817 



ATLANTIC OCEAN. 



ATLANTIC OCEAN. 



natural causes which produce the phenomenon of the perpetual or 

 trade wiuJj; for this we refer to the article TBADE-WISDS in the 

 division of ABTS and SOIESCES. Here it will be enough to describe 

 their limit* and peculiarities a they hare been recently determined. 



In tlio eastern part of the region of the trade-wind* them winds 

 Mow on the north side of the equator from north-east ; and on the 

 south from south-east, whence they have received respectively the 

 name* of the North-East and South-East trade-winds. More accu- 

 rately stated however, as has been determined by Lieutenant Maury, 

 the prevalent direction of the so-called North-East trade-winds is 

 about E.N.E. ; while that of the South-East winds is about K.K. l.\ 

 E., thug approaching the equator more directly than the North-East 

 winds. Hence the South-East winds are much more powerfully 

 affected by the calorific influence of the sun ; while the North-East 

 are much more influenced by the diurnal motion of the earth. 



The result of the investigations respecting the extent and limits 

 of the trade-winds is thus given by Lieutenant Maury in the 

 ' Appendix to the Washington Astronomical Observations,' 1851 : 



" The N.E. trade-winds occupy a belt or zone extending in length 

 from east to west across the Atlantic Ocean, having a variable breadth 

 of from 17 to 35 degrees of latitude. Its average mean breadth is 

 about 23 degrees ; and in its extreme range it extends from 3 S. to 

 35 N., according to the season of the year. This zone makes two 

 vibrations in a year. It reaches ita extreme northern decimation usually 

 in September. Then returning and following the mm it reaches its 

 southern extreme in March and April. Being stationary for two or 

 three months between 3 and 4" N., it commences to return north, 

 and in the months of August, September, and October, its other 

 stationary period, it is seldom or never found to the south of the 

 parallel of 9 N. The parallel of 9 N. may be taken as the mean 

 limit of the equatorial border of the zone of N.E. trade-winds. 



" The S.E. trade- winds occupy a similar zone in the South Atlantic, 

 with a like vibratory motion. The mean equatorial limit of this 

 cone, instead of being near the parallel of 9 S., to correspond with 

 the cone of the northern hemisphere, is in about 3 N. It is a 

 remarked phenomenon, discovered in the course of these investi- 

 gations, that the S.E. trade-winds blow with more force than do their 

 congeners of the northern hemisphere. They have force enough to 

 push the latter with their belt back towards the north, intruding 

 occasionally in the late summer and in the early fall months as far 

 as the parallel of 9 N. Whereas out of many thousands of records 

 examined, it does not appear that the belt of N.E. trade-winds is 

 ever found to cross the parallel of 3 S." 



The trade-winds are separated from one another by the region of 

 calms known as the Equatorial Calms. It has a mean average 

 breadth of about six degrees of latitude. This region is hot always 

 of the same extent, and does not occupy the same part of the ocean ; 

 and though it extends over the whole of it from the coasts of Africa 

 to those of America it is more felt on the eastern side. It sometimes 

 occupies not much more than two, and at other seasons up to ten 

 degrees of latitude. It is a remarkable circumstance that it does 

 not extend equally on both sides of the equator, but is rather situated 

 in the northern hemisphere. The position of the sun visibly influences 

 the position of the region of calms, as well as the range within 

 which the trade-winds blow. In the summer months, especially 

 in July, August, and September, the calms advance towards the 

 north and extend between the parallels of 8 and 14 N. ; in the 

 spring they range between 6* S. lat. and 4 N. lat These calms, 

 according to Lieutenant Maury, cover a region of constant precipi- 

 tation, this region vibrating up and down as a zone or belt, as the 

 trade winds vibrate. The calms which reign in this region would 

 oppose an insuperable obstacle to the progress of sailing vessels if 

 the water was not daily agitated by a squall, which spring* up 

 suddenly about noon and last* only for a short tune. It is only by 

 mean* of than daily squalls that the region of calms can be pawed 

 by vessels, but it always proves a very disagreeable navigati. 

 navigation is more pleasant than that with the trade-winds. They 

 are rather a breeze than a wind, and their Mowing is uniform, constant, 

 and not interrupted by squall*. The wave* raised by them are low, 

 and their swell is gentle. All these circumstance* induced Humboldt 

 to think that it is lea* dangerous to pas* from the Canaries to 

 America than to traverse one of the small lakes of Switzerland ; 

 and lie compares this navigation with the gentle descent on a slow- 

 flowing river, or rather a canal The Spaniards have called thin part 

 of the Atlantic Ocean il yoi/o de lat <fciM supposing tart even 

 kvdie* could muster courage enough to navigate it; for here the 

 paw-age to America may safely be effected in an open boat 



To theee general observation* we shall add a few other*, with 

 rwpeet to the difference between the northern and the southern trade- 

 win<i The northern trade-wind declines only from north-east to east 

 in it* progre** westward, but it is less constant towards it* northern 

 boundary than the southern towards the antarctic pole. Between 

 SO* and 80 N. lat, sometimes violent north-western winds prevail. 



Tho southern trade-wind i* of greater volume, more regular, and 

 always preserve* it* direction, and it is also lees boisterous toward* it* 

 *onth<-rn boundary. It extends, as we have already observed, in far 

 north that it i sometime* f.iiiul in force a* for a* even N. lat 

 Along the coast* of Southern Africa it blows from the S.W. ; but 



at a distance from the coast it becomes by degrees more southerly, 

 and a* it proceeds farther to the west it* direction becomes nearly due 

 K. The meridian of twenty degrees west of Greenwich may be 

 considered a* a line of separation between the winds which blow from 

 the S. and those from the E. To the east of that line, the direction 

 of the winds varies between S.W. and S.S.E., and to the west of that 

 line between S.S.E. and E.S.E. 



The trade-winds do not begin to blow on the coast of the continent*, 

 but only at a considerable distance from them. This is to be attributed 

 to the different degree of temperature of the land and of the sea, 

 which when the land is extensive must be considerable, and produce 

 a great change in the wind. Thus it ha* been long noticed that 

 between the northern trade-wind and the African coast, from the 

 Canaries to the Cape Verde Islands, the wind blows constantly 

 from the W. or S.W. This phenomenon is to be accounted for by the 

 nature of the great desert called the Sahara, whose surface composed 

 of loose sand is heated by the sun to an excessive degree, and rarefying 

 the superincumbent air causes it to rise. When this rarefied volume 

 of air comes in contact with the more dense strata of air covering the 

 sea, the latter expands over the deserts, and in this manner the west 

 wind along this coast is produced. 



Lieutenant Maury states that the charts and observations which 

 have been reduced under his direction show that the influence of th. 

 land upon the direction of the wind at sea is frequently traced for 

 more than 1000 miles at sea. In the following passage Lieutenant 

 Maury defines with much accuracy the extent of the constant wind 

 caused by the Sahara as just noticed. He styles it "a discovery which 

 has been fully and completely developed by the charts and the 

 investigations connected with them ;" and the claim is fully admitted 

 in the Report of the Royal Society and by other authorities. But it 

 ought to be remembered that English navigators were well aware of 

 such a monsoon, as the last paragraph, which was printed in the 

 ' Penny Cyclopaedia ' in 1835, is quite sufficient to show. The credit 

 of having fully and completely developed our knowledge of it is 

 however undoubtedly due to Lieutenant Maury. He says, "The 

 action of the sun's rays upon the great deserts and arid plains of 

 Africa in the summer and autumnal months is such as to be felt nearly 

 across the Atlantic Ocean between the Equator and the parallel of 

 13 north. Between this parallel and the Equator the trade-winds are 

 turned back by the heated plains of Africa, and are caused to blow a 

 regular southwardly monsoon for six months. This monsoon is a 

 discovery which has been fully and completely developed by the charts 

 and the investigations connected with them. They (the monsoons) 

 blow towards the coast of Africa from June to November inclusive. 

 They bring the rains which divide the seasons in these parts of the 

 African coast The region of the ocean embraced by the monsoon i* 

 cuneiform in its shape, having its base resting upon Africa and it* 

 apex stretching over till within 10 or 16 degrees of the mouth of the 

 Amazon." 



The changes which ore produced in the direction of the southern 

 trade-wind by the continent of South America are considerable. 

 Along the coast of Brazil a regular monsoon is formed, which between 

 September and March proceeds from between N. J E. to N.E. | K. ; 

 and from March to September from between E. i N. to E.S.E. This 

 monsoon blow* commonly with considerable force, and extends to a 

 great distance from the shore, especially in the month* of June and 

 July, when it is at its height, at which time it sometimes advances 

 nearly as far a* the middle of the Atlantic Ocean. To the north of 

 the equator the trade-wind does not undergo any change along the 

 coast of America. ThU is probably to be attributed to the lowness 

 of the coast of Guyana, and that of the extensive and frequently 

 inundated plains along the lower course of the Orinoco. It is even 

 observed that in this quarter the trade-winds extend so far OVIT tin' 

 land that their effect is felt at Angostura, 240 miles from the 

 mouth of that river. 



These ore the winds which blow on both sides of the equator t,. 

 80 lat in the region of the perpetual winds. To the north and 

 south of this region the winds arc variable; 'but it is observed that 

 westerly winds prevail in both hemisphere*, and according to tho 

 computation of Lieutenant Maury, the proportion between those that 

 blow from the W. to those from the K.. in in the northern hemisphere 

 as two to one from the parallel of 40 up towards the pole. Besides 

 being variable in direction they vary likewise extremely in tho degree 

 of fur with which they blow. 



In connection with the trade-winds of the Atlantic we ought 

 perhap* to mention the cloud* of dust which full like heavy shower* 

 if ruin on the Cape Verde Island* and on the Atlantic towards North 

 Africa, and which have been found by Ehrenberg to contain 

 innumerable siliceouiHihelled infunorio, which appear to be identical 

 with specie* found in South America. 



The current* of the Atlantic also contribute considerably to 

 accelerate or retard navigation, and on that account deserve the 

 greatest attention. Our knowledge of them is at present very 

 incomplete, but much additional information is being con- 

 tinually obtained. It would neem that there is hardly any con 

 mderablc portion of the ocean which always remains still. The tides 

 <l'i n<it occasion an absolute removal of the water from one place to 

 another, except near the coast ; and even there this motion is limited 



