246 Professor Forbes's Seventh Letter on Glaciers. 



Or view the matter thus — the movement of the ice stream 

 (considered just now solely as respects its surface), is eifected 

 against a varying resistance. The line of particles in the 

 direction a a presents a greater force of opposition to the 

 movement of the particle a, than the line of particles b (3 pre- 

 sents to the movement of b. This is owing to the lateral 

 friction acting more powerfully in retarding the first than the 

 second ; consequently the virtual w3i\\ of the glacier, or plane 

 of complete 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 shewn in the case of a stream 

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

 of the water, as shewn 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 mea- 

 sure 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 

 fissures produced by lateral friction, there will be little diffi- 

 culty in applying the same reasoning to the resistance of the 

 frontal dip, exhibited in the second figure of this letter. When 

 a fluid, or semi-fluid, is very viscous, there is a great resistance 

 to its onward motion in the direction which gravity and the 



* have lately identified completely the planes of separation in the 

 laya streams of Etna, which correspond perfectly to those of the glacier, 

 being nearly vertical at the slides, and directed slightly towards the 

 centre of the stream. 



