and Motion of Glaciers. 379 



torn of the trough is figured underneath the plan. A B E P is a 



box supported at the end of the trough, and filled with a mixture 



Fig. 8. 



O^i^ s::^:) s:::^ 



©Olo O 

 00\0 ^c 



CD 



o 



of water and fine pipe-clay. The front, A B, can be raised, like 

 a sluice, and the mud permitted to fiow regularly into the trough. 

 While the mud is in slow motion, a coloured circle, c, is stamped 

 upon the white clay between A B and ef; the changes of shape 

 which this circle undergoes in its passage downwards will indi- 

 cate the forces acting upon it. The circle first moves on, being 

 rather compressed, in the direction of the length of the trough 

 until it reaches ef, on crossing which, and passing down the sub- 

 sequent slope, it elongates as in the figure. Between ff h and i k 

 the figure passes through the circular form, and assumes that of 

 an ellipse whose shorter axis is parallel to the length of the trough. 

 It is manifest from this that the mud between ef and g h is in a 

 state of longitudinal tension, while between c/ h and i k its state 

 is that of longitudinal compression. On crossing i k and descend- 

 ing the second incline, the figui'e is again drawn out longi- 

 tudinally, while between m n and C D the ellipse widens on 

 account of the permission given to lateral expansion by the aug- 

 mented width of the trough. 



The side circles in the same figure will enable us to study the 

 influence of lateral friction upon the descending stream. These 

 circles are distorted into ellipses, whose major axes are oblique to 

 the direction of the trough's length. Above the line e/central fis- 

 sures perpendicular to the axis of the trough cannot be formed; for 

 here, instead of tending to open into fissures, the flattening of the 

 central circle shows that the mud is longitudinally compressed. 

 On the slope below ef, the distortion of the circles into ellipses is 

 very pronounced ; and as the longer axis of each ellipse marks the 

 2C2 



