180 TWELFTH LETTER ON GLACIERS. [1846. 



intense, and the movement slow (see Eleventh Letter), and 

 where the danger from falling stones is such as absolutely to 

 prevent the continuance of observations during summer, they 

 are less comparable arid complete than would otherwise have 

 been the case. Having explained fully to MM. Carrel and 

 Guicharda my methods of observing, and pointed out a more 

 convenient site, we may hope that we shall have a continuous 

 series of trustworthy observations of this most interesting glacier. 

 In the meantime I publish, with M. Carrel's permission, the 

 degree of motion of a stake fixed in the ice, near the lowest 

 extremity of the glacier, and little elevated above the soil. 



tion. Daily Metres. 



metres. 0'13 

 0-13 

 0-113 



0-12 



0-075 

 0-15 



0-10 



The mean daily motion is about 5 English inches, and it 

 is probable, from the discrepancy of the 5th and 6th observa- 

 tions, that the measurement of the 30th January was faulty. 

 If this be so, the general uniformity, as well as smallness of 

 the motion, is accounted for by the important retardation due to 

 friction.* 



In the absence of an exact geometrical plan of the glacier, 

 it was important to preserve some accurate record of its exten- 

 sion at the time of my visit in August 1846, which was done 

 in the following manner : The theodolite stationed at the point 

 B, Plate VIII. figs. 1 and 4, that is, on a promontory of limestone 

 a little to the west of the chapel of Notre Dame, and close to 

 the path from Courmayeur to the Allee Blanche. It is also 57 



* It is to he noted, that the advance of the glacier of 60 [70] feet in two 

 months, mentioned in page 179, is not the motion of a point in the ice, but the pro- 

 trusion of the front of the glacier after the effect due to melting has been deducted. 



