_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.