702 
ME. HOPKINS ON THE THEOEY OE THE MOTION OF OLACIEES. 
25. Let fig. 2 represent the section of a glacier by a plane parallel to its surface, and 
fig. 3 a vertical section through A x the axis of the glacier. Let A a:" be taken as the 
Fig. 2. 
Section parallel to the surface. 
axis of iT, and the surface of the glacier (nearly horizontal) as the plane of xy^ a trans- 
verse line perpendicular to A .r being the axis of and a perpendicular to the surface 
of the glacier the axis of z. Let Q' K' (fig. 2} represent a section, by a plane parallel to 
that of xy^ of an element of the mass lying between two planes perpendicular to the axis 
oi X', Q' E,', by the more rapid motion of the axial parts of the mass, will be brought into 
the position Q E. Also if Q' E' (fig. 3} represent a section of the same element by a 
vertical plane parallel to that of xz, Q' E' will be brought into the position Q E by the 
more rapid motion of the upper surface of the mass. Also the small elementary paral- 
lelepiped whose section parallel to (xy) is represented by P' y' r' s' in fig. 2, will be brought 
into the position 1^ qrs, while the section F'y'r's' (fig. 3) of the same element made by a 
Fig. 3. 
Vertical section. 
plane parallel to (xz) will be brought to the position P r s. Thus we see that there 
will be an angular distortion oiV qrs about an axis parallel to s, as represented in fig. 2, 
and a similar distortion about an axis parallel to y, as represented in fig. 3. These 
angular distortions are respectively due to the couples whose intensities are represented 
by F and E (art. 15). Also it is easily seen that there will be no twisting or angular 
distortion of P 5 about an axis parallel to that of x, and that consequently D will =0 
in all such glaciers as we are now considering. A becomes a longitudinal tension, and 
B a transversal one. C will be the pressure on the element parallel to z, and due to the 
superincumbent weight, and the inclination of the surface of the glacier to the horizon. 
26. The intensity of A and B will depend much on the form of the glacial valley. If 
the sides be parallel, A may be a tension or pressure according to the local variations in 
