the Hintereis Glacier. 159 



21*5 metres per annum, or 0*000068 cm. per sec., and ,the 

 slope to be 9 in 100, which corresponds to an average 

 shear force across the central two-thirds of the glacier bed 

 of 0'84 kg. per cm. 2 . Our experiments with the glass 

 bar oive a cutting velocity of 30 mm. in 24 hours, or 

 0*00000347 cm. per sec. under a pressure of 2*06 kg. 

 per cm. 2 ; so that, if the resisting area of the asperities 

 on the glacier bed amounted to 1 cm. 2 per 1 cm. 2 of bed, 

 the slip would be, using these figures as a basis, 



0-84x0-00000347 Arwwu , 1 , 1 

 cr?^" = 0-00000141 cm. sec. 



The actual rate of movement being 0" 00006 8 cm. per 



141 



sec, the asperities must, on this basis, amount to -77——- = 



obOO 



0*0207 cm. 2 per cm. 2 of bed, and their angle must be 

 tan" 1 (4x 0-0207 = tan" 1 0*0828 = 4° 44'. 



Small asperities having angles of 4° 44' would produce 

 an average roughness which cannot be considered as much 

 rougher than a glacier-bed. With the temperature at the 

 freezing-point the angle would be somewhat less. The 

 theory, therefore, gives results which are of the proper 

 order of magnitude. 



The very small irregularities of the glacier-bed are not, 

 of course, the only hindrances to motion. The glacier has 

 to change its form to accommodate itself to the bends and 

 larger irregularities of its channel. Here the resistance 

 to flow is a purely viscous one. Boulders, grit, and fine 

 material are embedded in the lower surface of the ice, 

 and this material is not held firmly in the glacier surface. 

 The viscous yield of the ice, as well as the thawing and 

 freezing due to varying pressure, allows the boulders &c. 

 to move in the ice surface as they score the rock below. 

 When the ice enters crevices and hollows it must be actuallv 

 sheared. 



We have called attention to the fact that the glacier 

 is held down by its weight, and the asperities are compelled 

 to cut their way through the ice surface. Now if the 

 angles of the asperities were all 4° 44', and the slope of 

 the glacier were much greater than this angle, there would 

 be a tendency for the glacier to slip forward, especially if the 

 surfaces of the asperities were smooth. In the case of the 

 Hintereis Glacier the slope at section V. is 5° 9', or a little 

 steeper than the figure obtained for the asperities. 



