710 
ME. HOPKINS ON THE THEOET OF THE MOTION OF GLACIEES. 
disappearance of all traces of them, and the resumption of complete crystalline con- 
tinuity in those parts of the mass in which these crevasses had previously existed, 
remained a difficulty of which no physical explanation had been given till the experi- 
ments of Dr. Tyndall afforded so happy a solution of it by proving that the renewed 
continuity was attributable to regelation resulting from the renewed contact of portions 
of the ice which had been previously separated. 
34. Formation of Crevasses in the deeper portions of a Glacier . — The formation of 
crevasses in this position must ever perhaps remain a matter of speculation ; for we can 
have little hope of making their existence a matter of observation. The general direc- 
tion of maximum tension at any point of a glacier at a great depth, cannot be exactly 
determined, because the general equations (c.), art. 16, cannot, for this case, be gene- 
rally solved. It will be easy, however, to determine this direction in terms of the 
internal forces for any point in the vertical plane through the axis of the glacier, and, 
consequently, the section made by that plane with the surface to which the direction of 
the maximum force is a normal — the surface along which the crevasse would be formed. 
For this purpose let hg. 5 represent the vertical section through the axis of the glacier. 
Fig. 5. 
The undistorted element Q' R' will be brought by the more rapid motion of the surface 
into the position Q R, as described in fig. 3. The internal forces acting on the small 
element pqrs will be A, B, C, and E, the latter producing the couple whose axis is 
parallel to that of y. It is manifest, however, from the conditions of symmetry, that the 
dhection of the maximum tension must lie in the vertical plane through the axis of the 
glacier, and will not be afiiected by B, which, as in other cases of transverse fissures, 
must be supposed either a pressure or a comparatively small tension. Hence, putting 
— C for B and E for F in our formulae (e'.), art. 20, we have 
tan 2 a ^ ^ * 
The angle d is indicated in fig. 5 (see art. 28 and fig. 2). Near the surface, E and 
therefore os, vanish, and the crevasse, if formed, will be vertical. At lower points of the 
mass os wiU increase with E, and the vertical section of the crevasse will resemble the 
curve M N in the figure. 
