Pièce 50 - James David Forbes to William Whewell

4 pp

Chaumoni - It is of great importance to examine the condition of one glacier at different seasons. JDF has been able to 'compare my general observations as to the structure and permanent phenomena on a great number of glaciers'. On one of his excursions he went with Studer [Bern. Studer] - 'the great Swiss geologist (and one whose merit has not been sufficiently recognized elsewhere partly owing to the jealousy of the French)'. JDF has made numerous drawings of the glaciers: 'I have arrived beyond all doubt at the normal type of the structure of all glaciers'. He has put his account in three letters which he has sent to Jameson [Robert Jameson], and which WW will receive when printed: 'I think I now see my way to a scheme of glacier movement...in which gravity continually acting on a semi solid - i.e. a solid always near its melting point - produces with time the same effect as it would speedily do with a viscous substance. I have no doubt that the glacier moulds itself to its bed like a viscous body - and falls down in the centre more rapidly than at the sides which are retarded by friction moulding itself into curves'. JDF will try to prove this by a comparison of two classes of facts: Firstly, the measures of motion in different parts of the ice at different seasons. Secondly, the statical facts of structure which JDF will try to show to be in accordance with these dynamical measures: 'I wish to explain to you that what I consider as original and important in my summers work is this: - that where as hitherto, so far as I know without exception (that is until the lectures which I delivered on glaciers last winter and my article in the Ed. Rev.) people have thought a theory of glaciers by looking at them simply, conceiving a cause a priori (as gravity, or dilatation) and have sought to prove it by direct experiments on the nature and structure of the ice, its temp. and the like, - I put the question of glacier motion as one of pure mechanics to be determined a posteriori from a knowledge of the empirical laws of that motion which are as definite and as determinable as the Elliptic Emperical Law of Planetary moton - thence to deduce the cause. I also was the first beyond any doubt to apply methods of determining the diurnal motion of the ice which involves the whole practical solution of the problem'.