338 
PEOFESSOES TT^sDALL AED HUXLEY OX 
the property of ice described in § 3, the resemblance between the motion of a substance 
like mud and that of a glacier is so great, that considerable insight regarding the 
deportment of the latter may be derived from a study of the former. From the manner 
in which mud yields when subjected to mechanical strain, we may infer the manner 
in which ice would be solicited to yield under the same circumstances. 
To represent then the principal accidents of a glacial valley, a wooden trough, 
ABCD, fig. 8, of varying width and incliaation, was made use of. From A to C the 
Eig. 8. 
trough measures 6 feet, and from A to B, 15 inches. It is dmded into five segments ; 
that between AB and ef is level, or nearly so, that between ef and gh is inclined ; from 
qli to ik is again nearly level ; from ik to mn inclined, while from mn to CD the inclina- 
tion is less than between ik and mn. 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. "\^Tiile the mud is in slow 
motion, a coloured circle, c, is stamped upon the white clay between AB and ef; the 
changes of shape which this circle undergoes in its passage doumwards will indicate the 
forces acting upon it. The circle first moves on, being rather compressed, in the direc- 
tion 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 gh and ik the figme 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 betAveen ef and gh 
is in a state of longitudinal tension, while between gh and ik its state is that of longitu- 
dinal compression. On crossing ik and descending the second mcline, the figme is again 
drawn out longitudinally, while between mn and CD the ellipse AAudens on account of 
the permission given to lateral expansion by the augmented Avidth of the trough. 
The side circles in the same figure Avill enable us to study the influence of lateral fic- 
tion upon the descending stream. These circles are distorted into ellipses, AA’hose major 
axes are oblique to the direction of the trough’s length. Above the line ef central 
fissures perpendicular to the axis of the trough cannot be formed ; for here, instead of 
