1846.] ICE WHEN BRUISED FORMS NEW ATTACHMENTS. 141 



My chief analogies for the illustration of glacier motion 

 have been drawn from the motion of a river, and by that 

 comparison it in a great measure stands or falls. Slight and 

 partial as is our knowledge of the mechanics of imperfect 

 fluids, the explanation which I have given is founded upon that 

 knowledge, and it appears to me to be sufficiently precise to 

 warrant the inference of an identity of the mechanism in the two 

 cases ; namely, that the movement is due to the internal pres- 

 sures, arising from the weight of the mass, communicated partly 

 or principally in the manner of hydrostatic pressure through- 

 out a body whose parts are capable of moving or being shoved 

 over one another (by that exertion of force which Dr. Thomas 

 Young calls Detrusive Force*, which overcomes what is com- 

 monly called the Friction of Fluids), so that the velocities vary 

 from point to point of the moving body, being most rapid near 

 the surface and centre, and least so near the banks and bottom. 

 So viscous fluids move, so bodies (even brittle solids, such 

 as hard-boiled pitch) possessing the ordinary properties of 

 solid bodies often do, if sufficient time and sufficient force be 

 allowed ; f the efficiency of time being chiefly this, that a 

 pressure insufficient to produce instant detrusion, will, sooner 

 or later, cause the particles to slide insensibly past one another, 

 and to form new attachments, so that the change of figure may 

 be produced without positive rupture, which would reduce the 

 solid to a heap of fragments. This change may either take 

 place without any loss of homogeneity, or by numerous partial 

 and minute rents not everywhere communicating, and therefore 

 not necessarily destructive of cohesion, which may be termed a 

 bruise. 



A glacier is not a mass of fragments. As the analogy of 

 the glacier to a river in which the fluid principle is greatly in 

 defect, and the cohering or viscous principle is greatly in 

 excess, is the theory which I maintain, it is evident that the 

 analogy of a stream of sand, or loose materials shot from a 



* Lectures, I. 135. 



f See Professor Gordon's Experiment, Philosophical Magazine, March 1845. 



