1844.] SURFACES OF DISCONTINUITY IN A VERTICAL PLANE. 59 



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 

 fall of the bed prescribe. Let L M be the surface, N the 

 bed of a glacier ; then the resolved force is usually considered 

 as acting on the particles m n, in the directions m m ', n ri , 

 parallel to the bed. But if we reflect that, owing to the length 

 of the glacier, and the toughness or consistency in its mass, the 

 resistance of the line of particles n v is enormous, the plane of 

 complete resistance N will virtually be twisted in the direc- 

 tion W 0', and the particle tends to be thrust forwards and 

 upwards, which will evidently produce the frontal dip. 



(3.) Bat there is a peculiarity in the vertical plane which 

 did not exist in the horizontal one. In the case we first con- 

 sidered, the veined structure exists almost entirely in the neigh- 

 bourhood of the sides of the glacier, and is lost towards its 

 centre, being due to the influence of friction, which varies with 

 the distance from the side; the central part, efgh (fig. 11), 

 moving nearly uniformly, would cease to exhibit a linear ar- 

 rangement. The completion of the curve is due to the influence 

 of the curvilinear bottom, combined with the opposing mass of 

 the glacier in front ; and this 

 influence will extend to the very ^^^ , 



surface, as a little consideration 

 will show. For, resuming the 

 construction of fig. 14, since a 

 vertical series of particles, m 1 . . 

 -m 6 (fig. 15) are supposed to be 

 acted on by a force partaking of 

 the nature of hydrostatic pres- 

 sure, derived from a great eleva- 

 tion, each particle is ready to Kg. 15. 

 move onward in the direction in which the effective pressure 

 is greatest ; and it is plain, that, owing to the diminishing rela- 

 tion between the weight of the superincumbent particles and 



