108 CHANGES OF CLIMATE DURING GEOLOGICAL PERIODS. 



by which the midwinter temperature was lowered = 44*3. Midwinter 

 temperature of Great Britain — 5-3. In the second (Miocene), these 

 figures were 347 ; 837 ; 45-3 ; — 6-3 ; and in the third (Glacial), 26-7 ; 

 864 ; 37-7 ; and 1-3 respectively. It must be borne in mind that 

 change of climate is not caused by difference in the mean annual 

 amount of heat received from the sun, but by the increase or decrease in 

 the difference between summer and winter temperature. Astronomically, 

 the summers at those periods were as much warmer than they are 

 at present, as the winters were then colder than now. Physically, 

 however, they were in reality much colder, notwithstanding the great 

 increase in the intensity of the sun's heat resulting from the diminished 

 distance of the sun. In the circumstances under consideration the 

 winters would not only be much colder than now, but they would be 

 considerably longer. The moisture of the air would be all precipitated 

 in the form of snow, and the presence of so much snow would lower the 

 summer temperature, and prevent, to a large extent, the melting of the 

 ice. This lowering of the summer temperature by a great accumula- 

 tion of snow and ice would be produced — first, by means of direct radia- 

 tion ; second, by the reflection by the snow of the rays of the sun ; third, 

 by chilling the air and condensing the vapour into thick fogs. It will 

 readily be seen, therefore, that the greater heat of a comparatively short 

 summer will by no means compensate for the greater cold of the longer 

 winter. Cover India with an ice sheet, and its summers would be colder 

 than those of England. 



But there is another agency brought into operation by the foregoing 

 condition of things, far more powerful than those hitherto noticed, viz., 

 the deflection of ocean currents. It his, I think, been proved that the 

 winds are the impelling cause of currents, and that the direction of the 

 current depends on the direction of the prevailing winds of the 

 globe, and on the conformation of land and sea. The ocean alone 

 can convey the equatorial heat to distant shores, and the heat 

 thus conveyed is distributed over the land by aerial currents. Trade 

 winds owe their existence to the great difference between the tempera- 

 ture of the equator and the poles. This difference is now greatest 

 between the South Pole and the Equator. Therefore, the south-eastern 

 trades are stronger than the north-eastern. But if the northern hemis- 

 phere were the cold one, as was the case at the period of the glacial 

 epoch, the north-east trade winds oa it would far exceei in strength the 

 south-east trade winds of the southern hemisphere. Tu3 effect of this 

 would undoubtedly be that the great equatorial currents would be shifted 

 southwards. 



The shifting of the equatorial current of the Atlantic only a few 

 degrees to the south of its present position would turn the entire current 

 away from the Gulf of Mexico into the Southern Ocean, and the Gulf 

 Stream woull consequently be stopped. This, combined with other 

 causes, would place Europe under glacial conditions. The Gulf Stream 

 (I quote from Mr. Croll) heats our island indirectly, by heating the 

 winds whioh blow over it to our shores. They are heated in two ways— 



