Professor Forbes's Fifteenth Letter on Glaciers. 147 



the upward and forward force due to the quasi-hydrostatic 

 pressure of the mass becomes insupportable, and gives rise 

 to the forced separation of the cohering substance by count- 

 less fissures, constituting the frontal dip of the veined struc- 

 ture of the glacier, whose position, taken in connection with 

 the wrinkles^ is shewn in fig. 5. 



I farther beg leave to direct your attention to a very 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 efi'ect of a steady pressure upon 

 a semisolid 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 like form, advanced in the centre, and withdrawn or retarded 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 

 uproards and forwards^ as I have maintained that the mass of 

 ice is, in consequence 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 equals 

 as ii;i one of the specimens herewith sent, — being nothing else 

 than the wrinkles exaggerated, and the bruise producing the 

 the veined structure pushed to an actual separation. (See 

 fig. 6.) These specimens (and such may be found in the work- 

 shop of almost any machine-maker)* have the higher degree 

 of interest, because 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 tenacious bodies, but even to streams no 



* They are not bo 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 

 mentioning recently these observations to Mr James Naysmith of Manchester, 

 he stated it to me, as a fact familiar to practical men, that, in turning ^ cylin- 

 der 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 mecha- 

 nism of the glacier ! It is this thickening, amounting to one-fifth part, which 

 compensates during winter, the summer's waste. 



