EFFECT OF FLUID PRESSURE IN THE CAPILLARY FISSURES. 
209 
of the former to all depths of the glacier ; and as it is matter of ocular demonstration, 
that these crevices, though very minute, communicate freely with one another to 
great distances, the water with which they are filled communicates force also to great 
distances, and exercises a tremendous hydrostatic pressure to move onwards in the 
direction in which gravity urges it, the vast porous crackling mass of seemingly rigid 
ice in which it is, as it were, bound up*.” 
Now the water in the crevices does not constitute the glacier, but only the princi- 
pal vehicle of the force which acts on it, and the slow irresistible energy with which 
the icy mass moves onwards from hour to hour with a continuous march, bespeaks 
of itself the presence of a fluid pressure. But if the ice were not in some degree 
ductile or plastic, this pressure could never produce any, the least, forward motion 
of the mass. The pressure in the capillaries of the glacier can only tend to separate 
one particle from another, and thus produce tensions and compressions, within the 
body of the glacier itself ’ which yields, owing to its slightly ductile nature, in the di- 
rection of least resistance, retaining its continuity, or recovering it by re-attach- 
ment after its parts have suffered a bruise, according to the violence of the action to 
which it has been exposed. 
The action of warm weather in accelerating the movement of the glacier is plainly 
due to the abundance of the water saturating its pores ; but this may act in two 
ways ; first, by rendering the frame-work of ice less brittle when it is in the very act 
of dissolving by the circulation of water in a perfectly fluid state through its pores-f~, 
and secondly, and more particularly, from the hydrostatic effect of gorging a porous 
mass with fluid. When an incipient frost dries even momentarily the surface of the 
glacier, the vast fibrous mass begins to drain. This is a very slow process, owing to 
the resistance to the passage of a fluid through very long and complicated canals. 
Were it not so, glaciers would be entirely dry after sunset and in winter, which is 
not the case. The hydrostatic pressure within the whole glacier is however sensibly 
diminished by the process of drainage ; this is evident from watching the level of 
water in a vertical hole of any depth made within the solid ice of the glacier. After 
much rain or heat this level is always higher than after dry cold. In the former- 
case the glacier may be said to be gorged, the supply of water from the surface ex- 
ceeding the power of the drainage to carry it off. The circulating vessels are there- 
fore overcharged. In the latter case the superficial supply is stopped, the drainage 
goes on slowly though uninterruptedly, and the level of the water in the vertical 
shaft slowly descends, indicating the diminution of internal pressure. If it were not 
for the capillarity of the ducts, it is plain that no effective hydrostatic pressure would 
* Sixth Letter. Appendix to Travels, 2nd edit., p. 428. 
t This I think is undeniable, from the appearance of the collapsed crevasses above referred to, notwithstand- 
ing the difficulty of imagining any variation in the sensible heat of water circulating in ice. It is not the only 
fact in the glacier theory which seems to require some modification of the commonly received laws of latent 
heat at the very limit of congelation and liquefaction. 
2 e 2 
