98 Professor Forbes's Twelfth Letter on Glaciers. 



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 ex- 

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

 done in the following manner : — The theodolite stationed at 

 the point B, Plate II., figs. 1 and 4, that is, on a pro- 

 montory of limestone a little to the west of the chapel of 

 Notre Dame, and close to the path from Courmayeur to the 

 AUee Blanche. It is also 57 feet to the eastward of the old 

 larch seen in the views, Plate I. From this point, then 

 (which cannot readily be mistaken), the telescope of the 

 theodolite was directed upon the steeple of the little church 

 of Entreves, the azimuth marked (286°), and the telescope 

 was exactly levelled. It was then turned in azimuth (tra- 

 cing out the horizontal plane), until it cut the contour of the 



^"*"~v. ^jn^ Horizon 



Entrfeves. 



glacier of La Brenva, which was at an azimuth of 184 1^°. 

 The difference of azimuth lOlf °- It is plain that if the gla- 

 cier advances bodily into the valley this angle will diminish ; 

 if it retreats, the angle will increase. Farther, from the 

 same station, B, the elevation of the highest part of the 

 front of the glacier, where it crosses the valley, was 11° 37'. 



* It is to be noted, that the advance of the glacier of 60 feet in two months, 

 mentioned in page 96, 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 de- 

 ducted. 



