
pA 
TRANSACTIONS OF THE SECTIONS. 63 
‘situate ; and he exp ained, also, how the motion would be facilitated by the effects 
of the internal heat of the earth, and of subglacial currents. When the attention of 
philosophers, however, was recalled a few years years ago to this subject, and more 
accurate observations and admeasurements were made, the inclinations of the beds of 
glaciers were found in many cases to be so small (in the glacier of the Aar, for 
example, not exceeding three degrees), that it appeared extremely difficult to conceive 
how the force of gravity alone could be adequate to overcome the friction on the 
bottom and sides of the glacier, and the numerous local obstacles to its movement. 
Numerous experiments on the descent of bodies along inclined planes had shown, 
that when the surfaces of the bodies and planes were perfectly hard and polished, no 
motion would ensue without an inclination considerably greater than that of many 
glaciers ; and, moreover, that the inclination required to produce motion was inde- 
pendent of the weight of the sliding body. These considerations led to the very 
general rejection of De Saussure’s theory, and to the adoption, by many persons, of 
the dilatation theory, of which M. Agassiz had been the principal advocate. According 
to this theory, a part of the water produced by the dissolution of the superficial por- 
tion of the glacier during summer passed, by infiltration, into the minute pores and 
crevices of the glacier, where it was again converted into ice, and by its expansion in 
the process of freezing produced a dilatation and consequent motion of the glacier. It 
was manifest, however, that the frequent alternation of freezing and thawing within 
the glacier which this theory assumed, could not possibly take place at depths be- 
neath its surface exceeding a very few feet, and therefore could not produce any 
sensible effect on the motion of the whole mass. This theory presented many other 
difficulties, of which no adequate solution had been given; and the author could not 
but consider it as contrary to the most obvious mechanical and physical principles. 
Another theory had also been put forward, which attributed the motion of glaciers 
to the expansion of water in the act of freezing, after it had filled, not the minute 
pores of the ice, but internal cavities of considerable dimensions. But, since the 
temperature of the glacier at considerable depths must be sensibly constant, how 
were new cavities to be formed when existing ones had been once filled up? The 
author, regarding both this theory and the preceding one as untenable, was thus led 
to examine how far the apparent objections to De Saussure’s theory were really valid, 
by a series of experiments on the descent of ice down inclined planes. The experi- 
ments were made in the following manner :—a slab of sandstone, prepared to be laid 
down as a part of a common flagstone pavement, was so arranged as to be easily 
placed at any proposed inclination to the horizon. The surface of the slab, so far 
from being polished, retained the grooved marks of the instrument with which the 
quarryman had shaped it. A quantity of ice was placed on the slab, within a frame 
nearly a foot square, intended merely to keep the ice together, and not touching the 
slab, with which the ice alone was in contact. The following were results obtained 
in one set of experiments, the ice being loaded with a weight of about 150lbs, The 
temperature of the air was about 50° Fahr. :— 
Inclination of the planes  ...scsscseceeeesesene Bo Os Our ao” Mn 
inches 
Mean space descended through in one hour 0°31 *62 "96 2° 2°5 
When the weight was increased, the rate of motion was also increased. The least 
inclination at which sensible motion would take place was not determined ; but it 
was ascertained that it could not exceed half a degree in the case of a smooth but 
unpolished surface. With a polished surface of a marble slab, the motion of the ice 
indicated a deviation from horizontality with as much sensibility as water itself. It 
will be observed, in the results above given, that (1) the motion was unaccelerated ; 
and (2) it increased with the inclination, and (when the inclination was not greater 
than nine or ten degrees) in nearly the same ratio; and (3) the rate of movement 
was of the same order of magnitude as in actual glacial motion, which may be stated 
generally, in cases yet observed, never to exceed two feet a day. The extremely small 
friction between the plane and the ice, indicated by the small inclination necessary 
to produce motion, was manifestly due to the circumstance of the lower surface of 
the ice being in a state of gradual disintegration, which, however, was extremely 
slow, as proved by the: small quantity of water proceeding from it. In the applica- 
tion, therefore, of these results to the case of actual glaciers, it was necessary to show 
