646 Professor Tyndall, [June 4, 



would undoubtedly lie on that side of the centre of the stream 

 towards which it turns its convex curvature. Can this be the case 

 with the ice ? If so, then we ought to have a shifting of the point 

 of maximum motion towards the eastern side of the valley, when 

 the curvature of the glacier so changes as to turn its convexity to 

 the western side. Such a change of flexure occurs opposite the 

 passages called Les Pants, and at this place the view just enunciated 

 was tested. It was soon ascertained that the point of swiftest motion 

 here lay at a diiferent side of the axis from that observed lower 

 down. But to confer strict numerical accuracy upon the result, 

 stakes were fixed at certain distances from the western side of 

 the glacier, and others at equal distances from the eastern side. 

 The velocities of these stakes were compared with each other, 

 two by two ; a stake on the western side being always compared 

 with a second one which stood at the same distance from the eastern 

 side. The results of this measurement are given in the following 

 table, the numbers denoting inches : — 



1st pair. 2nd pair. 3rd pair. 4th pair. 5th pair. 



West 15 \ West 17i\ West 2211 West 231) West 23f 

 ;East 12^1 East 15iJ East 15* j East 18ij East 19* 



It is here seen that in each case the western stake moved more 

 swiftly than its eastern fellow stake ; thus proving, beyond a doubt, 

 that opposite the Fonts the western side of the Mer-de-Glace 

 moves swiftest ; a result precisely the reverse of that observed 

 where the curvature of the valley was different. 



But another test of the explanation is possible. Between 

 the Fonts and the promontory of Trelaporte, the glacier passes a 

 point of contrary flexure, its convex curvature opposite to Trelaporte 

 being turned towards the base of the Aiguille du Moine, which 

 stands on the eastern side of the valley. A series of stakes was placed 

 across the glacier here ; and the velocities of those placed at certain 

 distances from the western side were compared, as before, with those 

 of stakes placed at the same distances from the eastern side. The 

 following table shows the result of these measurements; the numbers 

 as before denote inches : — 



! 1st pair. 2nd pair. 3rd pair. 



West . 12|1 West . 15 \ West . 17i 

 East . 14|[ East . 17Jj East . 19 



Here we find that in each case the eastern stake moved faster 

 than its fellow. The point of maximum motion has therefore once 

 more crossed the axis of the glacier, being now upon its eastern 

 side. 



Determining the points of maximum motion for a great num- 

 ber of transverse sections of the Mer-de-Glace, and uniting 

 these points, we have the locus of the curve described by the 



