Dr. Croll's Hypotheses on Geological Climates. 237 



well-known principle " post hoc, ergo propter hoc/' as it gave 

 a ready explanation of the former glacial periods of the north- 

 ern hemisphere. I am quite sure that Dr. Croll was also 

 influenced by the present differences of the two hemispheres. 

 Dr. Croll has long been an advocate of the wind theory of 

 ocean-currents and has proved that, at present, a considerable 

 quantity of warm water is brought by these currents from the 

 southern to the northern hemisphere and serves to warm the 

 latter. In these two questions he has rendered good service 

 to science. The transport of warm water from the southern 

 to the northern hemisphere is a fact ; but what is the cause ? 

 Dr. Croll believes the cause to be, at least indirectly, winter 

 in aphelion, which brings, especially during high excentricity, 

 but to a certain degree even now, a host of other indirect 

 results, by which the given hemisphere is cooled, its trade- 

 winds are strengthened, and bring the more warm water into 

 the other hemisphere, the higher the excentricity. I have 

 shown above that the cause assigned by Dr. Croll is inadequate 

 to produce any considerable lowering of temperature on the 

 ocean in winter, and that even the small difference perhaps 

 possible must be regained in summer. Thus whiter in aphe- 

 lion cannot cause the change in the velocity of the trade- 

 winds and their more southerly extension when winter in 

 aphelion exists in the northern hemisphere, and the reverse 

 during winter in aphelion i 1 the southern hemisphere. Why, 

 then, are the trades of the southern hemisphere blowing 

 into the northern at present, and also the conditions more 

 favourable to glaciation in the southern than in the northern 

 hemisphere ? There are certainly good reasons for that. 



1. The extent and depth of the oceans of the southern 

 hemisphere. This gives a greater steadiness and force to the 

 winds of that hemisphere ; and the difference is even more 

 marked in the westerly winds of middle latitudes than in the 

 trades, but is certainly 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 important 

 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 sometimes opposing winds. The 

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

 Even where the disturbances of the normal ocean gradients is 

 not large enough to cause monsoons, we see generally the 



