2T CLOUDS AND RIVERS, ICS AND GLACIERS. 



25. NEW LAW OP GLACIER MOTION. 



185. Let u* express these facts in another 

 va . Supposing the points of swiftest motion 

 m" n VL-CV great number of lines crossing the 

 fler de Glace 1o be determined ; the line 

 oining all those points together is what 

 mathematicians would call the locus of the 

 3oiut, of swiftest motion. 



ISO. At Tielaporte this line would lie cast 

 )f the centre ; at the Pouts it would lie^vest 

 if the centre ; hence in passing from Tiela- 

 porte to the Punts it would cross the centre. 

 But at the Montanvert it would again lie 

 jast of the centre ; hence between the Pouts 

 and the Mont an vert the centre must be 

 crossed a second lime. If there were further 

 sinuosities upon the Mer de Glace there 

 would he further ciossiugs of the axis of the 

 glacier. 



187. The points on the axis which mark 

 the transition from eastern to western bend- 

 ing, and the reverse, may be called jmnts of 

 contrary flexure. 



188. Now what is true of the Mer tie 

 Glace is true of ail other glaciers moving 

 through sinuous valleys : so that the facts 

 established in the Mer (le Glace may be ex- 

 panded into the following general law of 

 gl icier motion : 



YYiien a glacier moves through a sinuous 

 vsiiley. the locus of the point of maximum 

 motion does not coincide with the centre of 

 the glacier, but, on the contrary, always lies 

 on tue convex side of the central line. The 

 locus is therefore a curved line more deeply 

 sinuous than the valley itself, and crosses 

 the axis of the glacier at each point of con- 

 trary flexure. 



Is9. The dotted line on the Outline Plan 

 (Fig. G) represents the locus of the point of 

 maximum inotbn, the iirm line marking the 

 centre of the glacier. 



190. Substituting t he word river for glacier, 

 this law is also true. The motion of the 

 water is ruled by precisely the same condi- 

 tions as the motion of the ice. 



191. Let us now apply our law to the ex- 

 planation of a difficulty. Turning to the 

 careful measurements executed by ftl. Agas- 

 siz on the glacier of the Uuteraar, we notice 

 in the discussion of these measurements a 

 section of the " Syste"me glaciaire" devoted 

 to the "Migrations of the Centre." It is 

 here shown that the middle of the Untcraar 

 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 whifh we have just established on the 

 Mer de Glace. 



^ 23. MOTION OF Axis OP MER DE GLACE. 



192. We have now measured the rate of 

 motion of five different lines across the trunk 

 of the Mer de Glace. Do they all move 

 alike ? No. Like a river, a glacier at differ- 

 ent places moves at different rates. Coin- 



paring together the points of maximum mo- 

 tion of all rive lines, we have this result . 



MOTION OF MER DE GLACE. 



At Treliiporte 20 inches a day 



Mies Punts 



Above the Moutanvcrt. 



At the Montanvert 



lielow the Montanvert 33 



193. There is thus an increase of rapidity 

 as we descend the glacier from Treiaporle tc 

 the Montanvett ; the maximum motion at the 

 Montanvert being fourteen inches a day 

 greater than at Trelaporte. 



27. MOTION OP TRIBUTARY GLACIERS. 



194. So much for the trunk glacier ; lot 

 us now investigate the branches, permitting, 

 as we have hitherto done, reflection OD 

 known facts to precede our attempts to dis^ 

 cover unknown ones. 



195. As we stood upon our " cleft sta- 

 tion," whence we had so capital a view of the 

 Mer de Glace, we were struck by the fact that 

 some of the tributaries of the glacier were 

 wider than the glacier itself. Supposing 

 water to be substituted for the ice, how do 

 you suppose it would behave? You would 

 doubtless conclude that the motion down the 

 broad and slightly inclined valleys of the 

 Geant and the Lechaud wouid be compara- 

 tively slow, but that the water would forca 

 itself with increased rapidity through the 

 "narrows" of Tielaporte. Let us test this 

 notion as applied to ihe ice. 



19(3. Planting our theodolite in the shadow 

 of Mont Tacul,' and choosing a suitable 

 point at the opposite side of the Glacier civ 

 Geant, we fix on July 29th a series of teE 

 stakes across the glacier. The motion of tlu 

 line in twenty-four hours was as follows : 

 MOTION OF CLACIER DU GEANT. 

 SIXTH LINE: II II' UPON SKETCH. 



Stake 1 * 3 4 5 7 8 9 1C 



Indies 11 10 13 1) 1,J U ii 10 9 5 



197. Our conjecture is fully verified. The 

 maximum motion here is seven inches a day 

 less than that of the Mer dc Glace at Treia- 

 porte (192). 



198. And now for the Lechaud branch. 

 On August 1st we lix ten stakes across this 

 glacier above the point where it is joined by 

 the Talefre. Measured on August 3d, and 

 reduced to twenty-four hours, the motion was 

 found to be : 



MOTION OP GLACIER DE LECHAUD. 



SEVENTH LINE: KK'upo* SKETCH. 

 Stake... . 1 a 3 4 5 6 7 8 9 1C 

 Iaca 5 8 10 9 J) S 6 7 b 



j09. Here our conjecture is still further 

 verified, the rate of motion being even less 

 than that of the Glacier du Geant. 



28. MOTION OF TOP AND BOTTOM OF 

 GLACIEU. 



200. We have here the most ample and 

 varied evidence that the sides of a glacier, 

 like those of a river, are retarded by Jrictioa 

 against its boundaries. But the likeness doas 



