WCP4878

9, St. Mark’s Crescent

March 13th. 1869

Dear Sir Charles

Many thanks for your kindness in again speaking to Earl de Grey¹ about me. I can quite understand that with the immense amount of Government business now on hand, and especially the new Education bills which he & Mr. Forster² will especially have to attend to, they can give no time to such a comparatively small matter as the East London Museum.³

I should think that on consideration & enquiry the Duke of Argyll⁴ will give up his idea of an Indian Nat[ural]. History Mus[eum]. Our Indian Empire is now so extensive that any thing like a Complete Indian Museum would be a very large affair; — & an [2] incomplete or stationary one would be of very little use or interest.

I shall be glad to hear what answer Mr. Croll⁵ makes you, as I feel sure that in one of his papers he alludes to the erect trees as being "certainly since the glacial epoch".

I read Canon Moseley[’]s article about glaciers⁶ & Mr. Croll’s explanation, which I think a very poor & unsatisfactory one. It seems to me that Canon Moseley[’]s⁷ reasoning is quite inapplicable & his premises unsound. His experiment to determine the "shearing" resistance of ice, seems to me to have no application whatever to the actual conditions of a glacier or of glacier motion. His experiment only shows the force required to rend asunder by a lateral sliding motion the entire section of a glacier at the same [3] instant , — quite ignoring the fact that the ice of a glacier must be in varying states of tension, & the whole weight of a large portion of it may be concentrated to produce rupture at one determinate point or line. This rupture too, will be more or less successive, each simultaneous through the entire thickness or mass of the glacier:

Two diagams labelled Fig. I. and Fig. II. are drawn in the left-hand side of the page beneath the preceding text. Fig I. depicts two blocks of wood with a line labelled 'a' running down the middle. Fig II. depicts a rectangular box labelled 'a' supported by a curved surface labelled 'A']

Fig. I. shews⁸ Canon Moseley[’]s experiment. Force being applied to slide the two blocks of wood A & B over each other and rupture the ice; at the line a. to do which the cohesion of the whole sectional area must be overcome simultaneously. But if we take the same cylinder of ice and supporting it on a curved surface as at A. fig. II and then apply pressure [4] on each side so as to produce a fracture at a. the force required even with a very short leverage will be only a fraction of that necessary in the other experiment. Now owing to the irregular surfaces of the ground at the under side of glaciers and the constant melting away of the ice at its under surface, vast masses must often be rest on saddles or bridge over hollows, and the whole weight of the mass ^will cause a fracture on one determinate line of least resistance. A slight relief to the tension being thus given, a new line becomes that of least resistance & in turn gives way, and thus the glacier moves regularly downwards.

Canon Moseley goes on the principle that each sectional inch or foot of the glacier, can only be moved by a column of ice of the same sectional area behind it, and that [5]⁹ the "shearing" must take place over the whole sectional surface at once, — but it is clear that horizontal no less than vertical fracture, would occur, not over whole miles of the glacier at once, but in certain limited areas where unequal tension had reached its maximum, and the weight of [one word illeg. crossed out] ^{larger areas} of the glacier might act to produce successive fractures of a few feet or a few hundred feet at a time. If the whole glacier were ^{at any moment} as compact and uniform in its molecular condition as the square inch of ice on which Canon Moseley experimented, and everywhere in close contact with its bed & sides, his calculations might be available to some extent, though even then, owing to unequal slope and lateral inequalitites, the weight would [6] bear most unequally on different portions, and a number of lines and surfaces of tension & least resistance be immediately formed. The numerous crevasses on every glacier are the proof & the result of such tensions.

I have no doubt Tyndall¹⁰ will answer Canon Moseley in due time. Croll’s theory of molecular motions seems to me equivalent to Forbes¹¹ viscosity, — & if true there ought to be few or no crevasses.

We have made arrangements for a cheap edition of my [7] book as soon as this one is out.¹² I could have put a score more illustrations, but of course was obliged to have a limit. The one you suggest would be very effective,¹³ but I am afraid people would complain if there was more in the cheap than in the dear edition.

Believe me | Yours very faithfully | Alfred R. Wallace [signature]

Sir Charles Lyell, Bart.