i88o.] 
On Water and Air. 
327 
trust by means of the diagram (Fig. 35) to make the 
explanation evident to you. Let A c d b be a portion 
of the glacier, and, for the sake of simplicity, suppose 
the glacier flowing in the direction of the arrow. Sup- 
pose we take a transverse slice of the glacier (s T) and 
fix our attention upon a little square (T i) of this slice, 
that is a little square mass of ice, and I would ask the boys 
whether that slice of ice will remain straight after it has 
been two or three days subjected to the motion of the 
glacier. (Voices: No.) It will become curved, will it 
not (as at s' t) ? The centre will move forward because 
of the quicker flow of the centre of the glacier ; and there- 
fore when that slice of ice (T i) comes down we shall have 
it transformed into 6 the shape T i', and our little square will 
be distorted to the shape which, as my young mathematical 
friends know, has usually applied to it the term rhombus, 
or lozenge. Now I want you to realise the exaCt mechanical 
state of matters. When the square (T i) is distorted to the 
shape of the lozenge (t i') the diagonal T i of the square 
becomes the diagonal T i' of the lozenge, the diagonal of 
the lozenge being longer than the diagonal of the square. 
If the ice could freely stretch like treacle, one diagonal 
would stretch to the same length as the other, but the ice 
does not stretch in that way. It is strained, and it breaks 
across at right angles to the line of strain T i' } and forms a 
crevasse, or chasm, which, instead of being pointed downwards 
is pointed upwards, and thus you see that the motion of the 
glacier is exactly the reverse of what you supposed it to be. 
Now I have to point out to you a few examples of violent 
crevassing. You have seen the real mechanical origin of 
those crevasses. They are due to the strain imposed upon 
the glacier by the different motions of its parts. When the 
crevassing is very violent, and when the glacier is pulled in 
different directions, you have the crevass converted into 
the most fantastic peaks and towers of ice. You hear these 
called in Chamouni and elsewhere aiguilles (needles). 
Now we pass on to a final result of the glaciers. There is 
the glacier opposite which I spend a couple of months of 
each year of my life — the great Aletch Glacier — and at a 
certain place that glacier at one period of its history turned 
down a lateral valley. It now only fills the great trunk 
valley. The glacier has retreated from this lateral valley, 
and in its place we have a little lake called the Margelin 
See, one of the most picturesque parts of the Alps, and 
upon this lake you have floating little blocks of ice which 
are rendered by the snow quite white ; and they are, to all 
