VOLCANOES. 
73 
is an example of this kind. 2. A second class comprises such as 
constantl}’’ )>;ive evidence of their volcanic character by a cloud of 
smoke resting upon their top, and occasionally burst out with 
greater violence. 3. Others appear sometimes to be altogether 
extinct, then rage with great fury for a while and afterwards sink 
down again into repose. Of the last two classes which are much 
more common than the first, iEtna and Vesuvius may be cited 
as tolerable examples. This occasional awakening of the volcanic 
fires from eitlier partial or absolute repose, is called an eruption. 
The great regularity of the volcanic action at Stromboli, to¬ 
gether with the comparative minuteness of the scale on which the 
phenomena are there exhibited, admits of our approaching very 
near to the centre of activity, and observing the changes that take 
place. The crater itself is also commanded in such a way by a 
neighbouring eminence that the spectator can look directly down 
to its bottom. It is seen to contain a quantity of melted lava at 
a brilliant white heat, which is continually rising and falling. 
When at its maximum elevation, one or two enormous bubbles 
form upon its surface, swell lapidly, and finally explode with a 
loud detonation. A shower of liquid lava is thrown into the air, 
which cooling there, falls in the form of scoriae. The surface 
of the lava is depressed about twenty feet, but rises again in a 
few minutes, in consequence of the formation of new bubbles, 
which explode in the same way. The elastic fluid by the expan¬ 
sion of which the bubbles are created, appears to be simpl}’- steam. 
There is reason to believe that the nature of volcanic action in 
other parts of the world, does not difl’er greatly from what it is 
at Stromboli; except that in the case of this crater, the lava being 
constantly in a melted state, it offers but little resistance to the 
elastic fluid that escapes through it; whereas in other cases its 
passage is so obstructed as to create the most violent convulsions. 
It is very evident that the results must be widely different when 
there is a free communication between the centre of volcanic 
action and the external air; and where a volcano breaks out for 
the first time through the solid strata of the globe, or an ancient 
rent that has been obstructed and obliterated by masses of lava 
that have i)artly cooled and become consolidated within it, and 
partly fallen into it from the neighbouring heights, is opened 
anew. 
The indications of an approaching eruption from a dormant 
volcano, are, a commencement and gradual increase of smoke from 
the crater. Tremendous subterranean explosions like the firing 
of artillery, succeed, with tremors of the earth more or less 
violent. Often, it is said, the state of the atmosphere assumes 
a peculiar character, there being in it an unusual closeness, still¬ 
ness, and pressure. Springs disappear, wells are dried up and 
there is often a splitting and heaving of the strata in the neigh¬ 
bourhood of the volcano. The eruption generally commences 
with one tremendous burst, which shakes the moutain to its 
