272 PROPESSOE TYHDALL’S OBSEEVATIONS OM THE HER DE GLACE. 
Before enterinR upon the examination of this theory, I would ask permission to make 
the remarks .-I am aware that the paper puhUshed by Mr. Hr™d 
self has produced considerable diversity of opinion among scientific men Some 
opinions are entitled to every respect, regard the views there advocated, and the expe- 
laments there described, as consistent with and explanatory of the viscous theo y 
while others, of equal eminence, believe that if the news referred to be sound, the 
viscous theory can no longer be maintained. Under these 
to state distinctly the point of view from which I intend to examme the theory, submit 
ting myself completely to the public sense as to whether this point of new be the conec 
one or not. Both the terms and the illustrations made use of by Professm- Fobbes have 
difiused ideas regarding the physical quaUties of ice which render a stact 
of the subject essential. Let me here briefly state what I understand by nscosity 
what I, and other more competent persons, at one time beheve to e a emons ra 
B^vTsclity, I understand that property of a semifluid body which permits of its being 
drawn out when subjected to a force of tmdmi, the particles of the substance tak 
up new positions of equilibrium, so that when relieved from the stram the substance has 
no distortion to recover from. A capacity to change the form under crushing 
is not, I think, a test of viscosity; for this power is possessed by substances, 
we should never think of applying the term VISCOUS. ^ 
In examining whether glaciers possess the power of yielding to tension like viscous 
bodL l woulf refer :_1. To the shifting of the place of strain by the ciuvatin-e of the 
valley’ to which I have already referred. Let ABCD, fig. 3, embrace a curved portion 
of a glacial valley, and let AB be a linear element of the glacier transverse to its axis. 
Fig. 3. 
As the ice descends AB becomes curved in consequence of the quicker niovement ot its 
centre. Did the valley continue straight in the direction of E and F, the point of mm - 
mum velocity would, after a certain time, be found at a, midway betiveeii tie 
BF ; but the curving of the valley throws the point u to a , and thus ‘ 
upon the branch «'A' of the curve, while it diminishes the strain upon a B . 
persons. 
