Dr. CrolVs Theory of Glacial and Warm Periods. 87 



year than at another, enabling it to force back the other 

 trade wind and the calms several degrees. As we have 

 seen, there is no evidence of any pushing at all, and such a 

 postulate is contrary to all we know. 



The movement of the trades calms to and fro is in fact 

 due to another and a very simple cause. The fact is that 

 circulation of the two great constant winds has nothing 

 to do with the earth's equator of form at all. Their 

 circulation is induced by the heating effect of the sun, and 

 it is to where the furnace burns the hottest, that is to say, 

 to the latitude of greatest mean heat that these winds, by 

 the laws of pneumatics, must tend. Now the latitude of 

 greatest mean heat does not coincide with the equator, and 

 wherever it happens to be, there must be the focus of the 

 calms, and there the medial line between the trades. It is 

 the situation of the furnace of greatest heat for the time 

 being and not the relative strength of the trades that fixes the 

 situation of the combined belt of trade winds and equatorial 

 calms. This is not mere conjecture, but it agrees with 

 observed facts as well as with theory, and this must be true 

 whatever eccentricity we may postulate for the earth's orbit. 



I cannot, therefore, at all agree with Dr. Croll's re- 

 marks on the subject when he says :— " It has been shown 

 that an increase in the eccentricity of the earth's orbit tends 

 to lower the temperature of the one hemisphere, and to 

 raise the temperature of the other. The consequence 

 of this is that the aerial currents of the hemisphere, 

 in which the difference of temperature between the 

 equatorial and the temperate and polar regions is 

 greater, is stronger than those of the other." " This," he 

 adds, " would be more especially so with the trade winds. 

 The stronger trades would blow across the equator and 

 drive the median line between them, and the opposite trades 

 into the other hemisphere. If the north-east trades, for 

 instance, were the stronger, the equatorial waters would be 



