upon Glacier -motion. 419 



Mr. Reilly has not supplied me with any note of the motion 

 of the edge of the glacier during the interval ; but as the edge 

 of the Great Aletsch was found to move at the rate of only an 

 inch and a quarter in twenty-four hours, it probably did not ex- 

 ceed 2 feet. We shall therefore be justified in saying that while, 

 in the right-hand moiety of the glacier, a differential motion of 

 10 inches is distributed over a width of 100 yards from the line 

 of maximum velocity, a differential motion of at least 100 inches 

 must be distributed over the. remaining 70 yards up to the edge 

 of the glacier. 



Two of the 30-feet spaces were staked out into subdivisions of 

 2 feet each. Each of the intermediate stakes exhibited a differen- 

 tial motion, with occasional negative signs — the greatest relative 

 displacement observed being 2*25 inches in the twenty days, equi- 

 valent to the 2^3 of an inch in twenty-four hours for points 1 inch 

 apart. 



During the intervals of his labours on the Glacier of Bionassay 

 Mr. Reilly ranged a line across the Mer de Glace, on the Cha- 

 monix side of the Montanvert. His measurements on this 

 line during a period of nineteen days indicate a motion very 

 similar in its character to that of the Glacier of Bionassay. 

 The length of this line from the left-hand edge of the glacier 

 to the point of maximum velocity was about 1000 feet. The 

 central 500 feet had an absolute motion of 18 feet 7*75 inches 

 at its left-hand extremity, and of 21 feet 2 inches at its right, 

 equivalent to a mean daily motion of about 12 inches. Its 

 total differential motion was 28*25 inches, equivalent to a 

 mean daily differential motion of about 1-5 inch, or * of an 



. . 4000 



inch for points 1 inch apart. The nearest station to the edge of 

 the glacier was about 165 feet distant from it; and at this sta- 

 tion the absolute motion in nineteen days was found to be 10 feet 

 11*25 inches. This would indicate a marginal motion of about 

 4 feet, and would leave us 14 feet 6 inches of differential motion 

 to distribute over the lateral 500 feet of the line, or six times as 

 much as that of the central moiety. 



The law of variation of the differential motion indicated by 

 the observations above described is not new. It appears clearly 

 from the measurements made by Professor Tyndall on the Mer 

 de Glace, described in a communication to the Boyal Society, 

 read May 20, 1858, and published in vol. cxlix. of the Philoso- 

 phical Transactions. But nowhere is it brought out with more 

 striking prominence than in the observations made by Agassiz 

 upon the Unter-Aar Glacier, from 1842 to 1845, as described in 

 Chapter XII. of the Nouvelles Etudes, and in plate 4 of the 

 accompanying atlas, where curves showing the motion, for three 

 consecutive years, of a series of points originally in a straight 



2E2 . 



