HALF A CENTURY AGO AND TO-DAY. 



35 



forest. The process is now so complete that arfificuil re-afforestation 

 has become usual. The glacier world, too, has been driven back 

 with the forest to within its limits in the latter centuries of the 

 Eoman Empire. 



It is quite possible that the Aletsch glacier has not yet shrank 

 back to its size in the Roman days. There is no reason to deny 

 that at one time the cattle which now cross it on the ice may have 

 passed through a continuous forest from one side of the valley to 

 the other. 



REPLY BY THE AUTHOR. 



The communications by Mr. Hudleston and Professor Roget form 

 a valuable addition to my paper. I quite agree with the former, 

 that the shrinkage of the glaciers is mainly due to meteorological 

 causes, and these, again, to disafforesting of the mountain slopes. 

 When the surface of the ground has been deprived of the protection 

 of trees, the radiation of heat tends to disperse the clouds and 

 reduce the rainfall ; on the other hand the same process causes the 

 rain to flow down rapidly and suddenly to raise the rivers ; and it 

 is owing to this cause that in recent times the rivers descending 

 from the Alps have caused inundations of the plains of Northern 

 Italy. 



I cannot agree with Professor Roget in his explanation of the 

 differential movement of the central portion of the glacier as 

 compared with that of its sides. The true explanation is, as it 

 seems to me, that like a river flowing along its bed — the friction 

 of the sides of the glacier againstits basin retards the motion, 

 whereas along the centre this retarding agency is absent, and the 

 ice drags away from its sides, causing the crevasses to run upwards 

 this is generally recognized. 



