166 VISCOUS THEORY OF GLACIER MOTION. [1846. 



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 compres- 

 sions, within the body of the glacier itself, which yields, owing to 

 its slightly ductile nature, in the direction of least resistance, 

 retaining its continuity, or recovering it by re-attachment 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 per- 

 fectly fluid state through its pores,* 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 porous 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 

 exceeding the power of the drainage to carry it off. The cir- 

 culating vessels are therefore overcharged. In the latter case 

 the superficial supply is stopped, the drainage goes on slowly 



* This I think is undeniable, from the appearance of the collapsed crevasses 

 above referred to, notwithstanding 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. [See later in this 

 volume, the Sixteenth Letter on Glaciers, and the paper entitled " On some Pro- 

 perties of Ice near its Melting Point."] 



