wered, manifests the opposite. In some conjunctures we must keep
the conductor raised and in communication with the electroscope, in
order to observe certain phenomena which I shall presently describe:
this method I call observation with a _fixed conductor_.

I have also constructed a similar but portable apparatus for use on
eruptive cones, when required.

Having given this description of the apparatus, it remains for me to
relate the results obtained, especially on the occasion of the last
eruption of Vesuvius.

The Observatory is distant, in a direct line from the central crater of
Vesuvius, 2,380 metres, so that, when the smoke is copious, it is
properly situated for the study of electricity, particularly when the
wind inclines the pine-tree cloud in the direction of the Observatory,
as frequently happened on the last occasion.

With smoke alone, without ashes, we obtained strong tensions of positive
electricity; with ashes only, which sometimes fell while the smoke
turned in the other direction, we had strong negative electricity; when
the smoke inclined towards the Observatory, accompanied with ashes and
lapilli, we had sometimes one kind of electricity, and sometimes the
other, just as the smoke or the ashes predominated; and often with a
"fixed conductor" we obtained negative electricity, and with a "movable
conductor" positive electricity. In Naples, too, at the Meteorological
Observatory attached to the University, my colleague, Professor Eugenio
Semmola, observed negative electricity of strong tension whilst ashes
were falling there in abundance. The tensions on this occasion were so
strong as to equal those obtained at changes of weather or during storms
(_temporali_), and, being beyond measure with a delicate electrometer,
we marked them with the symbol for infinity: the same phenomena were
observed when lightnings flashed.

When there is but little smoke, it is necessary to approach the eruptive
mouths with a portable apparatus, in order to observe those phenomena
which, in great eruptions, may be studied from the Observatory itself.

The conditions under which (_folgori_) lightning flashes are seen from
the cloud of smoke are, that it is conveying great abundance of ashes.
In 1861, there were small flashes even from the line of eccentric mouths
above Torre del Greco, although the smoke was not very great; and when
these ceased to discharge, and the central crater became somewhat
active, with a moder