﻿504 M. A. Heim on Glaciers. 



Differential Motion. — It has already been endeavoured to 

 refer the structure of glacier-ice (the blue and white bands) 

 to surfaces of greatest differential motion. This, like the ice- 

 structure, takes place at surfaces which have the form of spoon- 

 bowls laid one in another, the drawn-out end downward. Such 

 surfaces can easily be obtained. If in the gypsum stream we 

 colour one layer differently, it will be seen, when in motion, 

 to assume this form. The differential motions, however, do 

 not take place in these bands, but cut them, as shown in 

 fig. 8. Professor Tyndall has already called attention to this 

 point. Whether the differential motion is greatest on these 

 spoon-shaped surfaces, or parallel with the sides of the valley 

 — in other words, whether " the direction in which the filaments 

 slide past each other will be obliquely directed toward the 

 middle," or parallel with the middle line, I have endeavoured to 

 prove by an experiment. I will develope it, because it presents 

 points of connexion in another direction. 



A number of lamellar or linear solid corpuscles in a thickly 

 liquid mass remain quietly in their position without any definite 

 arrangement when every part of the mass moves with equal ve- 

 locity; the lamellae are in no wise all arranged (as has recently 

 been said) perpendicular to the direction of the flow — for ex- 

 ample, in lavas. A definite arrangement is only brought about 

 by differential motion (when some parts move more quickly than 

 others). This is everywhere the case where the mass flows in a 

 fixed bed. There friction diminishes the velocity near the sides 

 of the bed (lava, mud-streams, glaciers). When a lamella lies in 

 a flowing mass, the end of it turned to the more slowly moving 

 side moves more slowly, is kept back relatively to the other ; 

 the lamella is turned till it falls into the surface of greatest dif- 

 ferential motion ; and in this position it remains. Were it by 

 accident to be turned beyond, it would immediately be tipped 

 over, and again be turned in the same direction into the same 

 surface, as at first. All lamellae in a flowing mass arrange 

 themselves in the surfaces of greatest differential motion. 



I cut an immense number of minute chips of wood, of about 

 a square line surface and the thickness of stout paper. These 

 were mixed with the gypsum, and the mass set flowing along 

 the valley-channel, and afterwards dried up solid (in which pro- 

 cess, it is true, the form was a little distorted); and now the 

 position of the chips was investigated. Fig. 13 shows the ar- 

 rangemeut in a longitudinal section through the middle line ; 

 therefore the surfaces of greatest differential motion are parallel 

 to the sides of the bed. In the middle of the higher parts, 

 where generally the differential motions are much smaller, no 

 arrangement had entered. If the other view were correct, the 



