250 ON GLACIERS IN GENERAL. 



this, it is sufficient to observe, in the first place, that during the 

 height of summer the portions of those glaciers which move 

 fastest are never reduced below the freezing point, and that 

 even in the most favourable cases of nocturnal radiation pro- 

 ducing congelation at the surface, it cannot (by well-known 

 laws of conduction) penetrate above a few inches into the in- 

 terior of the glacier. Again, the ascertained laws of glacier 

 motion are (as will be immediately seen) entirely adverse to this 

 theory, as it is always accelerated by hot weather and retarded 

 by cold, yet does not cease even in the depth of winter.* 



It is singular how slow observers were to perceive the im- 

 portance to the solution of the problem of glacier motion of 

 ascertaining with geometrical precision the amount of motion 

 of the ice, not only from year to year, but from day to day ; 

 whether constant or variable at the same point, whether con- 

 tinuous or by starts ; if variable, on what circumstances it 

 depended, and in what manner it was affected at different 

 points of the length and breadth of a glacier. 



This method of studying the question was taken up by the 

 writer of this paper. His observations were commenced on the 

 Mer de Glace of Chamouni, in June 1842. Between the 26th 

 and 27th of that month the motion of the ice opposite a point 

 called the " Angle" was found, by means of a theodolite, to be 

 16'5 inches in 26 hours ; between the 27th and 28th, 17'4 

 inches in 25J hours ; and from about 6 A.M. to 6 P.M. on the 

 28th the motion was 9'5 inches, or 17'5 inches in 24 hours ; 

 whilst even the proportional motion during an hour and a half 

 was observed. No doubt could therefore remain that the mo- 

 tion of the ice is continuous and tolerably uniform in short, that 

 it does not move by jerks. He also ascertained about the same 

 time that the motion of the ice is greatest towards the centre of 

 a glacier and slower at the sides, contrary to an opinion then 

 maintained on high authority. He next found that the rate of 



* The fullest exposition of the Dilatation Theory is to be found in De Char- 

 pentier, Essai sur les Glaciers (1841), and AgaSsiz, Etudes snr les Glaciers 

 (1840). 



