Pièce 146 - James David Forbes to William Whewell

10 pp

Pitlochry, Perthshire - JDF gives his answers to WW's questions. Firstly, 'crevasses' - 'it is now generally allowed...to have a very secondary importance in the theory of glacier motion'. The 'zig zag form of the glacier complicates matters very much by superimposing different systems'. JDF has always maintained that the direction of crevasses is perpendicular to the veined structure, which is often almost the same as perpendicular to the curves of motion. JDF gives an historical overview of the views of the principal scholars involved in glacier theory, and their views on glacier motion. Secondly, 'veined structure' - 'From the very first I affirmed that the veined structure 'appears' to be perpendicular to the lines of greatest pressure'. John Tyndall and William Hopkins simply reiterate this. 'Nor can I for the life of me make out that Tyndall has found or stated any physical reason why pressure produces the effect, beyond William Thomson's explanation of the fact, which I fear is too subtle to be true'. JDF has 'never pretended to be able to define with precision forms which on my theory of differential motion of a semifluid or plastic mass, the vents, producing by reattachment the veined structure, would assume'. He does not think anybody in this country - except perhaps William Thomson, George Stokes and maybe George Airy - 'could grapple with the problem. The quasi fluid pressure of a plastic mass and its comparatively small tangential resistance, cannot be left out of account'. JDF's mechanical principles are generally adequate to the explanation. Above all he relies on 'Plastic models, to the results of which (on this subject) my adversaries have never dared to allude'. The models show the forms of fluid motion and were reproduced by Tyndall in the Philosophical Transactions 'with an almost illusory acknowledgement to me'. However, his most impressive model in which powdered surfaces are 'broken up by tangential motion' and realigned into 'thread like bands' corresponding to those on glaciers, as well as possessing partial crevasses exactly perpendicular to them, 'have been kept studiously out of sight'. Tyndall and Hopkins no longer send him 'copies of papers or books in which I am most deeply interested, and in which my name is most freely used'.