Page:Popular Science Monthly Volume 2.djvu/413

Rh velocity and violence to the air-current. It is, therefore, in the air-current that the homeward-hound vessel from the Cape of Good Hope aims to steer, because she is sure of being wafted happily and swiftly to her destination.

It has long ago been demonstrated by meteorologic observations, taken both at sea and on land, that there is very much less atmosphere in the Southern Hemisphere than in the northern, and for a long time physicists were at a loss to account for the difference. It has been, however, very satisfactorily explained by the eminent American mathematician, Ferrel, in his work on the "Motions of Fluids and Solids, relative to the Earth's Surface," where he proves at length, and states in detail (p. 39): "As there is much more land, with higher mountain-ranges, in the Northern Hemisphere than in the southern, the resistances are greater, and consequently the eastward motion of the air, upon which the deflecting force depends, is much less; and the consequence is, that the more rapid motions of the Southern Hemisphere cause a greater depression there, and a greater part of the atmosphere to be thrown into the Northern Hemisphere" It is, doubtless, to this tendency of the Southern Hemisphere to throw off much of its atmosphere north of the equator that we may attribute in part the superior force and power of the southeast trades, and their well-known ability to battle with the northeast trades, and drive them from their own territory, at least all summer, and even in winter, as far back across the line as 3° or 4° north latitude. Mr. Ferrel, speaking of the principle just enunciated, well says: "This also accounts for the mean position of the equatorial calm-belt being, in general, a little north of the equator. But, in the Pacific Ocean, where there is nearly as much water north of the equator as south (and the resistances are usually equal), its position nearly coincides with the equator." In other words, just as a bucket full of water set to revolving on a perpendicular axis would show a depression in the centre, and the fluid be thrown from all sides of its rim, the Southern Hemisphere throws its water and its atmosphere into the Northern Hemisphere, all along the equator.

It is, therefore, a mathematical and mechanical certainty that there is an invasion of the northeast trade-wind belt from the southeast trades, and observation powerfully bears out the deduction of the mathematician. Auste states, in his cautiously-written "Physical Geography:" "The southern trade-wind region is much larger than the northern in the Atlantic Ocean. In this sea, the southeast trades are fresher, and blow stronger, than the others, and often reach to the 10th or 15th parallel of north latitude; whereas, the northern trade-wind seldom gets south of the equator, and usually ranges from 9° to 29° north latitude" (p. 253). It is easy to see how easily it happens that a very small atmospheric eddy found in the tropical Atlantic by the conflictory northeast and overleaping southeast trade-winds may soon become a hurricane of wide extent and of tremendous