1845.] ANALOGY OF GLACIERS TO LAVA STREAMS. 81 



been much neglected by writers on hydraulics ; but in one of 

 the most ancient hydraulic treatises, that of Leonardo da Vinci, 

 lately printed from the MS. in the Italian collection of writers 

 on hydraulics, they are very well described and figured. A 

 case parallel to the last mentioned, where a fixed obstacle 

 cleaves a descending stream and leaves its trace in the fan- 

 shaped tail, is well seen in several glaciers, as in that at Fer- 

 pecle, and the Glacier de Lys on the south side of Monte Rosa, 

 particularly the last, where the veined structure follows the law 

 just mentioned.* And I desire here to record that the views 

 just presented as to the origin of the veined structure of ice 

 were confirmed, but were not suggested, by the experiments on 

 viscous fluids just mentioned. The necessity of the tearing 

 up of a solid mass, if it moved at all in a bed presenting insur- 

 mountable resistances on all sides, in directions such as the 

 veined structure presents, was foreseen by me whilst dwelling 

 amongst the glaciers themselves, at a distance from books or 

 the means of experiment. . * . * My Third Letter to Pro- 

 fessor Jameson, written in 1842 from the remote village of 

 Zermatt, contains the substance of all that I have since deve- 

 loped and illustrated at greater length and in different ways, 

 rather to meet the difficulties of others, than to confirm what 

 was plainly fixed in my own mind.f 



In explaining the theory of the veined structure at a 

 meeting of the Royal Society of Edinburgh on the 20th 

 of March 1843, I stated that I had arrived at the conclusion 

 that crevasses resulting from tension in certain parts of a 

 glacier must be formed at right angles to the surfaces of 

 discontinuity or structural veins where they intersect the 

 surface : a law conformable to the empirical one discovered 

 by me on the glacier of the Rhone in 1841 4 since general- 

 ized in other cases, and which even the adversaries of my 



* [See Travels in the Alps, p. 328. J 



f [See page 23 of this volume. The quotation from it is therefore omitted.] 



^ Edinburgh PhilosophicalJournal, January 1842, [and p. 7 of this volume.] 



