116 VISCOUS THEORY OF GLACIER MOTION. [1845. 



a vertical plane.* I observed, however, a curious fact, plainly 

 indicated by the numerical results ; the curve crossed the axis 

 at the fourth pin, and attains its greatest convexity at the 

 twenty-fifth, "f 



Fourthly. That no information might be wanting as to the 

 precise condition of the mass of ice under experiment, I made 

 a very minute examination of the state of the transverse line 

 with respect to the occurrence of flaws in the ice. The most 

 important of these was one which returned into itself, crossing 

 the line towards the origin of the glacier between the twenty- 

 sixth and twenty-seventh marks, and returning backwards 

 between the fortieth and forty-first without extending further 

 upwards. Such a flaw, even if devoid of cohesion, could only 

 act by allowing the piece of ice, contained between the twenty- 

 seventh and fortieth mark, to slip bodily forwards, leaving a 

 vacuity behind. No such slip is recorded in the observations, 

 and no such gap or vacuity was left during the continuance of 

 the observations for seventeen days. Nothing approaching to 

 an open fissure occurred in any part of the space of ice under 

 observation, and the few flaws noted (see the column of remarks 

 in the Table, p. 112, and the indications of them by dotted lines 

 and arrows in Plate IX. fig. 2 [of Phil. Trans.], showing their 

 directions) were perfectly close, and all more or less of zigzag forms, 

 preventing the possibility of sliding. The veined structure was 

 developed in every part of the section, in some parts more admi- 

 rably than others, as near the seventh mark, and between the for- 

 tieth and forty- fifth. These countless blue veins may be con- 

 sidered as so many flaws or partial solutions of continuity, existing 

 or having existed ; they are almost perfectly straight, and (as will 

 be shown immediately) exactly in the direction in which the rela- 

 tive motion of the parts of the ice demonstrated by these 

 experiments takes place. It would require strong evidence to 

 convince us that these veins are not occasioned by, and the 

 mechanism of, the plastic motion of the ice. 



* An approximation to this effect will be obtained by stretching a fine thread 

 over the figure. 



f From my notes made at the time. 



