GEOLOGY. 295 



hy cold weather and retarded by hot; and mu=t cease altogether in the 

 winter; all conditions which are totally at variance with the now known 

 nature of glacier motion. Further objections are, that in the height of 

 summer, those parts of a glacier which move fastest are never reduced be- 

 low the freezing-point; and that the congelation produced by nocturnal 

 radiation cannot possibly extend more than a few inches below the surface 

 of the glacier. 



Such was the state of glacier theories at the time when Prof. Forbes ap- 

 proached the subject. He at once perceived that it was hopeless to attempt 

 any explanation of the phenomena of glacier motion, without previously 

 obtaining precise numerical data as to the nature and extent of that motion. 

 Accordingly, in the summer of 1842, he commenced a series of observations 

 0:1 the Mer de Glace, which were continued, principally on the same glacier, 

 during the following years, by which he was enabled to determine the daily, 

 and even the hourly, motion of the glacier at different parts of its course, as 

 Avell as that of different surface-points of the same portion of the glacier. 

 !>v mean-; of similar observations carried on under his direction by Auguste 

 B:\lmat in 1844-5, the mean annual motion was determined, together with 

 the effect produced on its velocity by changes of temperature. The result of 

 these observations was the establishment of the following facts: The 

 glacier moves continuously, day and night, winter and summer; the motion, 

 however, is not uniform, being accelerated by heat and retarded by cold. It 

 moves with varying velocity in different parts of its course, according to 

 variations in the slope, width, and other physical peculiarities of its bed. 

 The ice in the centre moves faster than that at the sides, and that at the sur- 

 face faster than that at the bottom; the variation in velocity from the sides 

 to :he centre is always gradual, and is greater or less according to the actual 

 velocity of the glacier at the time when, and the point where, the observa- 

 tion is made. Now, these are precisely the laws by which t!ie motions of any 

 viscous fluid (of which a river may be taken as the most familiar example) 

 are governed; and, according to Prof. Forbes, they can only be expressed 

 by the following theory, to which the name of the Viscous or Plastic Theory 

 is commonly given : A glacier is an imperfect fluid, or viscous body, which 

 is urged down slopes of certain inclination by the mutual pressure of its 

 parts. The ice of which a glacier is composed is not perfectly coherent, 

 but is traversed in all directions by capillary fissures, which are always more 

 or less charged with water, derived from the surface-melting of the glacier 

 itfelf, and of the snow-fields by which it is fed; and as the fluidity, and con- 

 sequently the velocity, of the glacier varies with the amount of water which 

 it contains, the retardation of its motion in the winter, and its acceleration 

 in the summer are fully accounted for. The lowering of the surface of the 

 glacier, which takes place during the summer, arising partly from superficial 

 melting, partly from the attenuation and collapse of the parts which move 

 most rapidly, is repaired during the winter, when the velocity of the whole 

 glacier is diminished, and, the higher regions of the glacier moving relatively 

 faster than the lower, the yielding mass of ice is pressed upwards in a ver- 

 tical direction. 



The above theory expresses so completely all the observed facts of glacier 

 motion, that it gradually acquired all but universal acceptance, notwithstand- 

 ing the startling nature of its assertion of the plasticity of a body which we 

 have always been accustomed to regard as one of the mo^t brittle of known 

 substances. Professor Tyndall, "however, while fully admitting that the 



