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



If it were not that the effects of slip assist viscous flow to 

 produce surface stresses, there would be less strain near the 



Kir. 11. 



Crevasses and Contour-lines on the Hintereis Glacier. 

 Heights in metres. 



sides to produce crevasses than is actually the case. How- 

 ever, by taking the actual velocity curve b, fig. 3, as we 

 have done in the above calculations,, the locus of the actual 

 greatest shear force is dealt with. 



The calculation shows that although a strain is set up in 

 the ice by shear, the tensile stress at an angle of 45°, which is 

 equal to the shear stress, it is not sufficiently great to produce 

 crevassing ; for ice will not rupture under a load of less than 

 2S'0 kgs. per cm. 2 , or about thirty times as much as the 

 shear stress gives rise to. 



The considerations which led Hopkins to promulgate his 

 theory are as follows : — 



Unit area of surface in moving down the glacier under- 

 goes compression in one direction and tensile stress in 

 another. If the longitudinal and the corresponding trans- 

 verse shears be / per unit area, then the stresses on any 

 section inclined upwards at an angle 6 with the side of the 

 g;lacier evidently consist of 



(a) a tension p = 2/ cos 6 sin 6 



=/sin 20, 



(b) a shear q ==/'(cos 2 6 — sin 2 6) 



=/cos 26. 



Crevasses may be considered to be produced by tension, 

 so we have to consider the variation of tensile stress with 

 angle. 



