78 THE FORMS OF WATER IN 



executed by M. Agassiz on the glacier of the Unteraar, 

 we notice in the discussion of these measurements a 

 section of the 6 Systeme glaciaire ' devoted to the 

 6 Migrations of the Centre.' It is here shown that the 

 middle of the Unteraar glacier is not always the point 

 of swiftest motion. This fact has hitherto remained 

 without explanation ; but a glance at the Unteraar 

 valley, or at the map of the valley, shows the enigma 

 to be an illustration of the law which we have just 

 established on the Mer de Glace. 



§ 26. Motion of Axis of Mer de Glace. 



192. We have now measured the rate of motion oi 

 five different lines across the trunk of the Mer de Glace. 

 Do they all move alike ? No. Like a river, a glacier 

 at different places moves at different rates. Comparing 

 together the points of maximum motion of all five lines, 

 we have this result : — 



MOTION OF MEE DE GLACE. 



At Trelaporte 



. 20 inches a day. 



At les Ponts 



• 23 „ „ 



Above the Montanvert 



. 26 „ 



At the Montanvert 



• 34 „ „ 



Below the Montanvert 



. 33* „ 



193. There is thus an increase of rapidity as we &3- 

 BceDd the glacier from Trelaporte to the Montanvert \ 



* This is probably under the mark. I think it likely that the swiftest 

 motion of this portion of the Mer de Glace in 1857 amounted to a yard in 

 twenty-four hours. 



