96 Prof. Tyndall on Ice and Glaciers. 



face, can, I think, unless influenced by previous theoretical views, 

 entertain any other opinion." Is it to be supposed that convic- 

 tions thus strongly uttered, based upon years of observation, 

 and established, according to the above quotation, by the testi- 

 mony of the senses themselves, are meant to be reversed by a 

 single observation which, after all, is essentially defective, in- 

 volving, in reality, not a fact, but an opinion ? The supposition is 

 unreasonable, and will appear still more so when it is remembered 

 that throughout the entire letter Prof. Forbes never once tells 

 us that he has changed his views regarding the origin of the 

 veined structure, though he does acknowledrje a change of view 

 upon a different subject. 



On tlic conversion of neve into ice the latter is sufficiently 

 clear; on the subject of the structure it is altogether vague and 

 unsatisfactory. Prof. Forbes refers to pressure, as he did in his 

 earliest communications upon this subject ; but by far the most 

 reasonable interpretation here is, that he regarded the pres- 

 sure as iutlueutial in producing the "differential motion," which, 

 he distinctly states, "necessarily takes place under intense pres- 

 sure." This interpretation is supported by the fact that we have 

 "lines of tearing" and "incipient fissures" invoked, as for- 

 merly, in this letter. " The imprisoned air," writes Prof. Forbes, 

 "is distributed in the lines of tearing, in the form of regular 

 globules, just as in the case of the banded lavas which have 

 been so well described by j\Ir. Darwin." The words " in the 

 lines of tearing" are put in italics by Prof. Forbes himself. 

 Now it is very remarkable that, in the passage of Mr. Darwin's 

 work to which reference is here made, Prof. Forbes's/rs^ theory 

 of the structure is referred to, and assumed to be correct. In 

 his reciprocal reference to Mr. Darwin's theory, Prof. Forbes 

 endorses the comparison of that eminent naturalist, and docs 

 not use a word lohich luould lead us to suppose that he wished to 

 modify Mr. Darwin's assumption. 



"With regard to the origin of the veined structure, the letter, 

 as already stated, is so vague that it is impossible to infer from 

 it, with any certainty, what the views of Prof. Forbes on the 

 subject really are. But those who, like myself, have taken the 

 trouble to acquaint themselves with the labour and the learning 

 expended in establishing the first theory, would, I am satisfied, 

 be the last to suppose that it was intended, in the letter inferred 

 to, to dispose of that theory in so indirect and summary a way. 



Royal Institution, January 1859. 



