Page:Popular Science Monthly Volume 7.djvu/742

722 Dr. Franklin suggested that the Gulf Stream has its origin in the trade-winds. It was a crude but sagacious remark. Lieutenant Maury, however, thought the cause wholly inadequate, and that the phenomenon is better explained by difference in the density and specific gravity of ocean-waters, arising mainly from differences of temperature. Dr. Carpenter, too, insists on difference in specific gravity as a cause of ocean circulation, but claims that the circulation is a diffused or general one of the ocean-waters between poles and equator, and attaches comparatively little importance to the Gulf Stream. Mr. Croll, however, revives the views of Dr. Franklin with surprising ability, and finds in the great wind-currents, and chiefly in the trade-winds, a cause adequate to the result. The wind and ocean currents coincide all over the globe. The waters move with the general set of the trade-winds—the direction of the one is a reliable exponent of the set of the other.

Now, it is obvious that any influence which changes the direction of the winds will also affect that of the currents, and in that way the climates of the globe.

At present the equatorial waters heated by the sun to a temperature of 83° move westward between the tropics at the rate of thirty miles in twenty-four hours. This is called the equatorial current. It impinges upon the coast of South America, a small portion going southward, the principal portion northward, and is discharged as the Gulf Stream, It is deflected every year by changes in the trade-winds; it is thrown northward when the southeast trade is at its maximum. We will confine our attention now to the northeast trades. Should these be increased in velocity and volume, they would also assume a somewhat more northerly direction at the equator, carrying the equatorial current southward beyond the median line, and increasing the volume of the southern at the expense of the northern flow. From this cause the temperature of the Northern Hemisphere would be greatly lowered, while the mildness of summer would prevail in the Southern Hemisphere.

Causes, therefore, which alter the force and direction of the trades are adequate to change the climates of the globe, and in the opinion of Mr. Croll these causes are found in variations in the earth's path around the sun, combined with the precession of the equinoxes. These affect not, indeed, the total volume of heat received by the earth in a year, but the distribution of it by the means already referred to. If it happen that during a vast period of time the winters of our Northern Hemisphere should occur when the earth is farthest from the sun, and its orbit at its greatest eccentricity, the result would be winters long and cold, with summers short but hot. The earth would in that case be 8,641,870 miles farther from the sun in winter than at present, and during that season would receive one-fifth less direct heat from it. At present the winters are eight days shorter than the summers, but