n Chamouni, for example, after being 2000
yards wide, passes through a strait only 900 yards in width. Such a
gorge, it is contended, would be choked up by the advance of any solid
mass, even if it be broken up into numerous fragments. The same acute
observer remarked, that water in the fissures and pores of glaciers
cannot, and does not part with its latent heat, so as to freeze every
night to a great depth, or far in the interior of the mass. Had the
dilatation theory been true, the chief motion of the glacier would have
occurred about sunset, when the freezing of the water must be greatest,
and it had, in fact, been at first assumed by those who favored that
hypothesis, that the mass moved faster at the sides, where the melting
of ice was promoted by the sun's heat, reflected from boundary
precipices.

Agassiz appears to have been the first to commence, in 1841, aided by a
skilful engineer, M. Escher de la Linth, a series of exact measurements
to ascertain the laws of glacier motion, and he soon discovered,
contrary to his preconceived notions, that the stream of ice moved more
slowly at the sides than at the centre, and faster in the middle region
of the glacier than at its extremity.[287] Professor James Forbes, who
had joined Mr. Agassiz during his earlier investigations in the Alps,
undertook himself an independent series of experiments, which he
followed up with great perseverance, to determine the laws of glacier
motion. These he found to agree very closely with the laws governing the
course of rivers, their progress being greater in the centre than at the
sides, and more rapid at the surface than at the bottom. This fact was
verified by carefully fixing a great number of marks in the ice,
arranged in a straight line, which gradually assumed a beautiful curve,
the middle part pointing down the glacier, and showing a velocity there,
double or treble that of the lateral parts.[288] He ascertained that the
rate of advance by night was nearly the same as by day, and that even
the hourly march of the icy stream could be detected, although the
progress might not amount to more than six or seven inches in twelve
hours. By the incessant though invisible advance of the marks placed on
the ice, "time," says Mr. Forbes, "was marked out as by a shadow on a
dial, and the unequivocal evidence which I obtained, that even while
walking on a glacier we are, day by day, and hour by hour, imperceptibly
carried on by the resistles