536 THE LIFE OF JAMES D. FORBVS. [API 



of the fact by Bendu, 1 and which I find to have been faithfully 

 transferred from the margin of my own copy of the The- 

 where I had thus recorded my incredulity by a pencil mark still 

 extant, on a first perusal of the passage^n 1843. 



In truth, I disbelieve the alleged observation of 40 feet per 

 annum for five years as much now as I did them. My own ob- 

 servations on that or any other part of the Mer de Glace lend no 

 countenance to it. The smallest annual rate which I have esti- 

 mated for that region is 486 feet, or in any other region of the 

 glacier, 260 feet. N6v6s and snow-beds have of course nothing 

 to do with the question. 2 But, what is much better evidence 

 in the present instance, Professor Tyndall himself, and Mr. Hirst, 

 his surveyor, who made it their business to measure the velocity 

 of this very region of the glacier 3 in no less than four transverse 

 lines, and at numerous stations in each line, have nowhere re- 

 corded a velocity of less than 6J inches a day in summer, which, 

 taking a mean with Professor Tyndall's own estimate of the 

 ratio of winter to summer motion, 4 must give us, at the very 

 lowest reasonable estimate, from 140 to 150 feet of advance per 

 annum. 



This station ' stood close to the moraine/ and the result is 

 interesting as giving us a velocity almost entirely due to the 

 drag of the ice over the bank of the glacier. Now, the motion 

 reported by the guides to M. Eendu was three, and a half I 

 less than this velocity of ice in contact with the soil, which 

 in fact, the minimum possible for that section of the glacier. 



1 See extract, marked (c), p. 12. 



2 In a passage at p. 306 of the Glaciers of the Alps, the author states that 

 the measurements of glacier motion, made wilh my own hands, vary from less 

 than 42 feet a year to 848 feet a year. The reader might very naturally infer 

 from the context thatthese measurements indicated the effect of plasticity, and 

 that they both referred to the Mer de Glace. As he might have difficulty in 

 correcting this view, or in discovering where I have recorded a velocity of only 

 42 feet a vear, and as references to my writings are here withheld, I may men- 

 tion in this place, that the only observation by me which at all warrants such a 

 statement, appears to be one which, I made on a high and dry little glacier, in the 

 state of nice, 8,000 feet above the sea, niched in a cavity near the Simplon Pass 

 (Occasional Papers, p. 122). The velocity which I found in summer was 1'4 

 inches in 24 hours, which, multiplied by 365, gives 42 feet and a fraction. It 

 is needless to add, that there was no plastic, connection between this little glacier 

 and the Mer de Glace, to which apparently the movement of 848 feet belongs. 

 (See Map in Johnston's Physical Atlas.) 



a See Philosophical Transactions for 1S59, p. 263, and Glaciers of tlie 

 Alps, p. 277. 



* I will allow with Professor Tyndall that the winter motion is | of the 

 summer motion, and 1 will take the mean of the two, or nearly of the sum- 

 mer motion, to be the average of the year. Now, at 6i inches per diem, the 

 annual motion would be 198 feet at the summer rate, or nearly 150 feet at tin- 

 average rate. 



