218 FIFTEENTH LETTER ON GLACIERS. [1848. 



curious illustration of these views which I lately noticed. In 

 the mechanical turning or planing of malleable iron, the spiral 

 shavings have a structure which is truly remarkable, and shews 

 convincingly that the effect of a steady pressure upon a semi- 

 solid or plastic body, is really such as to produce not merely 

 wrinkles or creases on the surface, in the usual wave-like form, 

 advanced in the centre, and withdrawn or retarded at the sides; 

 but that the shaving has its particles squeezed upwards and 

 forwards, as I have maintained that the mass of ice is, in con- 

 sequence of the intense frontal resistance, and when the tenacity 

 of the metal is pushed to its utmost limit of endurance, detrusion 

 takes place at intervals sensibly equal, as in one of the speci- 

 mens herewith sent, being nothing else than the wrinkles 

 exaggerated, and the bruise producing the veined structure 

 pushed to an actual separation. (See Plate IX. fig. 6). These 

 specimens (and such may be found in the workshop of almost 

 any machine-maker) * have the higher degree of interest, be- 

 cause the surface of detrusion makes so very large an angle with 

 the line of pressure. This process of heaping up by internal 

 sliding of the parts of a semifluid mass was pointed out by me, 

 I believe, for the first time, as applicable not only to very tena- 

 cious bodies, but even to streams no more viscid than common 

 water. But I concluded that when the frontal resistance (due 

 to friction and cohesion) becomes very great, the planes of least 

 resistance may assume an inclination of 60 or more, a notion 

 which has been treated as practically untenable by a mathema- 

 tical critic of my theory, whilst he admits that it is theoretically 

 possible. The iron shavings in question demonstrate the truth 



* They are not so common as I supposed when I wrote this ; they are princi- 

 pally to be found when coarse planings are made from iron of not the very best 

 quality, and not lubricated with water. The finest iron is too plastic. On men- 

 tioning recently these observations to Mr. James Naysmith of Manchester, he 

 stated it to me, as a fact familiar to practical men, that, in turning a cylinder three 

 feet in circumference, the shaving, owing to frontal condensation, and the over- 

 riding of the parts, is perhaps only two and a half feet long. How perfect the 

 analogy with what I have always maintained to be the mechanism of the glacier ! 

 It is this thickening, amounting to one-fifth part, which compensates during winter 

 the summer's waste. 



