Glacier Work. — Scott. 22^ 
might be sufficient to urge it down the slope of the valley if the 
sliding motion were aided by water flowing at the bottom. * 
Agassiz was the first to commence a series of exact meas- 
urements on glacier motion, in 184 1, and to substitute, fol- 
lowing Charpentier the idea of dilitation, for the "gravita- 
tion theory" of de Saussure. His theory was based on the 
idea that solid ice is always permeable to water, and contains 
innumerable capillary tubes. These tubes were supposed to 
imbibe water during the day. subsequently freezing at night, 
and expanding in such a way as to distend the whole ice 
mass. The immense force exerted by this distention was 
presumed tO' be sufficient to propel the glacier "down the 
valley." 
Later on, however, the theory of Agassiz was replaced 
by that of Forbes, or the viscosity theory. This theory sup- 
poses motion among the ultimate particles zcitlwut rupture 
and is regarded as the first to explain, in a partially satis- 
factory way at least, the differential motion of a glacier. 
Faraday showed in 1850, that if two pieces of ice having 
throughout a temperature of 2^° F. and each melting at its 
surface, be made to touch each other, the\4 will freeze together 
at the point of contact, the process being known as regela- 
tion. Upon this demonstration as a foundation Prof. 
Tyndal subsequently brought forward his regelation theory 
of the motion of glaciers, which, in brief, supposes motion 
among discrete particles, by rupture, chauge of position, and 
regelation. f CroU's Theory $ seems to be a modi- 
fication of Forbes', in attempting "a physical explanation of 
the viscosity of ice." Another theory that deserves mention 
is that of Thomson. It is based on the fact that the fusing 
point of ice is lowered, and the ice promptly melted by pres- 
sure. Compared' with Tyndal 's theory the result obtains 
that differential motion in the one case is by fracture, change 
of position, and regelation ; in the other by melting, change 
of position, and regelation. Many other theories have been 
advanced from time to time, but only those of apparently- 
most importance, have been considered. 
♦"Voyages dans les Alpes." 
t Le Conte, Elements of Geology. 
JCroll, Climate and Time. 
