100 : W. Mathews on Glacier-motion. 
3. This differential motion can only take place by the process 
which, in mechanics, is known by the name of shear. 
e resistance which ice offers to shearing, or its shearing. 
force, as ascertained by experiment in the shearing-apparatus 
devised by Canon ve a is not less than 75 lbs. per squareinch, _ 
5. But in order that the Mer de Glace may descend by its 
own weight, at the rate at which Professor Tyndall observed it 
descending at the Tacul, its shearing-force per square inch cal: — 
not be more than 13193 Ib. 
I propose in the present communication to examine thee — 
propositions. : 
he first has been challenged more than once in the coutse 
of the controversy, without eliciting any rejoinder from Canon — 
Moseley, no doubt from the absence of any materials available 
for the support of the hypothesis. The fact is, that while we 
ave numerous observations of the absolute motion of various — 
ao of the surfaces of glaciers, observers do not ee : 
’ 
; 
‘ 
‘ 
q Le 
which might safely be disregarded in a determination of aver’ 
age daily velocity becoming serious when relative and not abso- 
the glacier has melted away from the stakes, and from the con 
stant tendency of the stakes to heel over to the southward 
consequence of their heated faces enlarging the holes ©" — 
direction of the sun. 01 
During a short tour in the Alps in the autumn of Ee 
attempted, in concert with my friend, Mr. A. A. Reilly, t0 ‘te 
some observations upon differential motion, and selected, Te : 
side of the Great Aletsch Glacier as the field of our operation’ 
By means of a well defined station on the right b : 
was eleven yards from 1; and the remaining four stakes Ve 
p ne 
