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Mr. W. R. Browne. 



below. Let there be a rise in temperature, which does not extend 

 beyond the uppermost 10 feet. This layer will expand, and if it 

 were free would expand to the full amount due to the increase in 

 temperature. But its lower surface is not free. In expanding it will 

 therefore drag the next layer after it, or in otber words will cause it to 

 expand also. The amount of expansion, however, will not be so 

 great, because there will be a certain shearing extension at the plane 

 of division between the two. The second layer will similarly cause 

 an expansion in the third, and so on to the bottom. In consequence, 

 the energy which would all have been exerted on the top layer, had 

 that been free, will be distributed over the whole of the layers ; and 

 the extension of the top layers will of course be much smaller than it 

 otherwise would have been. Should the temperature then remain 

 constant, the layers will retain their position, and adapt themselves to 

 the new circumstances. If the temperature falls the layers will 

 contract ; but from the now opposing effect of gravity they will not 

 return to their original position. The top layer which has extended 

 furthest will be the furthest below its original position ; the second 

 layer next, and so on. If we suppose the layers to be indefinitely 

 thin, we have the condition of things in an actual glacier. The ice in 

 any vertical section will, on the whole, move down the slope, but the 

 top will move faster than the middle, and the middle than the bottom, 

 exactly as it is known to do. The same holds with regard to a hori- 

 zontal section. At the sides the ice will be held back, not only by 

 the friction, but also by the protuberances of the rock, which compel 

 the ice to shear over them. Hence the velocity there will be retarded, 

 and will be less than that in the middle, which is comparatively free. 



A more important objection remains to be considered, which is this. 

 On the present theory the motion at any point on the surface of a 

 glacier will be not continuous, but oscillatin g alternately downwards 

 and upwards, and the nett distance by which it has descended, say, 

 in a day, will be a mere fraction of the total distance through which 

 it has moved in that period. If so, this alternate motion ought to have 

 been noticed in the various observations which have been made upon 

 glaciers, and this does not appear to have been the case. But, in 

 reply to this, it may be remarked that most of the observations have 

 only given the nett movement of points on the glacier during 

 intervals of a day or more, and therefore would not show the oscilla- 

 tions. Again, such observations have always been at points near the 

 end of a glacier. Now the variations in temperature of a glacier will 

 be very different at different parts, and the motion of the end of the 

 glacier will, to a great extent, show the average result of these 

 different advances and retreats in different parts of the higher 

 regions. This average result will, of course, be a steady progression 

 down the valley, and the oscillatory movement at the end of the 



