396 THE POPULAR SCIENCE MONTHLY. 



The freshest trade-winds in the North Atlantic are generally found 

 between the parallels of 10 and 25, and by long-protracted experi- 

 ment in seamanship they have been found to have an average propel- 

 ling power, when the wind is taken just abaft the beam, of about six 

 knots an hour. But, of course, the northern boundary of the south- 

 east trade-wind likewise varies and vibrates with the seasons. So, 

 also, and under the same condition, does the southern boundary of 

 this trade vary and vibrate with the seasons. Its normal and mean 

 position is a little south of the parallel of 25 south, but in the winter 

 of our hemisphere it is pushed much farther south, and in the vicinity 

 of 35 south latitude. The charts of Captain Wilkes give easterly 

 winds for the east coast of Australia, and also for the south coast of 

 Africa. Sir John Herschel, speaking from knowledge gained by his 

 long residence at the Cape of Good Hope, tells us that there " the 

 southeasterly wind which sweeps over the Southern Ocean, infringing 

 upon the long range of rocks which terminates in the Table Mountain, 

 is thrown up by them, makes a clean sweep over the flat table-land 

 which forms the summit of that mountain (about 3,850 feet high), and 

 thence plunges down with the violence of a cataract " (" Meteorology," 

 p. 96). 



From these high southern latitudes, we must conceive the motion 

 of the southeast trades, extending northward in summer to the neigh- 

 borhood of the parallel of 10. 



From the Cape of Good Hope, in a straight line toward the pro- 

 jecting eastern coasts of Brazil, mariners have found a peculiar streak 

 of southeasterly winds. Between the island of Tristan da Cunha and 

 the Cape, and northward and westward to the island of Fernando 

 Noronha, this streak of powerful winds, with which nothing in the 

 trade-wind region of the North Atlantic can compare, has its atmos- 

 pheric current as sharply marked as the dark blue and rapid current of 

 the Gulf Stream in the Narrows of Bernini. It is, doubtless, the 

 region or band of most intensely acting southeast trades, and is 

 probably due to the peculiar configuration of the shores of the South 

 Atlantic, and to the wall of the South American Andes. It is a well- 

 known fact that the volcanic cone of Teneriffe, which lies in the zone 

 of northeast trades, intercepts the wind and gives it a lateral deflec- 

 tion ; so that, while the tirades are blowing strongly on the northeast 

 side of the island, on the opposite side there is a distinctly-marked 

 and carefully-measured calm shadow. Now, the chain of the Andes 

 endeavors to exert on the southeastern trades just such an influence 

 as is exerted by the Canary Islands on the northeast trades. This 

 influence, in the former case, suffices to throw off from the Continent 

 of South America a large body of the southeast trades, and to deflect 

 it to the eastward, giving it the character of a south-southwest wind, 

 and, at the same time, by forcing a greater or more concentrated body 

 of air into the regions northeast of Brazil, imparting an increased 



