ON THE STRUCTURE AND MOTION OF GLACIERS 15 
Before attempting to show the connexion between this theory and 
the case at present under consideration, a mode of experiment may 
be described which was found to assist in forming a conception of the 
mechanical conditions of a glacier, and which has already been 
resorted to by Professor Forbes in demonstration of the viscous 
theory. Owing to the property of ice described in § 3, the resem- 
blance between the motion of a substance like mud and that of a 
glacier is so great, that considerable insight regarding the deport- 
ment of the latter may be derived from a study of the former. From 
the manner in which mud yields when subjected to mechanical strain, 
a 
we may infer the manner in which ice would be solzcited to yzeld under 
the same circumstances. 
To represent then the principal accidents of a glacial valley, a 
wooden trough, ABCD, fig. 8, of varying width and inclination, was 
made use of. From A to C the trough measures 6 feet, and from 
Ato B 15 inches. It is divided into five segments; that between 
AB and é¢f is level, or nearly so, that between ef and g/ is inclined ; 
vertical line shall move laterally with the same velocity; but if they do not, the cohesion 
between them will be weakened or ruptured. The pressure will produce new contact, and if 
the new contact have a cohesive value equal to that of the old, no cleavage from this cause can 
arise. The relative capacities of different substances for cleavage, appears to depend ina great 
measure upon their different properties in this respect. In butter, for example, the new attach- 
ments are equal, or nearly so, to the old, and the cleavage is consequently indistinct; in wax 
this does not appear to be the case, and hence may arise in a great degree the pérfection of 
its cleavage. The further examination of this subject promises interesting results. 
