To explain this, it is supposed, that one reason of the sudden effect
produced by a long naked pointed wire is, that (by the repulsive
power of the positive charge in the prime conductor) the natural
quantity of electricity contained in the pointed wire is driven down
into the earth, and the point of the wire made strongly _negative_;
whence it attracts the electricity of the prime conductor more
strongly than bodies in their natural state would do; the _small
quantity of common matter_ in the point, not being able by its
attractive force to retain its _natural quantity of the electric
fluid_, against the force of that repulsion.--But the finger and
thumb being substantial and blunt bodies, though as near the prime
conductor, hold up better their _own_ natural quantity against the
force of that repulsion; and so, continuing nearly in the natural
state, they jointly operate on the electric fluid in the point,
opposing its descent, and _aiding the point_ to retain it; contrary
to the repelling power of the prime conductor, which would drive it
down.--And this may also serve to explain the different powers of the
point in the preceding experiment, on the slipping down the finger
and thumb to different distances.
Hence is collected, that a pointed rod erected _between two tall
chimnies_, and very little higher (an instance of which I have seen)
cannot have so good an effect, as if it had been erected on one of
the chimneys, its whole length above it.