90 Mr. H. H. Ho worth on 



the most heated zone of air is gradually brought back to 

 the south with the whole circulatory system of the trade 

 winds. At the end of March the northern limit of the 

 equatorial calms of the Atlantic is found on an average 

 towards the 2nd degree of north latitude, while at the end 

 of September the same limit attains ordinarily the 13th or 

 14th degree. As to the southern limit, it oscillates in the 

 same ocean from 1 to 4 degrees of north latitude " {id. 

 292-3.) 



Here, then, we have the real and efficient cause of the 

 movement to and fro of the trade winds and the equatorial 

 calms, and so long as the parallel of greatest mean heat,, 

 that is to say, so long as the furnace which causes the circu- 

 lation of the trades remains to the north of the equator, so 

 long will the south-east trades blow across the equator, not 

 because of their greater potency, but because they must 

 travel thither to reach the furnace which makes them move 

 at all. This seems to be a complete answer to the main 

 contention of Dr. Croll, which he states as follows : 



"... When the eccentricity of the earth's orbit attains 

 a high value, the hemisphere, whose winter solstice occurs 

 in aphelion has its temperature lowered, while that of the 

 opposite hemisphere is raised, let us suppose the northern 

 hemisphere to be the cold one, and the southern the warm 

 one. The difference of temperature between the equator and 

 the North Pole will then be greater than between the equator 

 and the South Pole ; according, therefore, to theory, the 

 trades of the northern hemisphere will be stronger than 

 those of the southern, and will consequently blow across the 

 equator to some distance on this southern hemisphere. 

 This state of things will tend to deflect equatorial currents 

 southwards, impelling the warm water of the equatorial 

 regions more into the southern or warm hemisphere than 

 into the northern or cold hemisphere. The tendency of 

 all this will be to exaggerate the difference of temperature 



