_What_ quantity of lightning, a high pointed rod well communicating
with the earth may be expected to discharge from the clouds silently
in a short time, is yet unknown; but I have reason from a particular
fact to think it may at some times be very great. In Philadelphia I
had such a rod fixed to the top of my chimney, and extending about
nine feet above it. From the foot of this rod, a wire (the thickness
of a goose-quill) came through a covered glass tube in the roof, and
down through the well of the stair-case; the lower end connected
with the iron spear of a pump. On the stair-case opposite to my
chamber-door, the wire was divided; the ends separated about six
inches, a little bell on each end; [and] between the bells a little
brass ball suspended by a silk thread, to play between and strike
the bells when clouds passed with electricity in them. After having
frequently drawn sparks and charged bottles from the bell of the
upper wire, I was one night waked by loud cracks on the stair-case.
Starting up and opening the door, I perceived that the brass ball,
instead of vibrating as usual between the bells, was repelled and
kept at a distance from both; while the fire passed sometimes in very
large quick cracks from bell to bell; and sometimes in a continued
dense white stream, seemingly as large as my finger, whereby the
whole stair-case was enlightened as with sunshine, so that one
might see to pick up a pin[87]. And from the apparent quantity
thus discharged, I cannot but conceive that a _number_[88] of such
conductors must considerably lessen that of any approaching cloud,
before it comes so near as to deliver its contents in a general
stroke:--An effect not to be expected from bars _unpointed_; if the
above experiment with the blunt end of the wire is deemed pertinent
to the case.