THE VARIATIONS OF GLACIERS. 283 
geological evidence has not led geologists to a uniform opinion. 
We know little about the velocity below the surface of the 
glaciers; can we find out how rapidly glaciers are sliding over 
their beds? This is certainly very difficult to do; in fact the 
only place where observations can be made is near the end of 
the ice or at the sides where the movement is slow. During the 
last thirty or forty years, a period of retreat, the movement has 
been slower than it will be during the advance now beginning ; 
observations made during the retreat would certainly lead to an 
underrating of the effect of a glacier on its bed; but during the 
more rapid movement of the advance we shall form a better 
estimate of it. A mathematical theory might enable us to infer 
‘ ee 
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SS, 
the velocity at the bottom from that at the surface; this would 
certainly be an immense gain in our knowledge of glaciers and 
ice-sheets. It is for this reason that experiments on the physical 
properties of ice, and observations on the rate of flow, rate of 
melting, and comparisons of the variations of glaciers with 
climatic variations, etc., are of so much importance to the glacial 
geologist. 
The study and observations of the variations of glaciers is 
being pushed with great zeal by the various Alpine clubs, and 
the results published in their journals, where they are apt to 
escape the notice of American geologists. Last summer the 
International Congress of Geologists at Zurich appointed a com- 
mittee to encourage, and to collect, observations on glaciers all 
over the world, with the special object in view of discovering a 
relation between the variations of glaciers and of meteorological 
