16 ON THE STRUCTURE AND MOTION OF GLACIERS 
from gh to 7k is again nearly level; from zk to mm inclined, while 
from mn to CD the inclination is less than between z# and mz. The 
section of the bottom of the trough is figured underneath the plan. 
ABEF is a box supported at the end of the trough, and filled with a 
mixture of water and fine pipe-clay. The front, AB, can be raised, 
like a sluice, and the mud permitted to flow regularly into the trough. 
While the mud is in slow motion, a coloured circle, ¢, is stamped upon 
the white clay between AB and ¢/; the changes of shape which this 
circle undergoes in its passage downwards will indicate the forces 
acting upon it. The circle first moves on, being rather compressed, 
in the direction of the length of the trough until it reaches ef, on 
crossing which, and passing down the subsequent slope, it elongates 
as in the figure. Between gf and zk the figure passes through the 
circular form, and assumes that of an ellipse, whose shorter axis 
is parallel to the length of the trough. It is manifest from this that 
the mud between é¢f and gf is in a state of longitudinal tension, while 
between gf and zé-its state is that of longitudinal compression. On 
crossing 2& and descending the second incline, the figure is again 
drawn out longitudinally, while between xz and CD the ellipse 
widens on account of the permission given to lateral expansion by the 
augmented width of the trough. 
The side circles in the same figure will enable us to study the 
influence of lateral friction upon the descending stream. These circles 
are distorted into ellipses, whose major axes are oblique to the direc- 
tion of the trough’s length. Above the line ef central fissures per- 
pendicular to the axis of the trough cannot be formed; for here, 
instead of tending to open into fissures, the flattening of the central 
circle shows that the mud is longitudinally compressed. On the slope 
below é, the distortion of the circles into ellipses is very pronounced ; 
and as the longer axis of each ellipse marks the line of maximum 
tension, and as the tendency of the mass is to form a fissure at right 
angles to such a line, we should have here, if the substance were not 
so plastic as to prevent the formation of fissures, the state of things 
observed upon the corresponding portion of the glacier; namely, 
central fissures perpendicular to the longitudinal axis of the trough, 
and side fissures inclined to the same axis because pointing in the 
direction of the shorter axis of each ellipse. Between gh and 7% the 
longitudinal tension is changed to compression; the central figure 
is flattened, while the side ones remain stretched. In the correspond- 
ing portion of the glacier we should expect the central fissures formed 
between ef and gh to be squeezed together and closed up, while the 
