TWOFOLD CLASSIFICATION OF CREVASSES. 
199 
on which the vis a tergo acts with most advantage ; and in a direction generally 
parallel to the blue bands, so far as they are due to inequalities of motion in the 
horizontal plane*. My earliest attempts to obtain clear views of the internal forces 
acting on a semi-rigid body, impelled by self-contained hydrostatic forces, convinced 
me how little could be founded on the completeness of any mathematical investiga- 
tion of them, which in our present state of knowledge may well be considered as 
hopeless ; and reserving to myself the not so difficult task of extricating at a future 
time the more important practical laws of these strains and thrusts, I very carefully 
avoided, in my first publication, any allusion to what might be considered as their 
actual distribution ; a distribution varying not only from point to point of the glacier 
surface, but throughout its thickness, and most undoubtedly varying also for the 
same point at different seasons of the year, or even changing its sign, so that a ten- 
sion at one season may become a thrust at another. 
I had no reason to repent of this caution, from which I only departed so far in my 
Seventh Letter on Glaciers, published subsequently, as to deduce in an approximate 
manner, from elementary mechanical laws, the directions of the surfaces of tearing 
within such a mass as I had described, upon the simple supposition that the hydro- 
static pressure acting uniformly, the tendency of motion of any particle will be in the 
direction of least resistance when all the resistances are taken into account, and that 
the surfaces of rupture will divide particles whose motions are dissimilar, but will 
not divide particles whose motions are alike. I repeat that I had no reason to repent 
of my abstinence from theorizing, when I found that a far better mathematician than 
myself, taking up the inquiry where I had left it, and after applying himself for a 
long time to the exclusive mechanical considerations which the viscous theory had 
suggested, left the subject, as I conceive, little more advanced than he had found it, 
and fell into some mistakes and inconsistencies, almost inseparable from this way of 
treating a problem which extensive observation and patient thought can alone dis- 
entangle. 
Formation of Crevasses . — It has been seen in the third section of this paper, that 
De Saussure, and almost all his successors, have regarded the crevasses as accidents 
of glacier motion, and not essential to it ; and in this view I of course concur. Never- 
theless, the study of crevasses is one of considerable, though secondary interest, and 
is very far indeed from being completed. It requires, among other things, a very 
sedulous attention to the state of the glacier at various seasons, and even whilst 
covered with snow ; and it requires further a two-fold classification of crevasses, into 
those which may be considered as proper to the mass of the glacier, and those which 
merely seam its surface. 
I will first speak of the last point. 
Though the formation of a crevasse betokens a local distending force, such a force 
cannot with any certainty be referred to the whole depth of the glacier below the 
* See Philosophical Magazine, May 1845, p. 408. 
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MDCCCXLVI. 
