The foregoing very sensible and distinct account may afford a good
deal of instruction relating to the nature and effects of lightning,
and to the construction and use of this instrument for averting the
mischiefs of it. Like other new instruments, this appears to have
been at first in some respects imperfect; and we find that we are, in
this as in others, to expect improvement from experience chiefly: but
there seems to be nothing in the account, that should discourage us
in the use of it; since at the same time that its imperfections are
discovered, the means of removing them are pretty easily to be learnt
from the circumstances of the account itself; and its utility upon
the whole is manifest.
One intention of the pointed rod, is, to _prevent_ a stroke of
lightning. (_See pages_ 283, 310.) But to have a better chance of
obtaining this end, the points should not be too near to the top
of the chimney or highest part of the building to which they are
affixed, but should be extended five or six feet above it; otherwise
their operation in silently drawing off the fire (from such fragments
of cloud as float in the air between the great body of cloud and
the earth) will be prevented. For the experiment with the lock of
cotton hanging below the electrified prime conductor shows, that a
finger under it, being a blunt body, extends the cotton, drawing its
lower part downwards; when a needle, with its point presented to the
cotton, makes it fly up again to the prime conductor; and that this
effect is strongest when as much of the needle as possible appears
above the end of the finger; grows weaker as the needle is shortened
between the finger and thumb; and is reduced to nothing when only a
short part below the point appears above the finger. Now it seems
the points of Mr. Maine's rod were elevated only (_a_) _six or seven
inches above the top of the chimney_; which, considering the bulk of
the chimney and the house, was too small an elevation. For the great
body of matter near them would hinder their being easily brought into
a negative state by the repulsive power of the electrised cloud,
in which negative state it is that they attract most strongly and
copiously the electric fluid from other bodies, and convey it into
the earth.
(_b_) _Nothing of the points, &c. could be found._ This is a common
effect. (_See page_ 312.) Where the quantity of the electric fluid
passing is too great for the conductor through which it passes, the
metal is either melted, or reduced to smoke and dissipated; but where
the conductor is sufficiently large, the fluid passes in it without
hurting it. Thus these three wires were destroyed, while the rod to
which they were fixed, being of greater substance, remained unhurt;
its end only, to which they were joined, being a little melted, some
of the melted part of the lower ends of those wires uniting with it,
and appearing on it like solder.
(_c_)(_d_)(_e_) As the several parts of the rod were connected only
by the ends being bent round into hooks, the contact between hook and
hook was much smaller than the rod; therefore the current through
the metal being confined in those narrow passages, melted part of
the metal, as appeared on examining the inside of each hook. Where
metal is melted by lightning, some part of it is generally exploded;
and these explosions in the joints appear to have been the cause of
unhooking them; and, by that violent action, of starting also most of
the staples. We learn from hence, that a rod in one continued piece
is preferable to one composed of links or parts hooked together.
(_f_) _No part of the chimney was damaged_: because the lightning
passed in the rod. And this instance agrees with others in showing,
that the second and principal intention of the rods is obtainable,
viz. that of _conducting_ the lightning. In all the instances yet
known of the lightning's falling on any house guarded by rods, it
has pitched down upon the point of the rod, and has not fallen
upon any other part of the house. Had the lightning fallen on this
chimney, unfurnished with a rod, it would probably have rent it
from top to bottom, as we see, by the effects of the lightning on
the points and rod, that its quantity was very great; and we know
that many chimneys have been so demolished. But _no part of this
was damaged, only_ (_f_)(_g_)(_h_) _at the foundation, where it was
shattered and several bricks torn out_. Here we learn the principal
defect in fixing this rod. The lower joint being sunk but three feet
into the earth, did not it seems go low enough to come at water,
or a large body of earth so moist as to receive readily from its
end the quantity it conducted. The electric fluid therefore, thus
accumulated near the lower end of the rod, quitted it at the surface
of the earth, dividing in search of other passages. Part of it tore
up the surface in furrows, and made holes in it: part entered the
bricks of the foundation, which being near the earth are generally
moist, and, in exploding that moisture, shattered them. (_See page_
358.) Part went through or under the foundation, and got under the
hearth, blowing up great part of the bricks (_m_)(_s_), and producing
the other effects (_o_)(_p_)(_q_)(_r_). The iron dogs, loggerhead
and iron pot were not hurt, being of sufficient substance, and they
probably protected the cat. The copper tea-kettle being thin suffered
some damage. Perhaps, though found on a sound part of the hearth,
it might at the time of the stroke have stood on the part blown up,
which will account both for the bruising and melting.
That _it ran down the inside of the chimney_ (_k_) I apprehend
must be a mistake. Had it done so, I imagine it would have brought
something more than soot with it; it would probably have ripped off
the pargetting, and brought down fragments of plaster and bricks. The
shake, from the explosion on the rod, was sufficient to shake down a
good deal of loose soot. Lightning does not usually enter houses by
the doors, windows, or chimneys, as open passages, in the manner that
air enters them: its nature is, to be attracted by substances, that
are conductors of electricity; it penetrates and passes _in_ them,
and, if they are not good conductors as are neither wood, brick,
stone nor plaster, it is apt to rend them in its passage. It would
not easily pass through the air from a cloud to a building, were it
not for the aid afforded it in its passage by intervening fragments
of clouds below the main body, or by the falling rain.
It is said that _the house was filled with its flash_ (_l_).
Expressions like this are common in accounts of the effects of
lightning, from which we are apt to understand that the lightning
filled the house. Our language indeed seems to want a word to express
the _light_ of lightning as distinct from the lightning itself. When
a tree on a hill is struck by it, the lightning of that stroke exists
only in a narrow vein between the cloud and tree, but its light fills
a vast space many miles round; and people at the greatest distance
from it are apt to say, "The lightning came into our rooms through
our windows." As it is in itself extremely bright, it cannot, when
so near as to strike a house, fail illuminating highly every room
in it through the windows; and this I suppose to have been the case
at Mr. Maine's; and that, except in and near the hearth, from the
causes above-mentioned, it was not in any other part of the house;
_the flash_ meaning no more than _the light_ of the lightning.--It is
for want of considering this difference, that people suppose there
is a kind of lightning not attended with thunder. In fact there is
probably a loud explosion accompanying every flash of lightning,
and at the same instant;--but as sound travels slower than light,
we often hear the sound some seconds of time after having seen the
light; and as sound does not travel so far as light, we sometimes see
the light at a distance too great to hear the sound.
(_n_) The _breaking some pieces of china in the beaufet_, may
nevertheless seem to indicate that the lightning was there: but as
there is no mention of its having hurt any part of the beaufet, or of
the walls of the house, I should rather ascribe that effect to the
concussion of the air, or shake of the house by the explosion.
Thus, to me it appears, that the house and its inhabitants were saved
by the rod, though the rod itself was unjointed by the stroke; and
that, if it had been made of one piece, and sunk deeper in the earth,
or had entered the earth at a greater distance from the foundation,
the mentioned small damages (except the melting of the points) would
not have happened.