ivity of the adjacent
Volcano.

If the very slightest eruption occurs, these instruments manifest slight
perturbation, increasing with the activity of the mountain. When the
Volcano attains a certain degree of activity, and the instruments are
proportionately disturbed, it is impossible to foresee a new phase of
increase without constantly watching the changes in the intensity of the
perturbations; and to effect this it is requisite to have upon the spot
a staff of assistants sufficiently numerous, scientific and intelligent.
If, therefore, on the night preceding the 26th of April the instruments
had been properly watched, they would have undoubtedly indicated the
great increase in the activity of the Volcano. The perturbations on the
23rd were steadily increasing, and on the evening of the 25th they were
much stronger than on the 24th, but on the morning of the 26th they had
become extraordinarily strong; they must, therefore, have increased
considerably during the night.

II.

NATURE OF THE LAVAS.

When the observer is near the source of the lava, he sees matter in a
state of fusion, which, like a torrent of liquid fire, runs along, with
more or less impetuosity, between two banks formed by itself. But as
soon as the surface of the torrent cools to the point of congelation, it
loses the splendour of its first incandescence. The part which begins to
harden breaks readily in some lavas into fragments which float on the
viscous fluid beneath; these, increasing in number with distance from
the source, conceal the molten matter beneath and retard its progress,
and at last nothing is seen but the more or less red-hot scoriae moving
along. These lavas I shall call "_Lavas with fragmentary scoriae_."

On other occasions, a skin forms on the surface of the lava, which,
gradually thickening, keeps flexible for some time, and then wrinkles or
swells or extends and breaks to give egress to the hot fluid within,
which, in its turn, skins over and repeats the same phenomena. This I
shall call "_Lavas with a united surface_."

These, in their course, discharge less smoke than the first, draw out
more easily into threads, and, when cold, have a dark colour, something
like bitumen or pitch. _The lava with fragmentary scoriae_, when
stretched, breaks easily, discharges smoke copiously, and, when
hardened, has a more bluish tint, like clods of upturned earth (_formato
di zolle_). It is noisy in its course, because the inc