PEOFESSOE TTISTDALL’S OBSEEVATIONS ON THE MEE DE OLACE. 
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§ 3. On the Cause of Glacier Motion. 
The various theories which have been advanced to account for the progression of 
glaciers are too well known to need detailed discussion here. Saussuee, and some 
before him, thought that the glacier slid along its bed*. CsAEPEiirTiER thought that 
the motion was due to the freezing of water in capillary fissures, and the consequent 
swelling of the contents of these fissures. Other hypotheses have been advanced with- 
out producing any deep impression. It has been objected to Saussure’s theory, that were 
it true, glaciers must slide down with an accelerated motion ; but reflection alone would 
deprive this objection of weight, and an experiment of Mr. Hopkins completely refutes 
itf. When incessantly checked by the surface over which they slide, even avalanches 
may, and do, sometimes descend with a uniform motion. The motion of a man in 
wal k i n g down stairs is on the whole uniform, but it is actually made up of an aggregate 
of small motions, each of which is accelerated. It is easy to conceive that ice moving 
over an uneven bed, will, when it is released from one opposing obstacle, be checked by 
another, and its motion thus be rendered sensibly uniform. So many obstacles exist 
along the bed of a glacier, that sudden slipping forwards of the mass through any con- 
siderable distance is not to be expected. But the real weak point of Saussure’s theory, 
though partly true, is its inability to account for many facts observed since his time. 
The theory of Charpentier, though not always fairly represented, has been shown to be 
untenable. 
The facts submitted to our consideration are briefly as follows: — We see the glacier 
winding through a valley, squeezing itself through a gorge, and widening where it has 
room. We see that the centre moves more quickly than the sides, and the top more 
quickly than the bottom ; and the next demand of the mind is for a general principle 
which shall unite these facts, and from which they shall follow as physical corollaries. 
Professor Forbes seeks this principle in the viscosity of the ice. Ice, according to him, 
is a substance resembling treacle, honey, or tar, and the observed phenomena are a conse- 
quence of this property. In this assumption consists what is called the viscous theory %. 
* I hardly think, however, that Sausstjbe would have subscribed to some of the interpretations of his 
theory now extant. 
t See Hopkixs in Philosophical Magazine, vol. xxvi. p. 4. Were it not that this objection is thought- 
lessly repeated in every work upon glaciers, I would not dwell upon it here. The objection drawn from the 
deportment of secondary glaciers lying on steep slopes is also very commonly dwelt upon, but it is equally 
without weight ; and applies with at least as much force to the viscous theory as to the theory of Saussuee. 
f The name of M. Eexdu will always be honourably associated with the theory of glacier motion. He 
first drew attention to the power of the glacier to move through a sinuous valley, to narrow and widen and 
behave like lava or like “ a soft paste.” He conjectured also that the centre would move more quickly than 
the sides. In fact he appears to have had a correct conception of almost all that the subsequent observa- 
tions of Professor Poebes established. I regret bo say that I have not been able to obtain M. Eendu’s 
original memoir. 
August 1859. — Thanks to my Zurich friends, I have recently had the pleasure of reading M. Eekdu’s paper, 
the perusal of which has confirmed my estimate of his sagacity. Had this gentleman been a philosopher 
instead of an ecclesiastic, we should doubtless have heard more about his claims than we have hitherto done. 
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