350 Pi'oft'ssur Furbes' Account of his recent 



rison of the two classes of facts, to decide upon the cause of 

 movement. 



What I have hitherto stated is matter oifact. I will state 

 very briefly what 1 am disposed to deduce by way of hypo- 

 thesis. 



It is impossible to consider these structural bauds on the 

 surface of the glacier, in combination with the fact established 

 in my former letters, that the centre of the glacier moves con- 

 siderably faster than its edges, without believing that the 

 bands are an indication of the motion, and that the motion 

 gives rise to the veined structure. These dirt-bands perfectly 

 resemble those of froth and scum which every one has seen 

 upon the surface of slowly-moving foul water; and their figure 

 at once gives the idea of fluid ^notion, freest in the middle, 

 obstructed by friction towards the sides and bottom. It will 

 be found that the analogies are entirely favourable : the gla- 

 cier struggles between a condition of fluidity and rigidity. It 

 cannot obey the law of semifluid progression (maximum velo- 

 city at the centre, which is no hypothesis in the case of gla- 

 ciers, but a fact), without a solution of continuity perpendicu- 

 lar to its sides. If two persons hold a sheet of paper, so as to 

 be tense, by tlie four corners, and one moves two adjacent cor- 

 ners, whilst the other two remain at rest, or move less fast, 

 the tendency will be to tear the paper into shreds parallel to 

 the motion ; in the glacier, the fissures thus formed are filled 

 with percolated water which is then frozen. It accords with 

 this view, 1. That the glacier moves fastest in the centre, 

 and that the loop of the curves described coincides (by ob- 

 servation) with the line of swiftest motion. 2. That the 

 bands are least distinct near the centre, for there the dif- 

 ference of velocity of two adjacent stripes parallel to the 

 length of the glacier is nearly nothing ; but near the sides, 

 where the retardation is greatest, it is a maximum. 3. It 

 accords with direct observation (see my last Letter), that the 

 difference of velocity of the centre and sides is greatest near 

 the lower extremity of the glacier, and that the velocity is more 

 nearly uniform in the higher part ; this corresponds to the 

 less elongated form of the loops in the upper part of fig. 5. 

 4. In the highest part of such glaciers., as the curves become 



