"blacks" falling to the ground on a sultry day are a sign of
thunder is traceable to a similar action.

The most important practical result of the early experiments with
frictional electricity was Benjamin Franklin's great discovery of
the identity of lightning and the electric spark. One day in June,
1792, he went to the common at Philadelphia and flew a kite
beneath a thundercloud, taking care to insulate his body from the
cord. After a shower had wetted the string and made it a
conductor, he was able to draw sparks from it with a key and to
charge a Leyden jar. The man who had "robbed Jupiter of his
thunderbolts" became celebrated throughout the world, and
lightning rods or conductors for the protection of life and
property were soon brought out. These, in their simplest form, are
tapes or stranded wires of iron or copper attached to the walls of
the building. The lower end of the conductor is soldered to a
copper plate buried in the moist subsoil, or, if the ground is
rather dry, in a pit containing coke. Sometimes it is merely
soldered to the water mains of the house. The upper end rises
above the highest chimney, turret, or spire of the edifice, and
branches into points tipped with incorrosive metal, such as
platinum. It is usual to connect all the outside metal of the
house, such as the gutters and finials to the rod by means of
soldered joints, so as to form one continuous metallic network or
artery for the discharge.

When a thundercloud charged with electricity passes over the
ground, it induces a charge of an opposite kind upon it. The cloud
and earth with air between are analogous to the charged foils of
the Leyden jar separated by the glass. The two electricities of