242 Professor Forbes on the Leading Phenomena of Glaciers 



part of both, but will undoubtedly rise towards the surface, 

 and thus slide upwards and forwards over the particles im- 

 mediately in advance. 



• Though I am not aware that this form of fluid motion has 

 been pointed out, its existence is scarcely to be doubted from 

 very ordinary mechanical considerations, and several obvious 

 phenomena also indicate it. Thus, such a viscid stream as 

 we have supposed, be it tar, mortar, treacle, glue, plaster of 

 Paris, slag, or cast metal, invariably presents wrinkles^ or 

 curvilinear arrangements of the floating matter, accompanied 

 by a crumpling or inequality of the surface. These inequali- 

 ties, these lines indicative of motion, which were the very first 

 indications which led me, when studying the " dirt bands," 

 to discover their nature, must be due to inequality of motion, 

 and to no other cause. It is vain to look for any original 

 linear or tabular arrangement of the particles when a fluid 

 is poured from a ladle into a sloping channel, and afterwards 

 becomes modified into the curves we have described. In the 

 scum or froth on a sluggish current of water, there was no 

 original arrangement of particles transverse to the stream, 

 which has become deformed into the elongated loops in which 

 the bubbles arrange themselves, as it were spontaneously, 

 during their progress. These curves are the direct result of the 

 unequal mutual pressures of the particles ; and the whole phe- 

 nomena, in the case of any of the semifluids I have mentioned, 

 are such as — combined with the evidence which I have given, 

 that the motion of a glacier is actually such as I have described 

 that of a viscid fluid to be — can leave, I think, no reasonable 

 doubt, that the crevices formed by the forced separation of a 

 half rigid mass, whose parts are compelled to move with differ- 

 ent velocities, becoming infiltrated with water, and frozen 

 during winter^ produce the bands which we have described. '^ 



* The wave-like figures of floating matter on a sluggish surface are not at all 

 to be confounded with the actual direction of motion of the fluid particles. They 

 are curves of differential velocity merely, and are always most perceptible near 

 the sides of the stream, where the variations of velocity are greatest. A stream, 

 like a mill-race, covered with saw-dust, will shew these linear markings inclin- 

 ing towards the centre of the stream ; but the motion of any floating body, as a 

 bit of cork; is sensibly parallel to the sides. I have proved the tame thing by 



