of the Descent of Glaciers by their Weight only. 1 45 



in other directions than that in which it is sheared. The forces 

 necessary to cause a glacier to descend on the pressure theory- 

 are therefore those necessary to cause it to shear plus other forces 

 necessary to complete its granulation ; and as its weight has been 

 shown to be insufficient to cause it to shear, as it ^actually does 

 shear, a fortiori they are insufficient by granulation to mould it. 



Mr. Matthews's experiment would moreover require to be 

 greatly varied to bring it into analogy with the case of a glacier. 

 If a glacier is supposed to descend by bending, the bending 

 must be supposed to be in the plane of its surface or in a direc- 

 tion parallel to that plane, and not perpendicular to it, as it 

 was in the case of the ice-plank. To make his experiment 

 apply to the case of the glacier, Mr. Matthews should have 

 placed his plank on the two bearers, not flatwise, but edgewise. 

 Indeed, looking at the proportion of the length to the width of 

 a glacier, and considering that it is in the direction of its length 

 that the bend must take place if it descends by bending, the 

 plank ought rather to have been placed vertically on one of its 

 ends, and then it should have been observed whether a deflection 

 showed itself endwise by lines on its surface similar to those of 

 the veined structure and dust-bands in glaciers ; and even if 

 these appeared, the analogy would not have been complete unless 

 they had been found also to exhibit themselves when the ice- 

 plank, instead of resting vertically on one of its ends, had been 

 inclined at an angle of 4° 53' to the horizon, being the inclina- 

 tion of a part of the Mer de Glace. 



Of Mr. CrolPs theory of the descent of glaciers I will speak 

 when I advocate my own. With reference to my present argu- 

 ment, it is sufficient to say that it agrees with my theory in this, 

 that it attributes the descent of glaciers to the work of their 

 weight plus the work of the heat they are constantly receiving 

 from without*. It acknowledges, therefore, the insufficiency of 

 their weight alone to cause them to descend, which is my pre- 

 sent argument. 



In Professor TyndalPs experiment, solid ice was crushed and 

 moulded by pressure ; and in his theory the moulding of the 

 solid ice of a glacier into its channel and its continuous motion 

 along it, like that of a liquid, is accounted for by the pressure of 

 its weight. 



In Professor Forbes' s theory, the insufficiency of its weight 

 so to mould solid ice and to give it this continuous motion like 

 that of a liquid is implied, and it is supposed to be a semiliquid. 

 It is its weight which, according both to the pressure theory 

 and the viscous theory, is the sole cause of its descent : the 



* Phil. Mag. September 1870, p. 159. 

 Phil Mag. S. 4. Vol. 42-. No. 278. Aug. 1871. L 



