MECHANICS OF GLACIERS. 



65 



the ice parts asunder in a number of planes, forming an equal num- 

 ber of crevasses, which proceed with gradually diminishing width 

 from the extreme lateral limits of the glacier towards the centre — 

 not, however, in a strictly transverse direction, but in that of the 

 tangent to the direction of the strain, tending upwards therefore to- 

 wards the source of the glacier. Finally longitudinal crevasses are 

 formed when a glacier has to force itself through a narrow gorge. 

 As it emerges from the gorge, the central portions move on faster 

 than the lateral portions, which are retarded by the sides ; and that 

 portion of energy (even here where ' the action against the rocky 

 sides is at a maximum) which is expended in parting the middle 

 portions from, and producing friction of them within the gorge 

 against, the lateral portions, cannot be expended at the same time 

 upon the work of erosion. Generally, we may say that the origin of 

 all these varieties of crevasse is the same property of glacier-ice 

 which makes it unable to yield to tensile force ; and the consequence 

 is, in each case, a breaking-up, more or less, of the glacier-mass, 

 and the consequent distribution of its force as a moving body. 



The whole weight of any given mass of the glacier may be re- 

 solved into two forces, the one acting parallel, the other at right 

 angles, to the inclined plane on which the glacier lies*. The former, 

 which will vary with the sine of the angle of inclination, and will 

 therefore be nil when the glacier rests on a horizontal bed, is, as 

 has been shown, partly used up within the glacier ; and the portion 

 thus used up, whatever it may be, is not exerted against the rocky 

 floor, and therefore can do no work in the way of erosion. The ice 

 moves on this floor, if it be inclined at a sufficient angle ; but it moves 

 with less velocity at its bottom than the centre of gravity moves. 



* The relation which subsists between the angle of inclination of the slope 

 on which a given mass of a glacier lies, and the pressure and shoving force due 

 to the weight of the given mass, will be made clearer by the following simple 

 mathematical reasoning. Suppose a given glacial mass to lie on a slope, repre- 

 sented in the accompanying 

 diagram by a line AA', which 

 makes an angle 9 with the hori- 

 zontal ; and let us suppose the 

 whole weight of the given mass 

 to be represented by one resul- 

 tant force W, acting vertically 

 through its centre of gravity, 

 upon the point P, as indicated 

 by the arrow. By a simple 

 " triangle of forces " it is easy 

 to see what parts of the weight 

 W are represented by the forces 

 acting (1) as pressure in the 

 direction of the normal P^ 

 upon the surface AA' at P, (2) as a shoving force parallel to AA'. For the first 

 we have 



WX 



Pry 

 Pr 



for the second, 



Q. J. G. S. No. 153. 



Vr 



W X cos 9 ; 



W X sin 0. 



