Mn. HOPKINS, ON THE MOTION OF GLACIERS. 



67 



retarding effects of the lateral action on the flanks of the glacier. The same explanation will 

 apply whether we suppose the cohesion along the planes of separation to be entirely destroyed or not. 

 It is only necessary that the tangential action between the central and contiguous portions should 

 not be sufficient to prevent the former from sliding past the latter. 



21. Position of the Surface of a Glacier 



Let P, Q be two points on the surface of a 

 glacier situated on the same line of motion. 

 C a point fixed in space in the vertical line 

 through Q. Draw Pp parallel to the bed 

 {AB) of the glacier. If the thickness BP of 

 the glacier at P remained constant while P 

 moved to the vertical line QC, P would come 

 to p, and the thickness of the glacier along AC 

 would he increaswl l)y Qp. Draw PM hori- 

 zontal, and let MPQ, = u, the inclination of the 

 surface of the glacier, and MPp = fi = that of 

 the bed of the glacier. Then if PM = a, 

 Qp = MQ - Mp 



= a (tan a - tan /3). 



i2 



