78 VISCOUS THEORY OF GLACIER MOTION. [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 which may be withdrawn at pleasure.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 abed 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 over with a coloured powder, and the sluice is withdrawn.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 sur- 

 face 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.] 



