The Theory of Glacier motion. 91 



to ask whether such a pressure can exist within an ordinary 

 glacier, while, on the other hand, glaciers undoubtedly move 

 at temperatures far below the freezing point, in the arctic 

 regions below zero" {Proceedings Royal Society^ xxxiv. 

 211 — 212). 



Professor Tyndall has also proposed some acute criticisms 

 of this theory. He first urges that the water in the supposed 

 case, when escaping, would escape upwards as surely as down- 

 wards, since the tendency to flow down by its own gravity 

 would be slight compared with the other forces acting on it, 

 and the ice above the melting portion would be less dense 

 and more permeable than that below it, and the glacier ought 

 to move uphill instead of down. Again, as Tyndall says : "The 

 difference between the length of the Mer de Glace at Montan- 

 vert, and at the summit of its principal tributary, the Col du 

 Geant, is about 4,846 feet. An atmosphere of pressure is 

 equivalent to about 40 feet of ice, which, according to 

 Professor Thomson, would lower the freezing point of 

 water by "0075 of a degree Centigrade. This being so, the 

 pressure of the whole column of ice referred to, 4,846 feet, 

 would lower it nine-tenths of a degree. ^^ Supposing thenl' as 

 Tyndall says, 'Hhe unimpeded thrust of the whole glacier, from 

 the Col du Geaftt downwards, to be exerted on the bed at 

 the Montanvert, or in other words, suppose the bed of the 

 glacier to be absolutely smooth and every trace of friction 

 abolished, the utmost the pressure thus obtained could per- 

 form would be to lower the melting point of the Montan- 

 vert ice by this quantity. Taking into account the actual 

 state of things, the friction of the glacier against its sides 

 and bed, the opposition which the three tributaries encounter 

 in the neck of the valley at Trelaporte, the resistance en- 

 countered in the sinuous valley through which it passes ; 

 and, finally, bearing in mind the comparatively short length 

 of the glacier, which has to bear the thrust, and oppose the 

 latter by its friction only, I think it will be evident that 



