BRITISH ASSOCIATION. 
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which attributes this motion to the resolved part of gravity acting 
along the inclined surfaces on which all glaciers in motion are 
situate ; and he explained^ also^ how the motion would be faci- 
litated by the effects of the internal heat of the earth, and of 
sub-glacial currents. T^Tien the attention of philosophers, how- 
ever, was recalled, a few years ago, to this subject, and more accu- 
rate observations and admeasurements were made, the inclina- 
tions 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 granty 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 des- 
cent of bodies along inclined planes had shown that, when the 
surfaces of the bodies and planes were perfectly hard and pohshed, 
no motion would ensue without an inclination considerably greater 
than that of many glaciers ; and, moreover, that the inclination 
required to produce motion was independent of the weight of the 
shding body. These considerations led to the very general re- 
jection of De Saussure^s theor}^, and to the adoption, by many 
persons, of the dilation 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 portion of the gla- 
cier 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 dila- 
tion 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 exceeding a very few feet beneath its surface, 
and therefore could not produce any sensible effect on the mo- 
tion of the whole mass. This theory presented many other diffi- 
culties, of which no adequate solution had been given ; and the 
author could not but consider it as contraiy to the most ob- 
VOL. II,— NO, XVIII. 2 A 
