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XII. 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. 
By James D. Forbes, Esq., F.R.SS. L. and E., Corresponding Member of the Institute 
of France, and Professor of Natural Philosophy in the University of Edinburgh. 
Received March 15, — Read April 10, 1845. 
^ 1. Plastic Models. 
§ 2. Analogy of Glaciers to Lava Streams. 
Note on the Velocity of Lava. 
§ 1. Plastic Models. 
In the concluding chapter of my “ 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, these 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 relative velocities 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 direc- 
tion 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 ex- 
ecuted as the printing of my volume approached its close, and which is cursorily de- 
scribed in a long note (page 3 77)*, 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 accu- 
mulating in one end of a long narrow box, AB, Plate IV. fig. 1, a deep pool of the viscid 
* In this paper reference is of course made to the first edition of my “ Travels,” the second not having been 
then published. 
MDCCCXLVI. U 
