180 PROFESSOR FORBES ON THE VISCOUS THEORY OF GLACIER MOTION. 
diately below the promontory, called the Abschwung, was found to be about 240 feet 
per annum, which though neither confirmed nor invalidated by the discordant mea- 
surements subsequently made by other observers on the same glacier, has at length 
been substantially corroborated by a professional surveyor, M. Wild, who has recently 
undertaken the verification at M. Agassiz’s request. 
After having myself observed the motion of several points of the Mer de Glace of 
Chamouni during the summer of 1842, 1 fixed the positions of two conspicuous 
blocks, one near Montanvert, marked D 7 ; and another opposite the Tacul, marked 
C, or the Pierre Platte (see my Map of the Mer de Glace), by means of which I 
hoped to ascertain the mean annual motion in succeeding years. With respect to 
the latter, or the Pierre Platte, I was successful ; for in September 1843 I ascertained 
geometrically its change of position, subject however to the uncertainty of a few 
yards, owing to the sliding of the block from the pedestal of ice upon which it was 
so picturesquely poised*, a circumstance which happens once or twice in the course 
of every summer. 
From the 17th of September 1842, to the 12th of September 1843, 
the advance was (in 360 days) 256*8 feet. 
Or reduced to the exact year of 365 days 260*4 feet. 
Mean daily motion 8*56 inches. 
Again, being enabled to repeat the measures in 1844, I found the advance — 
From the 12th of September 1843, to the 19th of August 1844 (342 days) 270 feet. 
Proportional motion for 365 days 288*3 feet. 
Mean daily motion 9*47 inches. 
In the case of the block D 7, J was less fortunate. It was very near the western 
side of the glacier, and though not thrown up on the shore, yet the ice on which it 
rested got in some manner so embayed or entangled, that though its motion had 
been steadily watched during the winter of 1842-43 by my able assistant, Auguste 
Balmat, it had scarcely moved since his last observation on the 8th of June 1843, 
when I visited it in September of the same year. It must be presumed that it had 
been much retarded previously, and hence it is clearly inadmissible to infer a pro- 
portional motion for the portion of the year when it had not been observed, as I did 
in the Postscript at the end of the first edition of my Travels, whilst in ignorance of 
the then unsuspected retardation. The motion actually observed was 432 feet in 
322 days, being at the rate of 483 feet per annum, or 15*88 inches per day. This is 
therefore undoubtedly below the true measure of the annual motion of the side-part 
of the glacier somewhat in advance of the Chalet of Montanvert (see the position of 
D 7 in the Map). It may at least be of some service as an inferior limit of the 
annual motion there. 
In 1843 I fixed approximately the position of a block marked P, higher up the glacier 
than the Montanvert, and near its left bank, exactly opposite the spot called Les Ponts. 
* See Frontispiece to Travels through the Alps of Savoy. 
