Professor Forbes on (he Leading Phenomena of Glaciers. 231 



tides of fluids, may have every conceivable variation ; the ex- 

 tremes are perfect fluidity on the one hand, and perfect rigi- 

 dity on the other. A good example is seen in the process of 

 consolidation of common plaster of Paris, which, from a con- 

 sistency not thicker than that of milk, gradually assumes the 

 solid state, through every possible intermediate gradation. 

 Even water is not quite mobile ; it does not run through ca- 

 pillary tubes ; and a certain inclination or fall is necessary to 

 make it flow. This may be roughly taken as an index of the 

 quality of viscidity in a body. Water will run freely on a 

 slope of 6 inches in a mile, or a fall of 1-10,000 part,* another 

 fluid might require a fall of 1 in 1000 ; whilst many bodies 

 may be heaped up to an angle of several degrees before their 

 parts begin to slide over one another. 



Thus, a substance apparently solid may, under great pres- 

 sure, begin to yield ; yet that yielding, or sliding of the parts 

 over one another, may be quite imperceptible upon the small 

 scale, or under any but enormous pressure. A column of the 

 body itself is the source of the pressure of which we have now 

 to speak. 



Even if the ice of glaciers were admitted to be of a nature 

 perfectly inflexible, so far as we can make any attempt to bend 

 it by artificial force, it would not at all follow that such ice is 

 rigid, when it is acted on by a column of its own material se- 

 veral hundred feet in height. Pure fluid pressure, or what is 

 commonly called hydrostatical pressure, depends not at all for 

 its energy upon the slope of the fluid, but merely upon the 

 difference of level of the two connected parts or ends of the 

 mass under consideration. If the body be only semifluid, this 

 will no longer be the case ; at least the pressure communica- 

 ted from one portion (say of a sloping canal) to the other, will 

 not be the whole pressure of a vertical column of the material, 

 equal in height to the difference of level of the parts of the 

 fluid considered ; the consistency or mutual support of the parts 

 opposes a certain resistance to the pressure, and prevents its 

 indefinite transmission. It must be recollected, that, in the 



• According to Dubuat (Hydraulique, tom.i., p. 64. Edit. 1816) at a slope a 

 great deal lower ; but its exact value does not now concern us. 



