58 



SEVENTH LETTER ON GLACIERS. 



[1844. 



resistance, will be no longer A B, but inclined (for the particle 

 a) in the direction A' B'. 



If this reasoning require support from experiment, it is 

 easily had. I have described, in a foot-note to my last chapter, 

 the experiment of dusting powder upon a moving viscous 

 stream; and our friend Heath has now a specimen of the 

 result, shewing the lines of separation in the direction I have 

 stated. The same is remarkably shown in the case of a stream 

 of water, for instance, a mill-race. Although the movement of 

 the water, as shown by floating bodies, is exceedingly nearly 

 (for small velocities, sensibly) parallel to the sides, yet the 

 variation of speed from the side to the centre of the stream 

 occasions a ripple, or molecular discontinuity, which inclines 

 forward from the sides to the centre of the stream at an angle 

 with the axis, depending on the ratio of the central and lateral 

 velocities. The veined structure of the ice corresponds to the 

 ripple of the water, a molecular discontinuity whose measure is 

 not comparable to the actual velocity of the ice ; and, therefore, 

 the general movement of the glacier, as indicated by the 

 moraines, remains sensibly parallel to the sides.* 



(2.) If I have explained myself distinctly as respects the 



Fig. 14. 



fissures produced by lateral friction, there will be little difficulty 



* I have lately identified completely the planes of separation in the lava 

 streams of Etna, which correspond perfectly to those of the glacier, heing nearly 

 vertical at the sides, and directed slightly towards the centre of the stream. 



