Wellington Philosophical Society. 483 



regions to fall as snow. Nor is this all ; the winters would not merely be 

 colder than now, but they would be also much longer. At present the 

 summer half-year in the northern hemisphere exceeds the winter half-year 

 by nearly eight days ; but at the period in question, supposing the eccentricity 

 of the earth's orbit increased, the winters would be longer than the summers 

 by upwards of thirty-six days. The heat of the sun during the short summer 

 would not be sufficient to melt the snow of winter, so that gradually year by 

 year the snow would continue to accumulate on the ground. At the same 

 time it is quite true that an increase of eccentricity does not give more solar 

 heat to the one hemisphere than to the other, but, nevertheless, it would bring 



about a different state of things. 



On the southern hemisphere the opposite condition of things would 

 obtain. Owing to the nearness of the sun during the winter of that hemi- 

 sphere, the moisture of the air would be precipitated as rains in regions where 

 at present it falls as snow. This and the shortness of the winter would tend 

 to produce a decrease in the quantity of snow. Thus you will observe that 

 the difference of temperature between the temperate and polar regions would 

 be greater on the northern than on the southern hemisphere, and as a 

 consequence the aerial currents of the northern hemisphere would be stronger 

 than those of the latter. This would be more especially the case with the 

 trade winds. The N.E. trade winds, being stronger than the S.E. trades, 



driv 



southern 



warm 



tend to increase the difference of temperature between the two hemispheres. 

 And it was this mutual reaction of those physical agents which led to that 

 extraordinary condition of climate in the northern hemisphere which 

 prevailed during the glacial epoch. 



and 



or 15° north of the equator ; so that the mean position of the median line lies 

 at least 6° or 7° north. But if Croll's views be adopted, the N.E. trades 



blowing across the equator, the median line would be 

 the south of the equator. The effect of this shiftii 

 the N.E. and the S.E. trades from the northern to the southern side of the 

 equator, and with it the equatorial current of the Atlantic, would be that 

 the whole of the waters would strike obliquely against the Brazilian coast, 

 and thus be deflected into the Southern Ocean. The effect produced on the 

 climate of the North Atlantic and North-Western Europe by the withdrawal 

 of the water forming the Gulf Stream may be conceived from what has 

 already been stated concerning the amount of heat conveyed by that stream. 

 The heat thus withdrawn from the North Atlantic would go to raise the 



