Dr. Croll' s Theory of Glacial and Warm Periods. 9 1 



already existing between the two hemispheres." {Climate 

 and Time, 13-17.) 



This position, it seems to me, fails at every point. The 

 circulation of the trades does not depend on the differ- 

 ence between the temperature of the equator and the 

 poles, for the warm equatorial winds and the cold polar 

 winds each have a separate and distinct circuit of their 

 own. There is no evidence that the south-east trades are 

 stronger than the north-east, nor, if they were, that they 

 could push back the region of equatorial calms, since, on 

 reaching the parallel of greatest heat, they must move verti- 

 cally and not laterally. On the other hand the fact that 

 they blow across the equator is accounted for by the situation 

 of the parallel of greatest mean heat being to the north of that 

 great circle, which necessarily causes them to blow across 

 the equator, and, what is more important, would do so 

 long as that parallel remained north of the equator, however 

 great the eccentricity of the earth. 



What Dr. Croll has to prove in order to substantiate his 

 position is, that during periods of greater eccentricity, 

 when our winter took place with the world in aphelion, the 

 equator of greatest mean heat was transferred to the other 

 hemisphere, and that it has been subjected to an alternate 

 movement north and south. That is to say, since the 

 position of the equator of greatest mean heat is dependent 

 upon the distribution of land and water, and the fact that 

 in the torrid zone there is at present a larger proportion of 

 land to water north of the equator than south of it, that 

 this proportion was different in so-called glacial times, and 

 that consequently there has been since those times a great 

 revolution in the contour of the continents. Against such 

 a conclusion the evidence of geology is very uniform, and it 

 is in fact fully admitted by Dr. Croll himself, who, speaking 

 of the so-called glacial epoch, says : — " Geologists almost 

 all agree that little or no change has taken place in the 



