This seems a proof, that though the small sharpened part of the wire
must have had a _less natural_ quantity in it before the operation,
than the thick blunt part; yet a greater quantity was _driven down
from it_ to the balls. Thence it is again inferred, that the pointed
rod is rendered _more negative_: and farther, that if a _stroke must
fall_ from the cloud over a building, furnished with such a rod,
it is more likely to be drawn to that pointed rod, than to a blunt
one; as being more strongly negative, and of course its attraction
stronger. And it seems more eligible, that the lightning should fall
on the point of the conductor (provided to convey it into the earth)
than on any other part of the building, _thence_ to proceed to such
conductor. Which end is also more likely to be obtained by the length
and loftiness of the rod; as protecting more extensively the building
under it.
It has been _objected_, that erecting pointed rods upon _edifices_,
is to _invite_ and draw the lightning into _them_; and therefore
dangerous. Were such rods to be erected on buildings, _without
continuing the communication_ quite down into the moist earth, this
objection might then have weight; but when such compleat conductors
are made, the lightning is invited not into the building, but into
the _earth_, the situation it aims at, and which it always seizes
every help to obtain, even from broken partial metalline conductors.
It has also been suggested, that from such electric experiments
_nothing certain can be concluded as to the great operations of
nature_; since it is often seen, that experiments, which have
succeeded in small, in large have failed. It is true that in
mechanics this has sometimes happened. But when it is considered
that we owe our first knowledge of the nature and operations of
lightning, to observations on such small experiments; and that on
carefully comparing the most accurate accounts of former facts, and
the exactest relations of those that have occurred since, the effects
have surprizingly agreed with the theory; it is humbly conceived that
in natural philosophy, in this branch of it at least, the suggestion
has not so much weight; and that the farther new experiments now
adduced in recommendation of _long_ sharp-pointed rods, may have some
claim to credit and consideration.
It has been urged too, that though points may have considerable
effects on a _small_ prime conductor at _small distances_; yet on
_great_ clouds and at _great distances_, nothing is to be expected
from them. To this it is answered, that in those _small_ experiments
it is evident the points act at a greater than the _striking_
distance; and in the large way, their service is _only expected_
where there is _such_ nearness of the cloud, as to _endanger a
stroke_; and there, it cannot be doubted the points must have some
effect. And if the quantity discharged by a single pointed rod may
be so considerable as I have shown it; the quantity discharged by a
number will be proportionably greater.
But this part of the theory does not depend alone on _small_
experiments. Since the practice of erecting pointed rods in America
(now near twenty years) five of them have been struck by lightning,
viz. Mr. Raven's and Mr. Maine's in South Carolina; Mr. Tucker's in
Virginia; Mr. West's and Mr. Moulder's in Philadelphia. Possibly
there may have been more that have not come to my knowledge. But in
every one of these, the lightning did _not_ fall upon the _body of
the house_, but precisely on the several _points_ of the rods; and,
though the conductors were sometimes _not sufficiently large and
complete_, was conveyed into the earth, without any material damage
to the buildings. Facts then _in great_, as far as we have them
authenticated, justify the opinion that is drawn from the experiments
_in small_ as above related.
It has also been objected, that unless we knew the quantity that
might _possibly_ be discharged at one stroke from the clouds,
we cannot be sure we have provided _sufficient_ conductors; and
therefore cannot depend on their conveying away _all_ that may fall
on their points. Indeed we have nothing to form a judgment by in
this but past facts; and we know of no instance where a _compleat_
conductor to the moist earth _has_ been insufficient, if half an
inch diameter. It is probable that many strokes of lightning have
been conveyed through the common leaden pipes affixed to houses to
carry down the water from the roof to the ground: and there is no
account of such pipes being melted and destroyed, as must sometimes
have happened if they had been insufficient. We can then only judge
of the dimensions proper for a conductor of lightning, as we do of
those proper for a _conductor of rain_, by past observation. And as
we think a pipe of three inches bore sufficient to carry off the
rain that falls on a square of 20 feet, because we never saw such a
pipe glutted by any shower; so we may judge a conductor of an inch
diameter, more than sufficient for any stroke of lightning that will
fall on its point. It is true that if another deluge should happen
wherein the windows of heaven are to be opened, such pipes may be
unequal to the falling quantity; and if God for our sins should
think fit to rain fire upon us, as upon some cities of old, it is
not expected that our conductors of whatever size, should secure our
houses against a miracle. Probably as water drawn up into the air
and there forming clouds, is disposed to fall again in _rain_ by
its natural gravity, as soon as a number of particles sufficient to
make a drop can get together; so when the clouds are (by whatever
means) over or under-charged [with the _electric fluid_] to a degree
sufficient to attract them towards the earth, the equilibrium is
restored, before the difference becomes great beyond that degree.
Mr. Lane's _electrometer_, for limiting precisely the quantity of
a shock that is to be administered in a medical view, may serve to
make this more easily intelligible. The discharging knob does by a
screw approach the conductor to the distance intended, but there
remains fixed. Whatever power there may be in the glass globe to
collect the fulminating fluid, and whatever capacity of receiving and
accumulating it there may be in the bottle or glass jar; yet neither
the accumulation or the discharge ever exceeds the destined quantity.
Thus, were the _clouds_ always at a certain fixed distance from the
earth, all discharges would be made when the quantity accumulated
was equal to the distance: But there is a circumstance which by
occasionally lessening the distance, lessens the discharge; to wit,
the moveableness of the clouds, and their being drawn nearer to the
earth by attraction when electrified; so that discharges are thereby
rendered more frequent and of course less violent. Hence whatever the
quantity may be in nature, and whatever the power in the clouds of
collecting it; yet an accumulation and force beyond what mankind has
hitherto been acquainted with is scarce to be expected[89].