204 ANNUAL OF SCIENTIFIC DISCOVERY. 



success ; and the main credit of attaining to this result i; unquestionably 

 due to Professor J. D. Forbes. lu the course of several successive journeys 

 to Switzerland, extending from 1811 to 187), he instituted a scries of minute 

 observations on the phenomena of glacier action, from which he was at 

 length enabled to elaborate a theory by which these phenomena were ade- 

 quately explained. Until very recently, Professor Forbes's hypothesis met 

 with all but universal acceptance; but early in 18-37 it was controverted by 

 Professor Tyndall, whose objections to it have not been removed by the 

 result of observations and experiments made by him chiefly during the last 

 two summers. The question being thus reopened, it became a matter of 

 interest to the public generally to ascertain more exactly the details of 

 Professor Forbes's theory, and the grounds on which it rests; and with 

 a view of supplying full information on this subject, the scattered papers of 

 Prof. F. have been collected and published during the past year, by Messrs. 

 Black, of Edinburgh. The appearance of this book furnishes a fitting op- 

 portunity for an attempt to estimate the contributions made by its author 

 to our knowledge of the laws of glacier action; for which purpose it is 

 necessary to state briefly the opinions which, before his time, were entertained 

 on the subject. 



The most striking and obvious phenomenon connected with glaciers is 

 unquestionably their continuous motion. That they do move, is a fact which 

 must have been evident to the earliest observers, from the barest considera- 

 tion of the circumstances of the ca.se. A glacier being a stream of ice, is- 

 suing from, and serving as an outlet to, the reservoirs of eternal snow, and 

 extending thousands of feet below the line of perpetual frost, it is obvious 

 that its lower extremity must be constantly thawing; so that, unless tho 

 glacier were as constantly advancing, its permanent existence below the 

 snow-line would be an impossibility. Motion, therefore, being the one 

 necessary condition of the existence of a glacier, the first task of the gla- 

 cier theorist was plainly to assign a cause for this motion. The earliest 

 theory on this subject was that known as the Gravitation Theory, originally 

 proposed by Gruner, in a work published at Berne in 1700, but more gen- 

 erally associated with the name of De Saussure, its most illustrious expo- 

 nent. According to this theory, the masses of ice which comprise the 

 glacier, slide bodily over their rocky bed, urged by their own weight, the 

 motion being facilitated by the melting of the ice at the bottom of the 

 glacier by contact with the warmer earth. The fatal objection to this 

 theory is, that a sliding motion of this kind, when once commenced, must 

 be accelerated by gravity, and the glacier must slide from its bed in zi.\ ava- 

 lanche. Other valid objections are also found in the small slope of most 

 glacier-beds, and their frequent contractions and changes of form through 

 which it would be impossible that a rigid, unyielding mass of ice could be 

 urged. The second hypothesis, known as De Charpentier's, or the Dilatation 

 Theory, ascribes the moving force to the well-known expansion which water 

 undergoes when converted into ice. A glacier is traversed by innumerable 

 minute fissures, which, during summer, are filled with water, during the 

 daytime; and this water, being frozen during the night, by its expansion 

 thrusts forward the whole mass of the glacier in the direction of its slope. 

 The same opinion was adopted by Agassiz, with the modification that the 

 water freezes, not in minute fissures in the glacier, but in the capillary ducts 

 by which its granular masses are traversed. According to this theory, the 

 motion of a glacier must take place by fits and starts; it must be accelerated 



