Page:Popular Science Monthly Volume 9.djvu/199

Rh This is the normal order of the wind-currents, and that which would prevail with nearly perfect regularity if the world were a uniform globe of water or of land, and equally heated on both sides of the equator. But the continents, and particularly mountain elevations, produce great disturbances—unequal rainfalls and ever-varying atmospheric pressures. When also, from any cause, one of the trade-winds, notably the southern, is increased in its violence, so as to push a tornado-tongue across the dividing line, into the opposite system of winds, there is started one of those cyclones, or great circular storms, which ravage the tropics and whirl through the temperate zones, finally exhausting themselves in the higher latitudes to the eastward.

The southern hemisphere is at the present time colder than the northern, owing primarily to the fact that the winters there are eight days longer than the northern, and the sun, during those seasons, about 3,000,000 miles farther from the earth than during the northern winters. The difference of temperature, therefore, between the warm air that rises at the equator and the cold air that comes in from the south is greater than that on the north side. And, as it is difference of temperature that produces the whole movement of the air-currents, of course the greater strength of that movement must be on the southern side. Hence the larger share of the equatorial current passes over to the south, and the southern trades are much the strongest. In accordance with this theory, it is a matter of observation that the southern trade-winds reach across the equator and into the northern hemisphere in some places ten to fifteen degrees.

In obedience to and perfect accord with this great system of winds, the waters of the oceans move. The strong southeast trades blow up from Southern Africa, cross the equator, and drive the waters of the South Atlantic into the Caribbean Sea. The lighter northeast trades, blowing between North Africa and the West Indies, assist and give direction to this movement, which finally impels through the Straits of Florida a tide of tropical waters a hundred times greater than the outflow of all the rivers in the world. This great flood of thermal waters spreads out in the Northern Atlantic, imparting to Europe a climate corresponding to countries twenty degrees south of it on this side of the ocean. There is, of course, an under-current from the Arctics to the equator, exactly compensating this enormous northward flow of the surface-waters. The same process and effect are repeated in the Pacific Ocean; and the great Japan Stream robs the southern hemisphere, for the benefit of our Pacific States, only in a degree less than does the Gulf Stream for the benefit of Europe.

A change in the relative strength of the trade-winds, such that the northeast trades would blow across the equator into the southern hemisphere, would entirely reverse the course of the warm ocean-currents, and carry to the southern continents the heat abstracted from the northern. Such a change in the course of ocean-streams has