390 CAUSES OF CLIMATAL CHANGE 



really to give it the first place as an efficient cause. This is the 

 varying distribution of ocean currents, in connection with the 

 differences in the elevation and distribution of land. The great 

 equatorial current, produced by the action of the solar heat on 

 the atmosphere and the water, along with the earth's rotation, 

 is thrown, by opposing continental shores, northward into the 

 Atlantic and Pacific in the Gulf Stream and Japan current, 

 giving us a hot-water apparatus which effectually raises the tem- 

 perature of the whole northern hemisphere, and especially of 

 the western sides of the continents. Mr. Croll imagined that 

 if his astronomical causes could, to ever so small an extent, in- 

 tensify the action of these currents, or their determination to 

 the north, we should have a period of warmth, while a similar 

 advantage given to the southern hemisphere would produce a 

 glacial age in the north. But this requires us to assume what 

 ought to be proved ; namely, that the position of aphelion, and 

 the increase or decrease of eccentricity, would actually so swing 

 the equatorial current to the north or south. It further requires 

 us to assume — and this is the most important defect of the 

 theory — that no change occurs in the distribution of land 

 and water ; because any important change of this kind might 

 obviously exert a dominant influence on the currents. Let us 

 take two examples in illustration of this. 



At the present time the warm water thrown into the North 

 Atlantic, co-operating with the prevalent westerly winds, not 

 only increases the temperature of its whole waters, but gives an 

 exceptionally mild climate to western Europe. Still the counter- 

 vailing influence of the Arctic currents and the Greenland ice, 

 is sufficient to permit numerous icebergs to remain unmelted on 

 the coast of Labrador and Newfoundland throughout the sum- 

 mer. Some of the bergs which creep down to the mouth of 

 the Strait of Belle-Isle, in the latitude of the south of England, 

 actually remain unmelted till the snows of a succeeding winter 

 fall upon them. Now let us suppose that a subsidence of land 



