CHANGES IN CLIMATE. 755 



of the globe. The Lyellian principle here appealed to, is thus briefly 

 expressed on page 43. Absence of land, and especially of high land, 

 from the higher latitudes, is equivalent to absence of a source of extreme 

 cold ; and absence of continental lands from tropical latitudes, that of 

 extreme heat ; and therefore, if the land existed only over temperate 

 latitudes, it would have but a small range in its temperature, and be 

 neither very warm nor very cold. The sinking of all lands would 

 diminish greatly both extremes, and perhaps give the whole globe 

 nearly the present mean temperature, 60°. 



In conjunction with these differences in the distribution of land? 

 there have been differences in the courses of ocean-currents ; and these 

 have probably more than quadrupled the effect from variation alone hi 

 amount and position of land-surface. 



In the northern hemisphere, each ocean has its great tropical cur- 

 rent, and its much smaller polar current. The polar current is much 

 the smaller, because a large part of the tropical waters make their 

 circuit without going into the Arctic regions ; and this must have been 

 the case in all time, for it would still be true if the earth were all 

 water, and of like depth throughout, since the cold Arctic areas are 

 even now small, compared with the rest of the surface of the globe. 

 Changes in the direction of flow of the Gulf Stream in the Atlantic, 

 and of the Japan Stream in the Pacific, must hence, as all admit, have 

 made wide diversities of climate over the earth. Croll has stated that 

 the Gulf -Stream, according to his calculations (based on the estimate 

 that the stream in the Florida Straits averages fifty miles in breadth, 

 and 1,000 feet in depth, is four miles an hour in velocity, and 65° F. in 

 temperature, and that, in its course northward, it cools down to at 

 least 40° F.), conveys from the Gulf 5,578,680,000,000 cubic feet of 

 water per hour ; and consequently that the total quantity of heat trans- 

 ferred from the equatorial regions per day by the stream amounts to 

 154,959,300,000,000,000,000 foot-pounds. 1 Reducing this one-half, to 

 accord with Mr. Findlay's estimate, the stoppage of the Gulf Stream, as 

 he says, would still deprive the Atlantic of 77,479,650,000,000,000,000 

 foot-pounds of energy in the form of heat per day, a quantity equal 

 to one fourth of all the heat received from the sun by that area. 



Speculations as to the way to divert the Gulf Stream from the 

 North Atlantic, in order to account for a cold era like the Glacial, are 

 alluded to on page 541. It is probable that the changes it has effected 

 have been brought about, not by a diversion of the current from the 

 ocean, and its restoration to it again, but by variations in the amount 

 and height of Arctic lands, in one case closing, and the other opening 

 the Arctic regions to the tropical stream ; and the same for the Pacific 



1 Phil. May., February, 1867: Am. Jour. ScL, ii. v., 118. 



