of the Ice of Glaciers, 245 



at the expiration of it they would be found in the points 1 ; , 2', 

 3', 4f, &c. If, instead of being placed apart, the stones had been 

 placed close together along the whole line, then 1', 2', 3', 4', 5', 

 &c, instead of being a polygonal line, would have been an irre- 

 gular curve. The irregularities in it may be supposed to be due 

 to accidental causes, as, for instance, unevenness in the bottom of 

 the channel along the line AB; so that, supposing these to be re- 

 moved, or their effect as exhibited in A B to have been obliterated 

 in the subsequent motion of the ice, the actual positions of the 

 stones would have been those shown by the dotted line 1 1. That 

 line would on this supposition represent the position of the ice at the 

 end of a year, over which at the beginning of it the line AB was 

 drawn. If, in like manner, l r 1", 2' 2", 3' 3", 4' 4", &c. be set off 

 equal to 1 1', 2 2', 3 3', 4 4', &c. respectively and in prolongation 

 of them, then the dotted line 2 2 will represent the position of 

 the ice which two years before was in A B ; and if this process be 

 repeated ten times, we shall finally arrive at the curve 1010, 

 representing at the end of ten years the position of the ice which at 

 the beginning of those ten years was in A B; supposing the mean 

 daily motion of the glacier to remain the same during that period. 

 There is no account of the motion of the ice underneath the 

 surface at Les Ponts, where these measurements of the motion of 

 the surface-ice were made; but at a point higher up, near theTacul, 

 the eastern face of the side of the Glacier du Geant was exposed, 

 and an ice precipice presented itself 140 feet high and nearly ver- 

 tical. And on the 11th of August, 1857"*, Professor Tyndall and 

 Mr. Hirst placed in it three stakes in the same vertical line — one 

 at the top, the second at 35 feet, and the third at 4 feet from 

 the bottom. The mean daily motions of these stakes respectively 

 they found to be 6 inches, 4*59 inches, and 2*56 inches. I have 

 multiplied these numbers by 365 ; and reducing the inches to 

 links, and also the feet in the heights of the several stakes from 

 the ground, I have set off in fig. 3, from the same scale as before, 

 E D and E F equal to the heights of the first and second stakes, 

 E 1 equal to the mean annual motion of the lowest stake, F 1' 

 equal to that of the next higher, and D 1 equal to that of the 

 highest; and I have repeated the distances E 1, F 1', and D 1 

 ten times along the lines E 10, F 10', and D 10. The points 

 1 l'l, 2 2' 2, 3 3'3, &c. show, therefore, the positions which the 

 stakes would have occupied at the ends of ten successive years if 

 the mean motion had remained the same ; and if the stakes had 

 been placed close together these polygonal lines would have be- 

 come curved lines, representing the position taken up by the ice 

 at the end of each successive year which started at first from A D. 

 Now let it be imagined, as an illustration, that the ice at the side 

 * Phil. Trans. 1859, part i. p. 270. 



