EFFECT OF FLUID PRESSURE IN THE CAPILLARY FISSURES. 
207 
the glacier of Bossons moves slower than its middle portion ; there is therefore a 
manifest longitudinal compression arising from the friction of the bed*. 
3. The next stage is that of the perfect bruise or veined structure, best seen in the 
most united and least fissured parts of glaciers with rocky sides and moving over a 
moderate slope. Whatever increases lateral compression (without however necessi- 
tating dislocation), such as the union of two or more glaciers in one, tends to develope 
the structure more perfectly'!'. Such cases are well seen on several parts of the Mer 
de Glace, and of the glacier of Miage^. 
4. In very wide glaciers, moving with feeble velocities, the veined structure is 
slightly developed, except near the sides, simply because the twist being small the ice 
is hardly bruised. Nor can we wonder to find the structure at the distance of many 
hundred yards from the sides of a vast slow-moving glacier of this description, if de- 
veloped at all, to be complex and irregular, exhibiting twists such as I have figured 
in my Travels, p. 164, and which are peculiarly conspicuous in the magnificent gla 
cier of Aletsch. This circumstance finds a precise analogue in the case of a great 
river, such as the Rhine, or indeed in any river moving with a very slight inclina- 
tion ; the excess of velocity of the central above the lateral parts, not very great at 
any rate, is distributed over such a space that the slightest casual disturbance of the 
current, from an irregularity in the bottom or sinuosities of the course, produces local 
differences of velocity, occasioning ripples and eddies in various parts of the breadth. 
If these ripples and eddies, in other words, differential motions of adjacent particles, 
could be visibly represented by using differently coloured fluids, they would un- 
doubtedly afford sections exhibiting undulations and contortions exactly like those 
which the ice presents in the cases mentioned above. We claim therefore the ap- 
parent exception as a real proof of our general rule. 
5. In the ndvd proper, no true veined structure is developed ; jirst, because, whilst 
the mass is snowy, its powdery nature yields without admitting of a fracture or 
bruise ; secondly, because the true n6v6 has rarely any lateral compression worth 
mentioning, being widely spread and not contained between steep barriers ; thirdly, 
because its motion is altogether very small ; lastly, because its extreme dryness does 
not afford water enough to percolate its substance and there to be frozen ; when it 
does so, it ceases to be ndvd. 
On these grounds I hope that the theory of the veined structure, so important to 
that of glaciers, may be considered as explaining a number of intimately connected 
phenomena. 
* The internal rents in the lava of Zafarana referred to in § 2 of this paper, and figured in Plate IV. fig. 8, 
present a perfect analogy with those of the glacier of Bossons, and appear to be due to the same cause. 
t Third Letter. Travels, Appendix, p. 407. 
♦ See the figures of the structure of the glacier of Miage. Travels, p. 197. 
2 E 
MDCCCXLVI. 
