48 
POPULAR SCIENCE REVIEW. 
Mr. Hopkins, that the lines of greatest tension in a glacier make 
an angle of 45° with the side of its valley ; along these lines 
(shaded in fig. 3), the ice is most severely strained, and as a 
necessary consequence its tendency to break is greatest in a 
direction perpendicular to them. 
Fig. 2. . 
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/111 2 ' 
Fig. 3. 
In fig. 3, which is a sketch in plan of part of the Lower 
Grindelwald Glacier, the arrow shows the direction of the 
motion ; the lines on each side, parallel with the barbs of the 
arrow, indicate the direction of the strain ; whilst the blacker 
lines across the former show the marginal crevasses. In some 
places in the drawing the crevasses are seen to stretch quite 
across the figure. This is caused by a sudden change of 
inclination in the bed of the glacier ; the ice, being unable to 
pass over the bend without rupture, snaps across, and deep 
transverse crevasses are the result. The union of these with 
the marginal crevasses creates in certain places continuous 
fissures, which swoop in immense curves across the glacier, 
the convex side of the curve being always turned upwards. 
The formation of the transverse crevasses is seen in fig. 2, 
each portion of which represents in section the parts shown 
directly beneath in plan. As the leaves of a book open when 
its back is bent, or as a stick snaps when strained across 
