122 Mr. J. Croll on the Eccentricity of the Earth's Orbit, 



colder than the opposite hemisphere which has its winter in 

 perihelion and enjoying an equable climate; and the conse- 

 quence is, the aerial currents from the pole to the equator 

 must be much stronger on the colder hemisphere than on the 

 warmer because the difference between the temperature of the 

 pole and the equator is greater on the former hemisphere than 

 on the latter. When the northern hemisphere, for example, is 

 under glaciation, the north-east trade-winds will be much 

 stronger than the south-east. The medial line between the 

 trades will consequently lie a considerable distance to the south 

 of the equator. The effect of the northern trades blowing across 

 the equator to a great distance will be to impel the warm 

 water of the tropics over into the Southern Ocean. And this, to 

 an enormous extent, will tend to exaggerate the difference be- 

 tween the temperature of the two hemispheres. 



But it is worthy of notice that a strong undercurrent of air 

 flowing from the north pole to the equator implies an equally 

 strong upper current flowing from the equator to the pole. 

 Now, if the effect of the undercurrent be to impel the warm 

 water of the tropics into the Southern Ocean, and thus raise 

 the temperature and melt the ice in the southern hemisphere, 

 the effect of the upper current will be to carry moisture raised 

 by evaporation in tropical regions away to the north, there to 

 be deposited in the form of snow on reaching the great ice- 

 sheet covering the arctic and temperate regions. The general 

 effect of the whole will be to produce an accumulation of snow 

 and ice in the northern hemisphere, and a diminution of these 

 in the southern. 



The effect of the aerial currents on the Gulf- stream during 

 the glacial epoch must have been peculiarly marked. It has been 

 shown * that if the equatorial current of the Atlantic, the feeder 

 of the Gulf- stream, were removed merely a few degrees to the 

 south of its present position, the entire current would be turned 

 into the Brazilian branch and flow into the Southern Ocean, 

 and thus probably stop the Gulf-stream altogether. But 

 during the glacial epoch, on the northern hemisphere, when the 

 medial line lay at a considerable distance to the south of the 

 equator, the greater portion of the equatorial current, if not 

 the entire current, must have flown into the Southern Ocean. 

 But let us assume, what will certainly be admitted to be a mo- 

 derate estimate, that 850,000 years ago, when the excentricity 

 was near its superior limit, that the Gulf-stream was reduced 

 to one-half its present volume, when the northern hemisphere 

 was under glaciation. The midwinter temperature in the 

 centre of Scotland, as is well known, is about 28° higher than 

 * Phil. Mag. for August 1864. 



