[After Dec. 10 and before Dec. 17, 1772]
The Committee appointed by you in July last on
an Application from the Royal Ordnance to consider of the most
proper Means of securing the Powder Magazines at Purfleet from
Danger by Lightning, made their Report to the Council during your
Recess, agreable to your Instructions, who immediately transmitted
it to the said Board. That Report together with the Committee who
sign’d it, having since been censured by Mr. Wilson in a Letter to
Sir Charles Frederic, Surveyor of the Ordnance, which Letter was
read at Sir Charles’s Request to the Society, and before the Report
it self, with the Reasons on which it was founded could in the
usual Course be laid before them, apparently tending to prejudice
the Society against their Committee, especially as Mr. Wilson has
stated his Objections with all the Strength they are capable of,
and has passed over very slightly the Answers made to them in the
Committee, and the Reasons on which it proceeded, only intimating
his Ability and Intention of examining and invalidating those
Reasons at some future time: We can not but think it a Piece of
Justice due, not to ourselves merely, but to the Philosophical Part
of the Question, that the Grounds of our Report in the Point
controverted should be more fully explained than it could properly
be in the Report itself. We therefore take the Liberty of
acquainting you, that Mr. Wilson’s Objections to the Use of Pointed
Conductors were heard and considered in the several Meetings of the
Committee with Patience and with Candour; that they were all
answered and obviated to the Satisfaction of every one of the
Committee but himself; and the Substance of what was said on the
Occasion being contained in a Paper of Experiments, Observations
and Facts, drawn up by Mr. Franklin, and read to the Committee in
the Presence of Mr. Wilson, several of the Committee having seen
the Experiments themselves, we here present the Society with that
Paper.
The Committee had also before them an
Experiment of Mr. Henley’s, related in a Letter from that Gentleman
to Mr. Franklin, which as it seems to have been misapprehended by
Mr. Wilson, is also here inserted, viz.
By this Experiment in which the Prime Conductor
[is] suppos’d to represent a Cloud, it appeared, that the Point
would operate at 12 times the striking Distance in drawing off and
lessening the Quantity of Electric Fluid, before the Cloud could
approach within that Distance; that a blunt Body drew off none of
it till it came within that distance; that therefore if it struck
on the blunt Body it would strike with its whole Force
undiminish’d; on the Point it would either not strike at all or
with a diminish’d Quantity for what is drawn off by pointed
Conductor, is at the same time convey’d away into the Earth. But
these Consequences did not strike Mr. Wilson; to him, as he says in
his Letter, “it appeared that the Difference in the Effects upon
this Fluid, between pointed and blunted Metal, is as 12 to 1.” And
thence he seems to conclude that in a Stroke upon a Point 12 times
the Quantity must be discharged, that would have fallen on a Body
which had no Point: For being persuaded from some Circumstances
observed by the Verger of St. Pauls, in and about the Conductors
upon that Building, the morning after the Thunder on March last;
and examined by himself and Mr. Delaval about a Week after, that “a
Bar of Iron near 4 Inches broad and about ½ an Inch thick,” had
been “made considerably hot if not absolutely red; by
a Stroke of Lightning;” he says, he “thinks it a happy Circumstance
that there was no Point fixed upon the Top of the Church to solicit
a greater Quantity;” for “the Difference in the Effects between
blunted and pointed Ends, in causing a Discharge in our Electrical
Experiments appearing to be as 12 to 1, it is easy to comprehend
the very great Danger this noble Fabric has probably escaped, by
having no pointed Apparatus upon it.” To your Committee this
Deduction still appears quite unconnected with the Premises, which
they conceive afford Conclusions directly contrary; and that,
supposing the Fact tho’ a doubtful one, that so great a Quantity
fell as to heat such an Iron Bar to the Degree imagined, yet if
there had been a pointed Apparatus to have operated on the
approaching Cloud while it was at 12 Times the Striking Distance,
drawing off continually and conveying into the Earth Part of the
Quantity it contained, all the Time of its Approach till within the
Striking Distance, the Remainder must be less than the original
Quantity, and strike therefore with less Force. And For another
Reason, drawn from Exp. IV. it seems rather likely that for Want of
Points on so great and lofty a Bulk of Metal as the leaded Roof of
that Edifice presents to a Cloud, the electric Atmosphere of that
Cloud might have been drawn from its more distant Parts, and
accumulated over the Building before the Stroke, so as to afford an
Explosion much larger than it could have done if such Accumulation
had been prevented, or its Quantity previously drawn off and
convey’d away by pointed Conductors.
Three of your Committee who signed the Report
in question, were also on the Committee you appointed to consider
of the securing St. Paul’s. They are now charg’d with
Inconsistency, for advising Pointed Conductors in the
present Case, and not in the former. “It is worthy of Note,” says
Mr. Wilson, “that those Conductors did not terminate in a Point;
nor was any Point put upon the Cross at the Top, and yet Dr.
Franklin was of that Committee. If Points are so essential
to our Safety, why was not the reason enforced at the Committee for
having them on that capital Edifice?”
To explain this, it may be proper to observe,
that the Advantages expected from Pointed Conductors have been
three, viz. 1. To prevent a Stroke as in Exp. V. by creating a
greater Distance between the striking Parts of a Cloud and the
Building. Or, 2dly. if the Cloud should come on so fast that a
Stroke could not be prevented, then to lessen its Quantity and
Force, by previously drawing off and conveying away into the Earth
silently a Part of it. And 3dly. to direct the Lightning precisely
upon the Conductor prepared to receive and convey it away without
Damage, rather than suffer it to fall on any other Part of the
Building, which it might damage in its Way from thence to a low
unpointed Conductor. Those who have a Pleasure in Disputation are
sometimes apt to make their Opponents say more or less, or
something different from what they have really said, in order to
obtain a little Advantage in Argument. No one of the Committee has
ever asserted that Points were essential to our Safety. Dr.
F. who first proposed them, and may therefore be supposed most fond
of them, has, as quoted by Mr. Wilson, mentioned a Case in which a
House might be safe without them, [that] is, to wit, “when the Roof
is covered with Lead or other Metal, and Spouts of Metal are
continued from the Roof into the Ground to carry off the Water.”
Now this was precisely the Case of St. Pauls. The Dome and the
whole Roof of the Church were covered with Lead, from whence
a Number of large leaden Pipes descended into the common Sewers;
but some Connections of Metal were wanting to unite the different
Leaden Coverings with each other, and make the Communication with
the Earth compleat. This being agreed to, none of the Committee
thought it necessary to propose Points. Dr. F. would not propose
them for a Dwelling House so covered, unless where some timid
Inhabitant dreaded the Noise of an Explosion, which a Pointed
Conductor might give a Chance of Preventing. For it being
indifferent on what Part of such a Roof a Stroke should fall, the
Conductors being compleat from all Parts, there would be no Reason
for desiring to draw it to a particular Quarter. This too was in a
great degree the Case of the Board House; another Instance given of
our Inconsistency: for tho’ not intirely covered with Lead, yet
every Angle of the Roof being well cop’d with broad Plates of that
Metal, all communicating with the Leaden Gutters within the
Parapet, and thence by large leaden Pipes quite down into the Water
of two Wells one on each side the Building, it was thought
sufficiently safe as a Dwelling House; and being no Powder Magazine
itself, and plac’d at a considerable Distance from them, some of
the Committee conceiv’d it not to be within our Charge, and
therefore were [remainder missing]