154 Messrs. R. M. Deeley and P. H. Parr on 



It is clear that the slip of a glacier is due to the force of 

 the downward component of gravity of the ice, acting all over 

 the glacier-bed, and it is important to ascertain how this force 

 is distributed across the section. The rate of shear in close 

 proximity to the glacier-bed is proportional to this force. 



Fig. 1 shows an ellipse with the surface-velocity curve 

 above, and also the lines of equal velocity within the ice. 

 Fig. 2 is the same ellipse with the lines of equal shear force. 



Fig. 3 shows a form of Parr's curve, similar to the Hintereis 

 Glacier profile at Section V., with the surface- velocity curves 

 and the lines of equal velocity within the ice. Fig. 4 shows 

 the lines of equal shear force within the glacier, across the 

 glacier bed and upper surface, all expressed in kg. per cm. 2 

 (See Appendix.) 



In these diagrams the viscous substance is regarded as 

 adhering to the walls of the channel. 



These curves show how very greatly the conditions of 

 stress and rate of shear differ in a channel of the form of 

 Parr's curve as compared with those in an elliptical channel. 



Slip of Glacier Ire. — In our paper previously communi- 

 cated to this Journal we write : "Very little is known con- 

 cerning the coefficient of friction of glaciers upon their beds. 

 Indeed, it is not clear why a glacier slips at all upon its bed 

 when we consider how slight the slope generally is." Under 

 the pressure to which the bottom ice is subjected it must 

 penetrate into every crevice and irregularity of the rock below, 

 and the interlocking of ice and rock might be considered as 

 tending to prevent any slip at all. 



The surface motion of glaciers has been found to be the 

 sum of two movements, one a bodily slip of the whole mass 

 of ice on its bed, and the other a result of the differential 

 motion of the ice of the glacier. The differential motion 

 which results in shear without fracture, and gives to a glacier 

 its peculiar structure, has been the subject of much discussion 

 and investigation. With regard to the nature of the slip little 

 is known. The grooving, scratching, and polishing effected 

 by glaciers might, at first sight, be regarded as due to a form 

 of slip similar to that which occurs between a surface of lead 

 into which emery has been pressed and the hard surface which 

 it grinds. A closer examination of the phenomena shows that 

 this resemblance is only partially true, for the cutting material 

 is not held firmly in the ice, as is the case with emery and 

 lead, and the ice, unlike the lead, penetrates into all the small 

 irregularities of the rock surface upon which it rests. 



In the case of a glacier the rate of shear at the slip surface 

 between the rock and ice is generally slow and steady, and 



