722 THE POPULAR SCIENCE MONTHLY. 



Dr. Franklin sugfjested that the Gulf Stream has its orioin 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 spe- 

 citic gravity of ocean-waters, arising mainly from difierences of tem- 

 perature. Dr. Carpenter, too, insists on diflerence 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 equa- 

 tor, 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 aflect that of the currents, and in that way the 

 climates of the globe. 



At present the equatorial waters heated by the sun to a tempera- 

 ture 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 equa- 

 torial current southward beyond the median line, and increasing the 

 volume of the southern at the exjjense 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 Ml'. 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 North- 

 ern 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 



