DISCUSSION AND ANALYSIS OF WINDS. 709 



more easterly in the western than in the eastern part of the ocean. This will be 

 readily seen by a reference to the map. (Plates 3, 5, 6, and 7.) 



There are in other places mnch greater differences in the limits of the S. E. 

 trade. Near the coast of America the winds are so irregular that the seamen do 

 not consider them true trades, thus on the meridian of 30° W. the polar limit is 

 set down at 16|° S. to 21° S. according to the seasons. Near the coast of Africa 

 (10° E.) the polar limit is south of 30° S. at all seasons. The S. E. trade advances 

 much beyond the equator, except in the months of February, March, and April. 

 In September it goes to 6° N. under 40° W. 



The narrowing of the ocean in its equatorial part between Cape S. Eoque in S. 

 America and Cape Verde in Africa does not allow of a determination of the 

 equatorial limits of the trade east of 17° W. and west of 40° W., as it is known 

 that the trades blow regularly only on the ocean. 



The greater breadth, however, of the S. E. trade and its regularity near the equa- 

 tor are well known. 



Along the coast of S. Africa there are prevailing S. W. winds the whole year. 

 They exist also on the ocean. This African monsoon is caused by the rarefaction 

 of the air in the interior of the continent, and, in the months from July to September, 

 extends far beyond the equator, and occupies much of the zone between the S. E. 

 and N. E. trade. Violent rains and thunder-storms are experienced at this season 

 in this region of S. W. winds. There is no doubt that the S. E. trade is drawn 

 far beyond the equator and gradually changed into a S. and then a S. W. wind. 

 Having passed over a broad expanse of warm sea it is copiously loaded with vapor. 



From January to March the Pilot Charts give the southern boundary of the S. W. 

 winds at 2|° N. and 15° W. It crosses the equator under 10° W. ; 7° S. under the 

 meridian of Greenwich; 10° S. under 4° E.; 20° S. under 10° E. Thus the belt 

 of S. W. winds has the greatest breadth opposite the Bay of Biafra, and is much 

 narrower North and south. From April to June the S. W. winds advance to 19° 

 W. opposite Sierra Leone, while the boundary is nearer to the coast of Africa 

 further southward. 



From July to September the belt of S. W. winds occupies a great space off the 

 west coast of N. Africa, between 17° and 32° W. and 6°-ll|° N. If the boundary 

 were traced for every month, it would be found to coincide much more closely with 

 the inner limits of the N. E. and S. E. trade ; as it is, it is near enough, as the 

 southern limit of the N. E. trade is 12° N. in September, near the coast of Africa, 

 while the S. W. monsoons begin about 11|° N. 



It is important also to obtain a knowledge of the minor characteristics of tlie 

 winds of the Atlantic, and this can best be done by studying the percentage of 

 winds in the different regions of the ocean, as presented in the following table: — 



