Professor Forbes's Fifteenth Letter on Glaciers. 145 



where described,* and in Professor Gordon's beautiful expe- 

 riment with brittle pitch in motion. t 



A body soft enough to convey a pressure partly hydro- 

 static, or one acting in any direction, — if it be tenacious 

 enough, — always tends to crease^ or have its surface near the 

 point of pressure pushed upwards and forwards relatively 

 to the surface farther off. A heavy weight laid on a tough 

 paste will raise a wrinkle round it, but at some distance ; 

 farther off in proportion to its toughness. So railway em- 

 bankments raised on a boggy bottom force out a crease, or 

 two or three successive parallel creases, on either side ; and, 

 I daresay, you recollect that one effect of the great land-slip 

 at Lyme, some yeart* ago, was to elevate a ridge of shingle 

 above the level of the sea at some distance from the shore. 

 A succession of equidistant wrinkles will be formed when- 

 ever different parts of a plastic body are subjected in succes- 

 sion to a pressure violent enough to produce detrusion, or 

 sliding amongst the particles. Thus, if a very viscid fluid is 

 poured in a gradual stream upon a flat surface, so that it may 

 spread uniformly, a succession of circular creases is formed 

 in consequence of the hydrostatic pressure from the heaped- 

 up centre becoming sufficient to overcome, for a moment, the 

 viscosity at a certain distance from the centre ; as the cir- 

 cumference rises in a crease, the centre falls, the central 

 pressure suddenly diminishes, and if the stream continue to 

 be poured uniformly upon the centre, although the circles 

 will expand slightly, it will not be until a sufficient head is 

 again raised to overcome suddenly the viscosity of the fluid 

 that a new wrinkle is formed in exactly the same relative 

 position as the first. J Treacle, mortar, tar, and similar bo- 

 dies, usually present such creases when poured out. What 

 has now been said of a viscid mass spreading uniformly from 

 a centre applies equally to one confined in a trough with pa- 

 rallel sides, if constantly fed from one end. A succession of 

 waves are thus formed, as in Mr Milward's mud-slides, or as 



* Phil. Trans., 1846. f Phil. Mag., 1845, vol. xxvi., p. 206. 



X The steps of this process are attempted to be illustrated by the curves in 

 Plate III., fig. 4. 



VOL. XLVI. NO. XCI. — JAN. 1849. K 



