XXVI RECENT PROGRESS OP THE GLACIER THEORY. 



nicated to the Royal Society of London. These papers are 

 reprinted here ; and as they form a supplement to the Theory, as 

 delivered in my Travels, they include not only many additional 

 facts, but answers to objections or difficulties given in a form 

 which has none of the unpleasantness of controversy. I ven- 

 ture to think that some parts of these papers contain valuable 

 matter, which makes it desirable that they should be republished 

 in this more convenient form. 



Subsequently to the composition of these papers, in 1846, 

 I once more returned to the Alps, and made the observations 

 referred to in a previous part of this preface, which altered some- 

 what my opinions on the consolidation of the glacier, and 

 afforded several other new observations, particularly as to the 

 more rapid surface motion of the ice stream. These gave rise to 

 the Eleventh, Twelfth, and Thirteenth Letters which follow. The 

 remaining Letters contain additional illustrations of the Theory 

 derived from various quarters, and the results of my latest visit 

 to Chamouni that of 1850. 



All these documents have been reprinted with scrupulous 

 attention to accuracy ; nor has the alteration even of a word 

 been made without an indication by a foot-note or brackets [ ] ; 

 the latter being exclusively reserved for additions made in the 

 course of the present impression. In the papers from the Philo- 

 sophical Transactions, only, I have consulted the reader's con- 

 venience by a few abridgements of formal and geometrical de- 

 tails (all noticed where they occur), but absolutely none which 

 affect the sense. 



The concluding paper is a popular sketch of glaciers in 

 general, and of my theory, being the article GLACIER from the 

 Eighth Edition of the Encyclopaedia Britannica, published in 

 1855. As it contains no novelty, and professes no originality, 

 I have here and there altered a few words, chiefly in consequence 



