Professor Forbes' s Seventh Letter on Glaciers. 



247 



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

 the bed of a glacier ; then the resolved force is usually con- 



Fig. \. 



sidered as acting on the particles m w, in the directions m m\ 

 n n\ 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 nv is enormous, 

 the plane of complete resistance N O will virtually be twisted 

 in the direction N' 0', and the particle tends to be thrust /<?r- 

 wards and upwards, which will evidently produce the frontal 

 dip. 



(3.) But 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 

 neighbourhood 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, efg h 

 (Fig. 1.), moving nearly uniformly, would cease to exhibit a 

 linear arrangement. 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 shew. 

 For, resuming the construction of Fig. 4, since a vertical series 

 of particles, m^ , , m^ (Fig. 5) are supposed to be acted on by a 

 force partaking of the nature of hydrostatic pressure, derived 

 from a great elevation, each particle is ready to move onward 



