394 Br. James Croll — On Geological Time. 



the equator. But if the equatorial regions were occupied by land 

 instead of water, the possibility of conveying heat to temperate and 

 arctic regions by means of ocean currents was completely cut off. 

 In fact, one of the most effectual ways of lowering the mean tempera- 

 ture of the globe would be to group the continents along the equator. 



The surface of the ground at the equator becomes intensely heated 

 by the solar rays, and this heat is radiated into space much more 

 rapidly than it would from a surface of water warmed under the 

 same conditions. Again, the air in contact with the hot ground 

 becomes more speedily heated than it would if it were in contact 

 with water, and consequently the ascending current of air over the 

 equatorial lands carries off a greater amount of heat than it could 

 have done from a water-surface. Now, were the heat thus carried 

 off to be transferred by means of the upper currents to high latitudes, 

 and there emjDloyed in heating the earth, then it might to a consider- 

 able degree compensate for the absence of warm ocean currents. In 

 such a case land at the equator might be nearly as well adapted as 

 water for raising the temperature of the whole earth. We know 

 very well, however, that the heat carried up by the ascending 

 current at the equator performs little work of this kind, but on the 

 contrary is almost wholly dissipated into the cold stellar space above. 

 Thus instead of warming the globe, this ascending current is in 

 reality a most effectual means of getting rid of the heat received 

 from the sun, and thereby reducing the temperature. Since then the 

 earth loses as well as gains the greater part of her heat in equatorial 

 regions ; it is there that the substance best adapted for preventing 

 the dissipation of that heat must be distributed in order to raise 

 the general temperature. Now, of all substances in nature water 

 seems to possess this quality in the highest degree ; and being a 

 fluid it is adapted by means of currents to carry the heat which it 

 receives to every region of the globe. 



It has been urged as an objection to any ocean-current theory that 

 while it provides the requisite amount of heat, it fails to remove the 

 three or four months' darkness of an Arctic winter, which must have 

 proved fatal to plants of the Miocene period. This objection seems, 

 however, to have no foundation in fact. Sir Joseph Hooker stated 

 to the Eoyal Society at the close of the reading of Mr. George 

 Darwin's paper that palms and other plants brought from the tropics 

 survived the winter in St. Petersburg without damage, though 

 matted down in absolute darkness for more than six months. And 

 he was of opinion that the want of sunlight during the Arctic winter 

 would not be very prejudicial to the plants. 



But a cause must be found as well for the cold of the Glacial Epoch 

 as for the warm climate of the Arctic regions that obtained in Miocene 

 times. According to Lyell the continents would require to be 

 moved to high temperate and polar regions to bring about a 

 glacial condition of things in Britain. But this is an assumption 

 which the present state of geological science will hai'dly admit. It 

 is perfectly certain that there have been no such vast revolutions 

 in physical geography in post-Tertiary times. 



