1842.] LAWS OF MOTION OF THE HER DE GLACE. 15 



In particular, you will recollect that I pointed out last winter 

 two experiments for distinguishing between the prevailing 

 theories of De Saussure and De Charpentier, those of gravita- 

 tion and of dilatation. One was the exact measurement of a 

 space along the ice to be measured after a certain time, in order 

 to ascertain whether any expansion had occurred. The other 

 was the determination of the linear velocity of the glacier at 

 any point, which, on the theory of Saussure, ought (if the 

 glacier be of nearly uniform section) to be uniform throughout ; 

 on the theory of Charpentier it ought to increase from nothing 

 at the upper extremity of the glacier, to a maximum at its 

 lower end. The former experiment had, I have since learned, 

 been suggested by Professor Studer to M. Escher last year, and 

 attempted to be put in practice (though unsuccessfully) by the 

 latter, on the glacier of Aletsch. Admitting Charpentier 's 

 theory, however, this dilatation would be too small to be suc- 

 cessfully observed in a moderate time, and with the geometrical 

 methods which the uneven and varying surface of the glacier 

 enables us to employ ; I have therefore not attempted it. The 

 other method, in fact, embraces both ends ; for if the movement 

 of the glacier in its upper and lower part be determined (by 

 upper I mean near its origin), the difference of the motions 

 determines the dilatation or contraction of the intermediate pail 

 of the ice, and is liable to none of the great errors arising from 

 the measurement of long distances. The observation, in the 

 simplest and best form which I employ, resembles perfectly that 

 of determining with the transit instrument the progress of a 

 planet. 



I have already said that my later observations confirm those 

 which I previously communicated ; any variations, indeed, arise 

 solely from a change of circumstances or season, and not from 

 errors of observation. (1.) The continuous imperceptible 

 motion of the glacier is entirely confirmed ; its bearing upon the 

 sliding theory is very obvious. (2.) This motion is not by 

 any means the same, however, from day to day and from week 

 to week, as indeed already appeared from my first results. (3.) 



