THE CAUSE OF THE GLACIAL PERIOD. 475 



occur in aphelion — that is, when the earth is farthest away 

 from the sun, and are seven days longer than the summers. 

 While admitting that there is not as yet enough known about 

 the laws governing the absorption and retention of the sun's 

 heat upon the earth's surface to permit us to say with confi- 

 dence that the acknowledged glacial condition of the southern 

 hemisphere is not produced by the astronomical cause under 

 consideration, it must be added that we are also unable to 

 prove the inadequacy of other causes to produce the same 

 results. In assuming the reality of Mr. Croll's cause, we are 

 in danger of resting on a theoretical may he rather than on 

 well-established premises. At any rate, Woeikoff, in the 

 ablest review that has yet appeared upon the subject, thinks 

 the glaciation of the southern hemisphere may readily enough 

 be accounted for without the aid of Croll's theory, and sums 

 up the case thus : 



The extent and depth of the oceans of the southern hemi- 

 sphere give a greater steadiness and force to the winds of that 

 hemisphere, and the difference is even more marked if we com- 

 pare the westerly winds of middle latitudes rather than the 

 trades, though also well seen in the latter. Now, land acts in 

 two ways on the trade-winds : it weakens them, first, by the 

 increase of friction. But this is not all. The trades, few 

 ocean regions excepted, are not strong winds ; they are impor- 

 tant on account of their extent and steadiness. The gradient 

 which causes them is small. Now, in such cases, land, even if 

 it is not a continent but only a cluster of small islands, has a 

 great influence on trade-winds in causing local gradients which 

 may have even an opposite direction to the general gradients, 

 thus causing different and even opposing winds. The land- 

 and sea-breezes and the monsoons are cases in point. Even 

 where the disturbances of the normal ocean gradients are not 

 large enough to cause monsoons, we see generally the trades 

 of tener interrupted in summer, when they are weaker and when 

 local thunder-storms and rains are more frequent on land. 

 For the two reasons given, the trades of the southern hemi- 

 sphere must be more extensive and stronger than those of the 

 northern. 



