190 PROFESSOR FORBES ON THE VISCOUS THEORY OF GLACIER MOTION. 
stately and regulated flow of a river, in which the slighter variations are absorbed 
by the predominant inertia of a comparatively stable mass. Now the glacier of 
Bossons is, as every one who has seen it knows, a mere icy torrent, “ a frozen cata- 
ract,” which descends in a continuous mass from the level of the Grand Plateau of 
Mont Blanc to that of the Valley of Chamouni with very little impediment, with no 
confining bulwarks of rock, no contracting straits; and throughout this great vertical 
height of at least 9000 feet, the angle of descent is very steep indeed for so vast a 
mass. On the other hand, though the part of the Mer de Glace, called the Glacier 
des Bois under the Chapeau, is very steep, its “regime” is regulated by the supply 
derived from the reservoir glacier above, and, precisely as in rivers of great magni- 
tude and length of course, and of moderate declivity, it yields sluggishly to impulsive 
or retarding forces which are checked and opposed by the multitude of sinuosities, 
the embaying of the ice in rock-bound expansions of the channel, the struggle of its 
passage through defiles and the enormous friction of its lower surface. Yet, lest 
we might attribute the irregularities of the torrential glacier to causes quite local and 
uncertain, we find them reflected more or less distinctly in the movements of the 
neighbouring one. Thus the anomalous retardation in the end of March and begin- 
ning of April appears in three stations out of four, as does that in the first half of 
June, showing clearly that it is not an error of observation. It appears that the 
thaw of the winter’s snow during the month of May, saturating the pores of the gla- 
cier with water, produced (as we know that a thaw always does) a sudden and violent 
march, especially of the more susceptible or torrential glacier. So completely had 
this sudden move forced on the glacier of Bossons, encumbered by the spring 
avalanches and loaded with all the fragments and snow masses which had remained 
temporarily suspended during the winter months, that the lower part of the glacier 
(as we read in the memoranda to the register) advanced and widened greatly, to an 
extent which it had not done for many years past, and seemed to change its whole 
character ; and in February a similar temporary increase of volume had taken place ; 
“ on ne s’y recommit presque plus,” writes Balmat ; thus accounting for the particular 
accession of speed which appears in that month. In both cases, after the rapid march 
in February and in May, a reaction takes place; the material is deficient, the exces- 
sive pressure has been removed by the previous overflow, and a lull occurs in March 
and in June. 
VII. These irregularities, such as they are, even should we fail in entirely explain- 
ing them, are at least not to be attributed entirely to errors of observation, since dif- 
ferent observations (which it is to be recollected were sent to England in so rough a 
state that they required to be reduced and computed before the variations of velocity 
could be deduced from them) agree amongst one another, and agree with the pheno- 
mena casually noted in the Meteorological Register. They are very trifling in the 
movement of the Glacier des Bois, which presents a curve of remarkable regularity, 
giving a minimum about the end of December, and a maximum in July. The coin 
