Notice of Prof. Forbes 1 s Travels in the Alps. 177 



be " partly owing to the dilatation of ice during winter, by the congela- 

 tion of the water in its fissures, producing at the same time the veined 

 structures." The glacier is, indeed, very far from being frozen to the 

 bottom during winter, for a great portion of the icy mass is still plastic ; 

 but the congelation extends to a considerable depth, and produces the 

 usual effects of expansion. This cause alone appears, however, to be 

 inadequate to the effect, and the author attributes the elevation of the 

 surface mainly to thq diminished fluidity of the glacier in cold weather, 

 which retards the motion of all its parts, and especially of those that 

 move most rapidly in summer. The ice is more pressed together, and 

 less drawn asunder ; the crevasses are consolidated ; " while the in- 

 creased friction and viscosity cause the whole to swell, and especially 

 the inferior parts, which are the most wasted." 



" Such a hydrostatic pressure, likewise, tending to press the lower 

 layers of ice upwards to the surface, may not be without its influence 

 upon the (so called) rejection of blocks and sand by the ice." 



It is well known that masses of stone, when below, rise to the surface, 

 and those on the surface remain there, and continue age after age to 

 travel downwards towards the plains. Prof. Forbes, although regarding 

 the views expressed above as only conjectural, has, however, no doubt 

 that the convex surface of the glacier (which resembles that of mercu- 

 ry in a barometer tube) is due to the hydrostatic pressure acting upwards 

 with most energy near the centre. " Exactly contrary is the case in a 

 river, where the centre is always the lowest; but that is on account of 

 the extreme fluidity, so that the matter runs off faster than it can be 

 supplied." This view is believed not to be inconsistent with the former 

 supposed great extension of glaciers, even from Mount Blanc to the Jura 

 mountains, of Geneva, more than fifty miles, in which case they must 

 have moved " with a superficial slope of one degree, or in some pai'ts 

 even of a half or a quarter of that amount, whilst in existing glaciers, 

 the slope is seldom or never under three degrees. The declivity requi- 

 site to insure a given velocity, bears a simple proportion to the dimen- 

 sions of a stream. A stream of twice the length, breadth, and depth 

 of another, will flow on a declivity half as great ; and one of ten times 

 the dimensions, upon one-tenth of the slope." 



The author finishes his view of the philosophy of the glaciers with 

 the following beautiful reflections : " Poets and philosophers have de- 

 lighted to compare the course of human life to that of a river ; perhaps 

 a still apter similitude might be found in that of a glacier. Iieaveu- 

 descended in its origin, it yet takes its mould and conformation from the 

 hidden womb of the mountains which brought it forth. At first, soft 

 and ductile, it acquires a character and firmness of its own, as an inev- 

 itable destiny urges it on in its downward career. Jostled and con- 



Vol. xlvi, No. 1.— Oct.-Dec. 1843. 23 



