212 



Mr. W. E. Browne. 



It is only '0075° per atmosphere. In other words, it will require a 

 pressure of 2,000 lbs. per square inch to liquefy ice at 31° instead of 

 32°. This is equivalent to the weight of a column of ice about 5,000 

 feet high. It is needless to ask whether such a pressure can exist 

 within an ordinary glacier, while on the other hand glaciers un- 

 doubtedly move at temperatures far below freezing point — in the 

 Arctic regions below zero. 



It seems to be generally supposed that the pressure in the lower 

 part of a glacier is due to the steeper upper portions : the glacier 

 channel is spoken of as a mould, through which the ice is forced by 

 pressure from behind. But in the upper glacier, slopes of ice or 

 neve are not uncommon at angles of 30°, or even more. Such slopes 

 usually do not even touch the more level parts of the glacier below 

 them 5 but are separated from them by a wide, deep crevasse called a 

 Bergschrund. Of this the well-known ice wall of the Strahleck is 

 a conspicuous example. In other cases such slopes do not end in a 

 glacier at all, but die away upon the mountain side. It is certain, 

 therefore, that ice or neve is able to maintain itself at a high angle 

 upon its slope of rocks, and therefore cannot possibly exercise pressure 

 upon the parts of the glacier far in advance of its foot. The fallacy 

 of this idea may be further illustrated by referring, not to modern 

 glaciers, but to those of the Great Ice Age. Can we suppose that the 

 pressure of the snows about the sources of the Rhone was sufficient to 

 drive that glacier down the valley to Martigny, round a sharp angle 

 to the Lake of Geneva, through the bed of that lake, and on to the 

 slopes of the Jura, a distance of more than 100 miles, in which the 

 average slope was about 1 in 200 ; giving a propelling force per ton 

 of ice of about 11 lbs. only ? 



All these theories have this in common, that they regard gravity as 

 the sole and direct agent in the movement of glaciers, and the above 

 considerations seem to prove that it is an agent far too weak for the 

 work it has to do.* 



The only other agent which has been suggested, or seems likely to 

 be suggested, to account for the motion of glaciers, is heat. This 

 suggestion, as is well known, is due to the late Canon Moseley, F.R.S., 

 and was to some extent worked out by him in papers published in 

 the " Phil. Mag." 1869 and 1870. 



The mode of operation, on this theory, is well known. Ice is here 

 considered merely as a solid body, obeying the ordinary laws of expan- 

 sion and contraction under differences of temperature. This it is 

 known to do, the coefficient of linear expansion, for 1° F., being 



* Another evidence against pressure from behind as a cause of motion is furnished 

 by the very small size of many glaciers. Some of these, notably those of the class 

 called " glaciers remanies," are only a few hundred yards long, and cannot be many 

 feet deep. 



