CLOUDS AND RIVERS, ICE AND GLACIERS. 75 



die middle line of the glacier. The demonstration that. 

 the point of swiftest motion wanders to and fro acrosa 

 the axis, as the flexure of the valley changes, is, there- 

 fore, — shall I say complete ? 



180. Not yet. For if surer means are open to us we 

 must not rest content with estimates by the eye. We 

 have with us a surveying chain : let us shake it out and 

 measure these lines, noting the distance of every stake 

 from the side of the glacier. This is no easy work 

 among the crevasses, but I confide it confidently to Mr. 

 Hirst and you. We can afterwards compare a number 

 of stakes on the eastern side with the same number of 

 stakes taken at the same distances from the western 

 side. For example, a pair of stakes, one ten yards from 

 the eastern side and the other ten yards from the 

 western side ; another pair, one fifty yards from the 

 eastern side and the other fifty yards from the western 

 side, and so on, can be compared together. For the 

 sake of easy reference, let us call the points thus com- 

 pared in pairs, equivalent points. 



181. There were five pairs of such points upon our 

 fourth line, D D', and here are their velocities : — 



Eastern points ; motion in inches . 



. 13 



15 



16 



18 



2() 



Western points „ ,, 



. 15 



17 



22 



23 



23 



Tn every case here the stake at the western side moved 

 more rapidly than the equivalent stake at the eastern 

 side. 



182. Applying the same analysis to our fifth line, 



7 



