in Scotland and Parts of England. 267 



of gravitation to pull a glacier along in its bed ; for it so 

 happens that the main effect of Professor Forbes' s investiga- 

 tion, has been to impress such a notion, while the actual 

 terms of his proposition are forgotten or overlooked. These 

 were, that a glacier is " urged down a slope of a certain in- 

 clination, by the mutual pressure of its parts." As far as I 

 can understand the views of our learned associate, there is a 

 hydrostatical pressure from a column of the same material 

 acting on a superior level, and thus pushing along what is at 

 the lower level. Mr Forbes says, " 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." It appears that the 

 less fluid the body, the less is this the case, from the resistance 

 which the viscosity presents ; but, at the same time, the greater 

 the viscosity, the more will the retardation due to friction be 

 distributed throughout the mass ; so that the sliding of the 

 bottom of the fluid over its bed, will be the more facilitated. 

 The glacier, of course, being a highly viscous body, will be 

 comparatively slow to yield to the hydrostatical pressure of the 

 more elevated parts ; but it will, and does yield in a certain 

 reduced degree, and its comparative viscosity ensures that 

 its base shall not be left greatly behind its middle and super- 

 ficial parts — that it must, in short, slide bodily, and so graze 

 the bed or surface over which it moves. 



The question occurs, over how small an inclination will a 

 mass constituted in the manner of a glacier slide'? We see 

 glaciers in the Alps, moving at the rate of above two hun- 

 dred yards in a year, over a bed with walls and impeding 

 projections, which has a mean slope of 5°. Will it move at all 

 over a very much smaller inclination 1 We have an answer 

 on this point from Professor Forbes : — " Large and deep 

 rivers," says he, " flow along a much smaller inclination than 

 small and shallow ones. . . . The most certain analogy 

 leads us to the same conclusion in the case of glaciers. We 

 cannot, therefore, admit it to be any sufficient argument 

 against the extension of ancient glaciers to the Jura, for ex- 

 ample, that they have moved with a superficial slope of one 



