502 THE LIFE OF JAMES D. FORBES. [CHAJ-. 



theorist, the water already in the crevices and fissures of 

 the ice, and in contact with ice, instantly freezes. Not at 

 all ; for where is it to deposit the heat of fluidity, without 

 which it cannot, under any circfim stances, assume the 

 solid form ? The ice surrounding it cannot take it ; for, 

 being already at 32, it would melt it. It can only, 

 therefore, be slowly conveyed away through the ice to 

 the surface, on the supposition that the cold is sufficiently 

 intense and prolonged to reduce the upper part of the 

 ice considerably below 32, The progress of cold and 

 congelation in a glacier will therefore be, in general, 

 similar to that in earth, which, it is well known, can be 

 frozen to the depth of but a few inches in one night, 

 however intense the cold. Such a degree and quantity 

 of freezing as can be attributed to the cold of a summer's 

 night must therefore be absolutely inefficient on the mass 

 of the glacier. 



* I will not stop to consider the attempt made by M. de 

 Charpentier to show that the friction of any length of a 

 glacier upon its bed may be overcome as easily as the 

 shortest, from a consideration of the forces producing 

 dilatation ; but it is as indefensible on mechanical grounds 

 as the preceding theory is on physical ones (Essai, p. 106). 

 I quote from M. de Charpentier, not because his defence 

 of the theory of dilatation is more assailable than that of 

 others, but because his work is the only one in which an 

 attempt is made to explain its physical principles with 

 precision. 



4 1 cannot admit, then, that either the sliding or dilata- 

 tion theory can be true in the form which has hitherto 

 been given to them. When I first began to study the 

 subject minutely, under the auspices of M. Agassiz, in 

 1841, its difficulty and complication took me by surprise ; 

 and I soon saw that to arrive at any theory which, con- 

 sistent with the rigour of ph} T sical science at the present 

 time, would be worthy of the name, a very different 

 method of investigation must be employed from that 

 which was then in use by any person engaged in studying 

 the glaciers. 



