248 ON GLACIERS IN GENERAL. 



of it is by far the most important application of mechanical 

 physics connected with the subject. 



Obvious as the fact itself must appear by what has been 

 already stated, manifest confusion has obtained in .the minds of 

 intelligent persons regarding it. Thus Ebel, in his well-known 

 Swiss guide-book, affirms the motion of the glaciers of Cha- 

 mouni to be 14 feet, and those of Grindelwald 25 feet in a year ; 

 quantities which, if they have any meaning, must refer to the 

 apparent advance of the lower termination of those glaciers 

 into the valley, which therefore only indicate the difference of 

 the real motion, and of the waste in any particular season, and 

 which may become null, or even negative, if the summer be 

 more than usually warm. The peasants, however who are 

 inevitably made aware of the progressive motion of the ice by 

 observing the progressive advance of conspicuous blocks on its 

 surface commonly ascribe to the glaciers the more correct 

 measure of several hundred feet per annum. 



M. Hugi, of Soleure, measured, with some accuracy, year by 

 year, the progress of a conspicuous block on the glacier of the 

 Aar, which he found to be 2200 feet in nine years, or about 

 240 feet per annum.* M. Agassiz continued some of these 

 annual measures, but only in a rough way, by causing his 

 guides to reckon the distance of a block on the moraine by 

 lengths of a pole or rod from a fixed rock some thousand feet 

 off. These measures appear not to have been altogether trust- 

 worthy. 



The principal theories to account for the progressive motion 

 of glaciers which were prevalent previous to 1842, may be briefly 

 characterised as De Saussure's and De Charpentier's, though 

 each had been maintained in times long antecedent by the 

 earlier Swiss writers. The first may for brevity be called the 

 gravitation theory, the latter the dilatation theory. Both sup- 

 pose that the motion of the ice takes place by its sliding bodily 

 over its rocky bed, but they differ as to the force which urges 



* Agassiz, Etudes surles Glaciers, p. 150. 



