In which B is the impulsive arc and A the definite arc, showing that A
comes out nearly equal to 1/2 B. In dry weather all goes perfectly
within the limits of proportion, and I can tell whether, during the time
in which the index traversed the impulsive arc, there were any
_dispersions_ and of what nature; for if the definite arc is not close
to the limit of the impulsive arc, it is a sign of _dispersions_ having
taken place during the motions of the index. Every degree less in the
definite arc denotes two degrees of loss for the impulsive arc; but as
the index employs double the time traversing the definite as it does the
impulsive arc, we may consider the loss of one equal to the loss of the
other.
In excessively damp weather the index gives no definite arc, and it is
necessary to resort to artificial heat in order to dry the insulators.
The most simple means I know of is to hold the instrument over some
hollow vessel, which, for the time, is converted into a stove by the
introduction of a spirit lamp.
From Gauss's formula for the bifilar system of instruments of this
class, we learn that the maximum sensitiveness of such instruments is
given when the length of the suspending fibres is greatest, and the
distance between them is smallest, with the weight of the movable or
rotating member a minimum; and these elements being the same, the
sensitiveness of the instruments is invariable.
To some electrometers, in order to avoid errors of parallax, a small
telescope, with a micrometer wire, has been added; but, with a little
practice, we can read accurately without this refinement. In order to
obtain comparative measurements, it is necessary to select some given
unit of tension. I have observed that by making a galvanic pile of
copper, zinc and distilled water, and insulating it well, each pole has
a tension which remains the same for many days, if the conditions of
temperature and the moisture of the surrounding atmosphere are not very
different. With thirty pairs of this pile, each element having
twenty-five square centimetres of surface, I have on the electrometer a
definite arc of 15°, with the temperature of the atmosphere at 20° C.,
and with the difference of 4° to 5° C. between the thermometers of the
psychrometer of August's construction. The first observation was made
twenty-four hours after mounting the pile. For unit of tension I took
that which corresponded to a single pair, that is, the thirtieth part of
the total tension. Other electrometers may be compared with one already
properly adjusted, without always having recourse to the pile.
This done, let us see the arrangement of all the apparatus:
_H H_ (Plate VIIa, Fig. 1) is the ceiling of a well-situated lofty
room, with an opening, _o o_, at the upper part.
_M M_, a bracket or table fastened against the wall, about a metre
distant from the ceiling, _H H_.
_N N_, a wooden platform for the observer.
_A_, the bifilar electrometer.
_B_, Bohnenberger's electroscope.
_a a_, a movable conductor formed of a brass rod 15 to 18 millimetres in
diameter, insulated below by means of a glass rod, well varnished with
gum lac, having a suspending pulley, _c_, and a wooden guide-rod
underneath it, _l_, within the guiding tube, _k_. At the upper part of
this conductor, _a a_, there is a sliding roof, _b_, which can be
adjusted so as to prevent rain entering at the opening, _o o_. The
conductor terminates in a disc made of a sheet of thin brass, _d_, 24
centimetres in diameter. Upon this disc, or even in place of it, we may
use metallic points.
As a support to the conductor at the upper part, I have made use of a
triangular ring, _x_, drawn at its full size in Fig. 2. The conductor
passes between three springs, and the triangular ring is held in place
by three silk cords, _m m m_. Their material should not be mixed with
any cotton, and it may be advisable to saturate them with an alcoholic
solution of gum lac.
_f f f_ is a hempen cord, which is used to raise and lower the
conductor.
_i_ is a copper wire covered with silk, by means of which the triangular
ring, _x_, and through that and its springs the conductor communicates
with either the electrometer or the electroscope.
Quickly raising the conductor by pulling the cord, _f_, the index of the
electrometer will describe a more or less large impulsive arc, and,
after two oscillations, will stop at the definite arc. Having thus
measured the electric tension of the air, and having lowered the
conductor, I next place the wire, _i_, in communication with the
electroscope, _B_, and by again raising the conductor, I ascertain
whether the electricity be positive or negative. It is scarcely
necessary to say that the conductor, when raised, gives electricity of
the same nature as that prevailing at the moment in the atmosphere; and
when lowered, 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 moderate amount of smoke but a great deal of ashes, small
and frequent lightning flashes were observed in the twilight darting
through the smoke, which was dark in colour. In 1850 the eruption was
more vigorous, the smoke more abundant, and the ashes scarce, but the
flashes were very rare. In 1855, 1858, and 1868, with a scanty supply of
ashes and at intervals, no flashes were observed, and the electricity
remained constantly positive. But having regard to the facts of
antecedent eruptions, one sees that the flashes are always derived, from
the midst of smoke accompanied with ashes and lapilli, which separate
like rain from the rolling volumes of smoke, in the midst of which they
were ejected.
But how can we account for the positive electricity of the smoke, and
the negative electricity of the falling ashes? Without denying the
probability that a part of the positive electricity depends upon the
elevation of the smoke, as in the case of every other conductor we raise
aloft, or with a jet of water sent from a vessel by compressed air, I
think that the greater part of the electricity proceeds from the rapid
condensation of vapours, which are changed from the gaseous condition
into dense clouds; for even when the smoke issues tranquilly and does
not rise, because carried away horizontally by the wind, it gives signs
of positive electricity. From all my studies of atmospheric electricity,
and from some experiments made specially, it follows that the
condensation of vapours is the origin of this development of positive
electricity.
The negative electricity of the falling ashes certainly arises from the
fact itself of their fall; for if we place a metallic vessel full of
ashes upon an elevated and well-situated terrace, while the atmospheric
electricity is positive, and cause the ashes from the vessel to fall
gradually into an insulated metallic cup, communicating with
Bohnenberger's electroscope placed at three or four metres distance from
the vessel, the electroscope will manifest negative electricity. If the
upper vessel be insulated, and the ashes permitted to fall upon the
ground, we shall obtain, from the vessel, positive electricity. The
intensity of these electric manifestations depends (other things being
equal) upon that predominant at the moment in the air; so that if the
experiment be made while negative electricity prevails, the falling
ashes will manifest positive electricity, the upper vessel then showing
negative electricity. Now, as the ashes separate from the positively
electrified smoke in order to approach the ground, which is negatively
electrified, it follows that they must manifest negative electricity
upon touching the ground, leaving the positive electricity in the smoke
above. For this reason, the electric tension of the smoke is increased
by the descent of the ashes and lapilli, so that discharges between the
upper and lower part of the pine-tree cloud, or the surface of the
crater, are rendered possible. Hence it follows that the flashes of
lightning of Vesuvius play through the smoke, and with difficulty strike
bodies upon the earth; and from this circumstance our ancestors believed
the thunderbolts of Vesuvius to be harmless. However, if the smoke were
very great, and driven by the force of the wind to some distance from
the crater, with an abundant fall of ashes, it would be possible to have
lightning flashes proceed from the smoke to the earth. I possess some
documents which relate that, in 1631, thunderbolts fell upon the Church
of Santa Maria del Arco, and other places on the coast of Sorrento.
After upwards of twenty years' study and observation of meteoric
electricity, I am enabled to prove that atmospheric electricity is never
manifested without rain, hail or snow, and that manifestations of light
are always accompanied by thunder--manifestations of light (_lampi_),
thunder and rain being most closely connected. We may have rain without
manifestations of light, but never the latter without rain or hail. I
cannot here repeat what I have demonstrated in other memoirs; I can only
say that the lightnings of Vesuvius, erroneously believed to be not
accompanied by thunder, are really not accompanied by rain, but are
induced by the descent of ashes and lapilli.[6]