234 
THE GEOLOGIST. 
vious mecliaiiical 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 con- 
siderable 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 once been filled 
up ? The author, regarding both this theory and the preceeding 
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 
experiments were made in the following manner : a slab of sand- 
stone, prepared to be laid down as a part of a common flagstone 
pavement, was so arranged as to be easily placed at any pro- 
posed inclination to the horizon. The surface of the slab, so far 
from being polished, retained the grooved marks of the instru- 
ment with which the quarry-man 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 15 01b. : — 
Inclination of the planes 3° 6° 9" 12° 15° 
inches 
Mean space for one hour 0*31 '62 '96 2* 2*5 
When the weight was increased, the rate of motion was also in- 
creased. 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 un- 
polished surface. With a polished sm-face 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 re- 
sults above given, that (1) the motion was unaccelerated ; and 
(2) it increased with the inclination, and (when the inchnation 
