Item 57 - James David Forbes to William Whewell

4 pp

Salerno - Thanks WW for his letter and proceeds to answer his query regarding the veined structure of glaciers: WW objects that 'the shells produced by the rupture of the parts of the ice, caused by excessive friction should get all parallel to the sides and bottom of the trough of the glacier, instead of being inclined from the sides inwards and forwards towards the centre and from the bottom upwards and forwards'. JDF has tried to explain his argument for the veined structure in the final chapter of his book [Travels through the Alps of Savoy and other parts of the Pennine Chain with Observations on the Formation of Glaciers, 1843]. It is the most difficult and least obvious part of his theory. JDF gives a detailed outline with drawings of his argument: 'The tearing asunder of the particles of the glacier owing to the friction of the sides is nearly but not quite parallel to the sides, for this reason, that the lines of greatest strain are determined naturally by the force of gravitation which urges the particles forwards, but there is a drag towards the centre of the stream in consequence of the greater velocity there...The veined structure of the ice corresponds to the ripple of the water, a molecular discontinuity whose measure is not comparable to the actual velocity of the ice, and therefore the general movement of the glacier, as indicated by its moraines, remains sensibly parallel to the sides'. The same thing happens in lava flow.