze. 

 These currents, in reverse directions, can be easily ac- 

 counted for when we consider the heating agency of the 

 sun. Necessarily, land becomes hotter than water under 

 an equal power of calorific rays ; whence it follows, that 

 the surface of the ground, becoming heated after sunrise, 

 determines the ascent of an atmospheric current verti- 

 cally ; thence, proceeding oceanward, the same current 

 returns from the sea to the land. No sooner does the 

 sun set, than this current is reversed. 



In the preceding explanations of the cause of winds as 

 being owing to inequalities of temperature, it will be ob- 

 served that reference has alone been made to the calorific 

 effects of the sun's rays. This is, in point of fact, the 

 only source of heat which has to be noticed in meteoric 

 considerations ; for, although the earth's own tempera- 

 ture gradually increases as we pierce downwards below 

 the surface, so imperfect are the conducting powers of 

 the materials of which the crust of our planet is com- 

 posed, that all consideration of them may be safely 

 omitted in accounting for the present phenomenon. 



PROCESS OF REVERSE ATMOSPHERIC CURRENTS. Al- 

 though the existence of atmospheric currents, proceeding 

 in a direction reverse to those we meet with on the 

 earth's surface, is forced upon the mind by inferential 

 reasoning, and the fact must be accepted, even though 

 no further evidence of it could be adduced, nevertheless 

 direct proofs are not wanting. The direction of upper 

 layers of clouds afford their testimony to the truth of 

 the opinion. Where the trade winds prevail, the higher 

 strata of clouds may be seen taking a direction opposite 

 to that of the wind itself ; and travellers, during their 

 ascent of high mountains, have frequently proved the 

 existence of a superior wind pursuing a course opposite 

 to the wind below. This has been especially obvious on 

 the Peak of Teneriffe, a mountain situated in the belt of 

 the trade winds. On the summit of this peak, it has been 

 always '.ound that a south-west wind prevails ; whereas 

 the trade wind, at the base of the same mountain, blows 

 from the north-east. 



A similar remark has been made by travellers who 

 have ascended Mona Kca, in Owyhee, the height of 

 which is 18,000 feet But, perhaps, the most striking 

 illustration is the following : The island of Barbadoes 

 lies eastward of St. Vincent, and between the two the 

 trade wind continually blows, and so forcibly, that it is 

 only with difficulty, and by making a long circuit, that a 

 ship can sail between the latter and the former. Never- 

 theless, on one occasion, during au eruption at St. 

 Vincent, dense clouds formed over Barbadoes, and large 

 quantities of ashes fell on the island. A similar result 

 was observed after an eruption of the volcano of 

 Cosoguina, on the shores of the Pacific, in Guatemala, in 

 January, 1835, some of the volcanic ashes falling in 

 Jamaica, more than 800 miles in a direct lino distant, and 

 directly opposed to the prevailing lower current. At the 

 same time, another portion of ashes was carried 

 westward, or in an opposite direction, falling on Her 

 Majesty's ship Conway, in the Pacific, more than 1,200 

 miles distant. 



The trade winds are, for the most part, only recognis- 

 able at sea; the solid material of land, developing local 

 aerial currents of their own. The extent of prevalence 



