[80] Peter Franklin. _Editor._
_Of Lightning, and the Methods (now used in America) of securing
Buildings and Persons from its mischievous Effects._
Experiments made in electricity first gave philosophers a suspicion,
that the matter of lightning was the same with the electric matter.
Experiments afterwards made on lightning obtained from the clouds by
pointed rods, received into bottles, and subjected to every trial,
have since proved this suspicion to be perfectly well founded;
and that whatever properties we find in electricity, are also the
properties of lightning.
This matter of lightning, or of electricity, is an extreme subtile
fluid, penetrating other bodies, and subsisting in them, equally
diffused.
When by any operation of art or nature, there happens to be a greater
proportion of this fluid in one body than in another, the body
which has most will communicate to that which has least, till the
proportion becomes equal; provided the distance between them be not
too great; or, if it is too great, till there be proper conductors to
convey it from one to the other.
If the communication be through the air without any conductor, a
bright light is seen between the bodies, and a sound is heard. In
our small experiments, we call this light and sound the electric
spark and snap; but in the great operations of nature, the light is
what we call _lightning_, and the sound (produced at the same time,
though generally arriving later at our ears than the light does to
our eyes) is, with its echoes, called _thunder_.
If the communication of this fluid is by a conductor, it may be
without either light or sound, the subtle fluid passing in the
substance of the conductor.
If the conductor be good and of sufficient bigness, the fluid passes
through it without hurting it. If otherwise, it is damaged or
destroyed.
All metals, and water, are good conductors.--Other bodies may become
conductors by having some quantity of water in them, as wood, and
other materials used in building, but not having much water in them,
they are not good conductors, and therefore are often damaged in the
operation.
Glass, wax, silk, wool, hair, feathers, and even wood, perfectly dry
are non-conductors: that is, they resist instead of facilitating the
passage of this subtle fluid.
When this fluid has an opportunity of passing through two conductors,
one good, and sufficient, as of metal, the other not so good, it
passes in the best, and will follow it in any direction.
The distance at which a body charged with this fluid will discharge
itself suddenly, striking through the air into another body that is
not charged, or not so highly charged, is different according to
the quantity of the fluid, the dimensions and form of the bodies
themselves, and the state of the air between them.--This distance,
whatever it happens to be between any two bodies, is called their
_striking distance_, as, till they come within that distance of each
other, no stroke will be made.
The clouds have often more of this fluid in proportion than the
earth; in which case, as soon as they come near enough (that is,
within the striking distance) or meet with a conductor, the fluid
quits them and strikes into the earth. A cloud fully charged with
this fluid, if so high as to be beyond the striking distance from the
earth, passes quietly without making noise or giving light; unless it
meets with other clouds that have less.
Tall trees, and lofty buildings, as the towers and spires of
churches, become sometimes conductors between the clouds and the
earth; but not being good ones, that is, not conveying the fluid
freely, they are often damaged.
Buildings that have their roofs covered with lead, or other metal,
and spouts of metal continued from the roof into the ground to carry
off the water, are never hurt by lightning, as, whenever it falls on
such a building, it passes in the metals and not in the walls.
When other buildings happen to be within the striking distance from
such clouds, the fluid passes in the walls whether of wood, brick or
stone, quitting the walls only when it can find better conductors
near them, as metal rods, bolts, and hinges of windows or doors,
gilding on wainscot, or frames of pictures, the silvering on the
backs of looking-glasses, the wires for bells, and the bodies of
animals, as containing watery fluids. And in passing through the
house it follows the direction of these conductors, taking as many
in its way as can assist it in its passage, whether in a strait, or
crooked line leaping from one to the other, if not far distant from
each other, only rending the wall in the spaces where these partial
good conductors are too distant from each other.
An iron rod being placed on the outside of a building, from the
highest part continued down into the moist earth, in any direction
strait or crooked, following the form of the roof or other parts of
the building, will receive the lightning at its upper end, attracting
it so as to prevent its striking any other part; and, affording it a
good conveyance into the earth, will prevent its damaging any part of
the building.
A small quantity of metal is found able to conduct a great quantity
of this fluid. A wire no bigger than a goose-quill has been known
to conduct (with safety to the building as far as the wire was
continued) a quantity of lightning that did prodigious damage both
above and below it; and probably larger rods are not necessary,
though it is common in America, to make them of half an inch, some of
three quarters, or an inch diameter.
The rod may be fastened to the wall, chimney, &c. with staples of
iron.--The lightning will not leave the rod (a good conductor) to
pass into the wall (a bad conductor) through those staples.--It would
rather, if any were in the wall, pass out of it into the rod to get
more readily by that conductor into the earth.
If the building be very large and extensive, two or more rods may be
placed at different parts, for greater security.
Small ragged parts of clouds, suspended in the air between the
great body of clouds and the earth (like leaf gold in electrical
experiments) often serve as partial conductors for the lightning,
which proceeds from one of them to another, and by their help comes
within the striking distance to the earth or a building. It therefore
strikes through those conductors a building that would otherwise be
out of the striking distance.
Long sharp points communicating with the earth, and presented to
such parts of clouds, drawing silently from them the fluid they are
charged with, they are then attracted to the cloud, and may leave the
distance so great as to be beyond the reach of striking.
It is therefore that we elevate the upper end of the rod six or eight
feet above the highest part of the building, tapering it gradually to
a fine sharp point, which is gilt to prevent its rusting.
Thus the pointed rod either prevents a stroke from the cloud, or,
if a stroke is made, conducts it to the earth with safety to the
building.
The lower end of the rod should enter the earth so deep as to come
at the moist part, perhaps two or three feet; and if bent when under
the surface so as to go in a horizontal line six or eight feet from
the wall, and then bent again downwards three or four feet, it will
prevent damage to any of the stones of the foundation.
A person apprehensive of danger from lightning, happening during the
time of thunder to be in a house not so secured, will do well to
avoid sitting near the chimney, near a looking glass, or any gilt
pictures or wainscot; the safest place is in the middle of the room
(so it be not under a metal lustre suspended by a chain) sitting in
one chair and laying the feet up in another. It is still safer to
bring two or three mattrasses or beds into the middle of the room,
and, folding them up double, place the chair upon them; for they not
being so good conductors as the walls, the lightning will not chuse
an interrupted course through the air of the room and the bedding,
when it can go through a continued better conductor, the wall. But
where it can be had, a hammock or swinging bed, suspended by silk
cords equally distant from the walls on every side, and from the
cieling and floor above and below, affords the safest situation a
person can have in any room whatever; and what indeed may be deemed
quite free from danger of any stroke by lightning.