1845.] VISCOUS THEORY OF GLACIER MOTION Illustrated. 77

stated (under Head III. above) that the velocity of any glacier is proportional to the completeness of its saturation with water at the time. The bas-névés, or old avalanches, furnish very little water at their lower extremities.

I have now gone through these observations, made by persons, it may be assumed, not particularly desirous to find results confirming a theory which they have opposed, but which it may be hoped they will oppose no longer, when their own results speak in language so unequivocal. My analysis has been succinct, but complete and impartial. The facts are stated as given, without selection or suppression.

XI. ILLUSTRATIONS OF THE VISCOUS THEORY OF GLACIER MOTION.-PART I. CONTAINING EXPERIMENTS ON THE FLOW OF PLASTIC BODIES, AND OBSERVATIONS ON THE PHENOMENA OF LAVA STREAMS.*

§ 1. Plastic Models. § 2. Analogy of Glaciers to Lava Streams.-Note on the Velocity of Lava.

§ 1. PLASTIC MODELS.

In the concluding chapter of iny "Travels in the Alps of Savoy," I have shown how the obscure relations of the parts of a semifluid or viscous mass in motion (such as I have attempted to prove that the glaciers may be compared to) may be illustrated by experiment.

The larger models, there described and figured, showed very clearly the precise effects of friction upon the motion of such a mass. They were formed of plaster of Paris, mixed with glue, and run in irregular channels, and the relativ

* From the Philosophical Transactions for 1846, p. 143. Received Society of London March 15-Read April 10, 1845.

cities of the top and bottom, the sides and centre of such a pasty mass were displayed by the alternating layers of two coloured pastes, which were successively poured in at the head of the model valleys. The boundaries of the coloured pastes were squeezed by the mutual pressures into greatly elongated curves, whose convexity was in the direction of motion; and in a vertical medial section, the retardation of the bottom and the mutual action of the posterior and anterior parts shaped the bounding surface of two colours into a spoon-like form.

Now these models convey a very palpable commentary upon the effects of friction on a plastic mass, and likewise on the influence of the mutual pressures of its parts; but in further illustration of the same thing I constructed another model, only executed as the printing of my volume approached its close, and which is cursorily described in a long note (page 377),* whence its real importance may perhaps have been pretty generally overlooked.

The models in question, of which I have since made many, are formed by accumulating in one end of a long narrow box AB, Plate I. fig. 1, a deep pool of the viscid material already mentioned, which is retained there by a sluice or partition C, which may be withdrawn at pleasure.

The surface of the pool abcd is then pretty thickly dusted over with a coloured powder, and the sluice is withdrawn.

The pasty mass subsides slowly under its own weight into the lengthened form efgh. The film of colour on the surface is therefore broken up so as to cover three or four times the surface it did at first; and its new distribution marks the lines of greatest separation of the superficial particles of the mass. The appearance of such a model when run is shown in fig. 2 of the same plate, and it manifests in the plainest manner the twofold tendency to separation in such a case where the channel is narrow and confined, and there is a certain mass of matter in

In this paper reference is of course made to the first edition of my "Travels," the second not having been then published. [The substance of the note referred to was re-written and introduced into the text at page 381 of the Second Edition.]

1845.] MODELS ILLUSTRATING THE VEINED STRUCTURE.

79

front. Plate V.* shows a more accurate drawing taken from such a model.

The lines of sliding separation occur most distinctly marked near the sides, where the friction is greatest, and the central parts are forced past the lateral parts, on account of the less embarrassed and consequently swifter motion of the centre; and they incline to the centre although the breadth of the channel be perfectly uniform. But the forces which tear asunder the parts (when such exist) act perpendicularly to the former,

and produce dislocations and fissures, which perfectly correspond to the direction and appearance of the crevasses of a glacier, that is, they are convex upwards or towards the origin of the glacier. It is the former of these lines of separation, or differential motion, which constitute and trace out with an exact parallelism the veined structure which I have described as forming the normal structure of all true glaciers. Plate VI.* is a representation of a very beautiful plaster model of more consistence than the other, in which the swelling of the surface and the direction of the open cracks produced by direct thrusts are

* [Plates V. and VI. of the Philosophical Transactions for 1846, which exhibit more elaborately the markings shown in the diagram of Plate I., fig. 2, of this volume, are not reproduced on account of their elaborate nature, but fig. 19 of this page gives a somewhat rude idea of one of them.]