The Theory of Glacier motion. 145 



course of some months." Professor Pfaff then details some 

 experiments confirming this one, and like others I have 

 already quoted. " I next endeavoured," he says, "to determine 

 the amount of extension of ice by traction, and he attached a 

 weight to a prism of ice. After seven days, signs of stretching 

 were clearly visible. It is therefore shown," he says, " that 

 a pull continued for a long time, even when it is slight, 

 stretches ice, that near its melting point it shews itself like 

 other bodies yielding to pressure as well as to pull, and 

 at a temperature in the vicinity of zero, it is to be regarded 

 as an eminently plastic substance" {Phil. Mag., L. 



333—336). 



In 1883, Mr. C. Trotter tried some experiments on the 

 shearing of ice in an ice grotto at Grindelwald, the apparatus 

 being placed about 1 8 metres from the edge of the glacier,. 

 25 or 30 metres below its upper surface, and about the same 

 above its bed, so as to have conditions of temperature like 

 those of an actual glacier, and the ice used was cut from 

 the glacier itself The result was to show that under a 

 shearing force rather more than double that which, according 

 to Canon Moseley's calculations, is exerted by gravity in 

 the Mer de Glace, near the Tacul, but ^r^^th only of his 

 smallest value of the shearing force of ice, the amount of 

 shear was actually larger than that implied in any of the 

 ordinary cases of glacier motion, and he concludes that 

 there is little doubt that under conditions closely resembling 

 those of the interior of a glacier, and under the influence of 

 forces comparable with those of gravity, hand specimens of 

 ice shear in the same manner as a truly viscous solid would 

 {Pro. Roy. Soc. xxxviii. 100 — 10 1). 



The observations of Tyndall upon crevasses do not 

 prove that ice is not extensible, but that it is incapable of 

 any appreciable elastic extension before it gives. 



"I believe, therefore," says Mr. Trotter, "that the weight of 



