Ch. XVI.] 



MOTION OF GLACIERS. 



371 



middle region of the glacier than at its extremity.* Prin- 

 cipal J. D. Forbes, who had joined Professor Agassiz during 

 his earlier investigations in the Alps, undertook himself an 

 independent series of experiments, which he followed np 

 with great perseverance, to determine the laws of glacier 

 motion. These he found to agree very closely with the laws 

 governing the course of rivers, their progress being greater 

 in the centre than at the sides, and more rapid at the surface 

 than at the bottom. This fact was verified by carefully 

 fixing a great number of marks in the ice, arranged in a 



straight line, which gradually assumed a beautiful curve, the 

 middle part pointing down, the glacier, and showing a velo- 

 city there, double or treble that of the lateral parts. t He 

 ascertained that the rate of advance by night was nearly the 

 same as by day, and that even the hourly march of the icy 

 stream could be detected, although the progress might not 

 amount to more than six or seven inches in twelve hours. 

 By the incessant though invisible advance of the marks 

 placed on the ice, ' time/ says Mr. Forbes, ' was marked out 

 as by a shadow on a dial, and the unequivocal evidence which 

 I obtained, that even whilst walking on a glacier we are, day 

 by day, and hour by hour, imperceptibly carried on by the 

 resistless flow of the icy stream, filled me with admiration.' J 

 In order to explain this remarkable regularity of motion, and 

 its obedience to laws so strictly analogous to those of fluids, 



in 



first hinted by Rendu, that the ice, instead of being solid 

 and compact, is a viscous or plastic body, capable of yielding 

 to great pressure, and the more so in proportion as its tem- 

 perature is higher, or as it approaches more nearly to the 

 melting point. He endeavoured to show that this hypothesis 

 would account for many complicated phenomena, especially 



for a ribboned or veined structure which 



is everywhere 



observable in the ice, and might be produced by lines of 



* See Systeme Glaciaire, by Agassiz, of the sides, and was, therefore the first 



iTTrvt- onrl TIdcat -rvrx IQft A 3 7 AAZ +^ «~„ i. 1 • . . ' 



Guyot, and Desor, pp. 436, 437, 445. 

 M. Agassiz, at p. 462, states that he 

 published in the Deutsche Vierteljahr- 

 schrift for 1841, this result as to the 



central motion being greater than that p. 133. 



to correct his own previous mistake. 



f J. D. Forbes. 8th Letter on Gla- 

 ciers, Aug. 1844. 



+ J ; D - Forbes. Travels in the Alps, 



B n 2 



