164 VISCOUS THEORY OF GLACIER MOTION. [1846. 



structure, so important to that of glaciers, may be considered 

 as explaining a number of intimately connected phenomena. 



" The glacier struggles between a condition of fluidity and 

 rigidity."* " A glacier is not a mass of solid ice, but a com- 

 pound of ice and water more or less yielding, according to its 

 state of wetness or infiltration." f " The pressure communi- 

 cated from one portion to the other will not be the whole 

 pressure of a vertical column of the material equal in height to 

 the difference of level of the parts of the fluid considered ; the 

 consistency or mutual support of the parts opposes a certain 

 resistance to the pressure, and prevents its indefinite trans- 

 mission. . . A glacier is not coherent ice, but a granular 

 compound of ice and water." { " When the semi-fluid ice 

 inclines to solidity during a frost, the motion is checked ; if 

 its fluidity is increased by a thaw, the motion is instantly 

 accelerated. . . It is greater in hot weather than in cold, 

 because the sun's heat affords water to saturate the crevasses." 

 Such were the terms in which, within a few months after sug- 

 gesting the viscous theory, I expressed my opinion of the 

 influence of the compound structure of the glacier, a mass 

 composed, not of ice alone, but of ice including water in its 

 countless capillaries, never frozen || even in winter. The quality 

 of plasticity or viscosity resulting from the union of a nearly 

 perfect fluid with an imperfect solid is seen in very numerous 

 and familiar instances, as for instance in sand, which is itself 

 devoid of any tenacity until its interstices have been saturated 

 with just so much water as to cause it to flow ; or in the still 

 more familiar instance of water-ice prepared for the table, in 

 which the varying proportion of the solid and fluid ingredient 

 gives to it every shade of consistency, from a brittle solid to 

 a liquor, including suspended solid grains. The prodigious 

 effect of capillary infiltration in determining the motion of even 



* Third Letter on Glaciers, August 1842, [p. 23 above], 

 f Travels, 1st Edit., 1843, p. 175. 



j Travels, p. 367., Edit, 1843. $ Ibid, p. 372. 



U Ibid, p. 361, 372. 



