but
encroach still lower into the region of cultivation, and trespass on
fields where the tobacco-plant is flourishing by the side of the
peasant's hut.

The cause of glacier motion has of late been a subject of careful
investigation and much keen controversy. Although a question of physics,
rather than of geology, it is too interesting to allow me to pass it by
without some brief mention. De Saussure, whose travels in the Alps are
full of original observations, as well as sound and comprehensive
general views, conceived that the weight of the ice might be sufficient
to urge it down the slope of the valley, if the sliding motion were
aided by the water flowing at the bottom. For this "gravitation theory"
Charpentier, followed by Agassiz, substituted the hypothesis of
dilatation. The most solid ice is always permeable to water, and
penetrated by innumerable fissures and capillary tubes, often extremely
minute. These tubes imbibe the aqueous fluid during the day, which
freezes, it is said, in the cold of the night, and expands while in the
act of congelation. The distension of the whole mass exerts an immense
force, tending to propel the glacier in the direction of least
resistance--"in other words, down the valley." This theory was opposed
by Mr. Hopkins on mathematical and mechanical grounds, in several able
papers. Among other objections, he pointed out that the friction of so
enormous a body as a glacier on its bed is so great, that the vertical
direction would always be that of least resistance, and if a
considerable distension of the mass should take place, by the action of
freezing, it would tend to increase its thickness, rather than
accelerate its downward progress. He also contended (and his arguments
were illustrated by many ingenious experiments), that a glacier can move
along an extremely slight slope, solely by the influence of gravitation,
owing to the constant dissolution of ice in contact with the rocky
bottom, and the number of separate fragments into which the glacier is
divided by fissures, so that freedom of motion is imparted to its
several parts somewhat resembling that of an imperfect fluid. To this
view Professor James Forbes objected, that gravitation would not supply
an adequate cause for the sliding of solid ice down slopes having an
inclination of no more than four or five degrees, still less would it
explain how the glacier advances where the channel expands and
contracts. The Mer de Glace i