1846.] RELATIVE MOTIONS OF SURFACE AND BOTTOM. 187 



lateral face of a glacier, is open to the possible objections which 

 I anticipated in my former letter, and which I carefully endea- 

 voured to avoid (see p. 173 of the Eleventh Letter, and fig. 2 of 

 Plate VI.), so that even the most scrupulous might be, if possible, 

 satisfied. It is pleasing to me to find that the French observers 

 corroborate my result ; but I must remark that they obtained it a 

 fortnight later, and my publication preceded theirs nearly a month. 



I shall not seem to insist too much upon the collusiveness 

 of this observation, when it is recollected that an opponent of 

 the Viscous Theory has virtually staked the question of the cause 

 of glacier motion upon such an experiment (notwithstanding that 

 I think he has attached to it an undue importance), in the fol- 

 lowing passage : 



* * The claims of the two theories (the sliding theory 

 and that of plastic motion) will undoubtedly be determined by 

 other means. The observations required are such as shall de- 

 termine, as far as possible, the motions of the upper and lower 

 surfaces of a glacier. We may never hope to have access to the 

 bottom of a glacier in its deeper portions, but at the extremities 

 of glaciers the amount of sliding may easily be ascertained, as 

 well as at many other points, probably, if sought for, along their 

 flanks ; fissures also, of considerable depth, are not unfrequently 

 met with, in which the deviation from verticality, if it exist, 

 might be easily determined ; and though the evidence thus ob- 

 tained might not afford positive demonstration with respect to 

 the deepest portions of a glacier, still, should it all concur in 

 showing an approximate equality in the motions of the upper and 

 lower surfaces, every candid and impartial mind must admit, I 

 conceive, the sliding in preference to the viscous theory; but if, 

 on the contrary, it should be proved that the velocity of the 

 upper bears a large ratio to that of the lower surface, the claims 

 of the latter theory must be at once admitted."* It is very 

 fortunate that independent observers on different glaciers should 

 have arrived, in ignorance of each other's results, at conclusions 

 which permit only the alternative favourable to the viscous 



* Mr. Hopkins, in London Philosophical Magazine, March 1845, p. 250. 



