on the Motion of Glaciers. 



409 



motions of a common fluid, as determined by my experiments 

 and models, the lines of separation are almost mathematically 

 ferpendicular to the open crevasses, which are convex towards 

 the origin in fig. 2, which represents the lines of relative mo- 



Fig. 2. 



tion of a viscous fluid, reduced to the horizontal plane. Such 

 I have also found to be the case in lava streams. Hence it is 

 plain that when the horizontal forces are alone considered (as 

 in all Mr. Hopkins's reasonings), the statement or empirical 

 law is even more accurately true than I ever alleged it to be*. 

 In the second place, Mr. Hopkins has not had sufficient re- 

 gard to accuracy in stating facts so as to support his own views, 

 or to invalidate mine. I will not return to the question of 

 the verticality of the transverse crevasses, which has been an- 

 swered sufficiently ; but 1 will take a more striking instance, 

 lying at the very root of the matter. In the Phil. Mag. for 

 March, p. 248, Mr. Hopkins affirms that "no experiments 

 have been adduced to show that the plasticity of glacial ice is 

 really greater than common inspection would lead us to sup- 

 pose" (that is, quite insignificant); and yet in the next page 

 he admits that the velocity of the centre of a glacier sometimes 

 is double that of its lateral parts ; and he does not deny that 

 this is the result of plasticity, but he argues that it is not stiffi- 

 cient to explain the more rapid motion of the surface than the 

 bottom. It comes therefore, as Mr. Hopkins has in more 

 than one place admitted, to a question of deforce', and if the 

 degree of plasticity be sufficiently great, Mr. Hopkins will 

 admit it to be the caiise of motion^', but half iXie total motion 

 is not a sufficient fraction in Mr. Hopkins^s estimation. But 

 could Mr. Hopkins be ignorant of the fact stated in the second 

 paragraph of my Eighth Letter on Glaciers, that I had ob- 

 served points in the transverse section of the glacier of Aletsch 

 only 1000 feet apart, which moved with the relative velocities 



* There are circumstances in which the crevasses of glaciers approach 

 the forms of figure 1, but in which the combination of several systems pro- 

 duces a general upward convexity. These I shall explain in another place. 



t Phil. Mag., February 1845, p. 168. 



