232 Professor Forbes's ^ixth Letter on Glaciers. 



which untoward circumstances permitted me to make, last 

 autumn, upon the glaciers of Switzerland and Savoy. I have, 

 however, had leisure to reflect maturely upon the theory of 

 glaciers, which I have been occupied for two years in endea- 

 vouring to mature ; and, without pretending to find in it a 

 complete solution of every problem which might be proposed 

 respecting these wonderful bodies, I am perfectly satisfied that 

 it is fundamentally conformable to the laws by which they are 

 governed. Some new analogies, to which your Lordship has 

 referred in your last letter, such as that between glaciers and 

 lava streams, may serve to render the subject more popu- 

 larly intelligible ; and in explaining them, I may have an 

 opportunity of removing, in some degree, the difficulties which 

 have arisen in the minds of candid and intelligent persons, 

 who have studied this theory for the first time — difficulties 

 which would probably disappear of themselves by a more pro- 

 longed attention. 



I have not had the advantage of seeing the eruption of 

 Etna, to which your Lordship alludes, which was indeed over 

 before I arrived at Naples, and of which I did not even hear 

 for a considerable time after ; so small is the sensation which 

 such events excite in the country. I have, however, had an 

 opportunity — probably not less favourable, though far less 

 imposing — of studying the mechanism of plastic lava, in the 

 small currents which, during the months of November and 

 December, were very frequently flowing from mouths 7vithin 

 the crater of Vesuvius. On the SOth November, in particular, 

 I descended to the bottom of the crater, in order to examine 

 a current of very liquid lava, fifteen or twenty feet wide, which 

 issued from a cavity near the foot of the small cone which 

 occupied the centre of the crater, and from whose top (in the 

 shape of an inverted funnel, or of a blast furnace) there issued 

 smoke and flames,* occasionally accompanied by a discharge 



* I am able to add my distinct testimony to that of M. Pilla, as to 

 the emission of Jiames by the crater of Vesuvius. I spent part of the 

 evening of the 1st January on the top, and had not the least doubt that 

 what I saw were actual flames, which issued from time to time from the 

 orifices of the small cone, and which were of a pale colour, often inclin- 

 ing to blue. 



