Page:Popular Science Monthly Volume 2.djvu/412

396 The freshest trade-winds in the North Atlantic are generally found between the parallels of 10° and 25°, and by long-protracted experiment in seamanship they have been found to have an average propelling 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 neighborhood of the parallel of 10°.

From the Cape of Good Hope, in a straight line toward the projecting 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 atmospheric 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 deflection; 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