Considerations on Volcanoes. 35? 



With respect to the other case, the author proceeds to investi- 

 gate the phenomena of active volcanoes, dividing them into three 

 classes, which he calls phase of permanent eruption, phase of 

 moderate activity, and phase of prolonged intermittences. 



In the first class, Stromboli is an unquestioned example ; and 

 the same appears true of the volcano in the Lake of Nicaragua. 

 The second class includes the great mass of volcanoes known as 

 such ; and among those, Vesuvius and iEtna are the most familiar 

 and the best studied, from the free and frequent access which they 

 have permitted, for so many years, to persons endowed with the 

 capacity for observation. This is also the character of the vol- 

 canoes of the Pacific, of those of Kamtschatka, and the Molucca 

 and Philippine Islands, and indeed, generally of many more whose 

 histories are to be found without end in works, and of which the 

 enumeration here would be too long for our analysis. 



In the third class, or under the phase of prolonged intermit- 

 tences, the number is even greater than in the preceding ; and as 

 novelty, in the case of eruptions of this nature, is added to the 

 naturally terrific circumstances attending them, these are the 

 eruptions which have excited the greatest attention, and which 

 make the greatest figure in history. The author proceeds to 

 sketch the general appearances attending an eruption of this 

 nature. 



They are commonly preceded by earthquakes of different degrees 

 of intensity and duration, and with loud sounds or detonations 

 resembling the noise of ordnance and musketry, apparently pro- 

 duced by the disengagement of aeriform fluids, and the increase 

 of bulk in the fluid rocks ; and their sounds are conveyed through 

 the solid earth ; not by means of the air. The atmosphere at this 

 time is remarked to be in a peculiar state of stillness, attended by 

 a sense of oppression. 



During this period also, springs are apt to disappear, so that 

 wells become dry ; and it is known that the extent of this affection 

 is sometimes very considerable. 



When the eruption first appears, it is generally with sudden 

 and great violence. Explosions, apparently from confined air, 

 take place with loud noises, and succeeding each other with 

 rapidity, and often with increasing force ; the vent being, com- 

 monly, the central point or crater of the mountain. And in its 

 attempt to escape, this air throws up fragments of rock, which 

 sometimes fall back into the crater, and are again repeatedly 

 projected, together with clouds of aqueous vapour. And as the 

 fragments also are often broken into small pieces, and even into 

 dust, this, uniting to the vapour or mixing with it, produces 

 dense black clouds, or smoke, often assuming the form of a 

 column of entangled or successively formed clouds. 



Having arrived at a certain height, this column generally 



