HISTORY OF THE PAPERS IN THIS VOLUME. XXV 



vours to place the Theory of Glaciers in a clear point of view, 

 with the aid of such lights as the science of the time enabled 

 me to use. I trust that the circumstances of their detached 

 composition and publication will be taken into account in judg- 

 ing of them. There will be found amongst them many things 

 besides those already referred to, which, owing to the same 

 cause, have been inevitably overlooked.* 



I will now say a few words as to the contents of this 

 volume. They are, with the exception of the last paper, 

 arranged accurately in chronological order, beginning with 1841, 

 and ending with 1858. 



The First Article on the Veined Structure of Glaciers pre- 

 ceded any attempt made by myself to account for the pheno- 

 mena of glaciers. 



The first four of the Letters on Glaciers, which follow next, 

 were written from Switzerland in 1842, and contain the origi- 

 nal draft of the Plastic or Viscous Theory, which was expounded 

 in 1843 in a more methodical and detailed manner in my 

 Travels in the Alps, which appeared in that year. 



In 1843 and 1844 I again visited Switzerland, as well as 

 Italy and Sicily, though not in favourable circumstances for 

 making extended observations. The Fifth to the Ninth Letters 

 inclusive contain the results of these journeys, with the reflections 

 which farther consideration of the Theory had suggested to my 

 own mind, or which the objections made to it had called forth. 



The discussion of these objections continued in 1845, and 

 it then appeared to me desirable to bring down the subject of 

 the Glacier Theory to that period in a methodical form, which I 

 did in three papers on the Viscous Theory, which were commu- 



* Besides the Thirteenth Letter already quoted, I would mention the " Sum- 

 mary of Evidence," at p. 168 of this volume, and the ohservations on plastic bodies 

 in the Fifteenth Letter, as not perhaps generally known. 



