Pieces of the red trachyte from the
summit, a foot long, have been brought to Europe, perforated all over
with these natural bullet marks, each of them lined with black glass,
due to the fusion of the rock by the passage of the spark. Specimens of
such thunder-drilled rock may be seen in most geological museums. On
some which Humboldt collected from a peak in Mexico, the fused slag from
the wall of the tube has overflowed on to the surrounding surface, thus
conclusively proving (if proof were necessary) that the holes are due to
melting heat alone, and not to the passage of any solid thunderbolt.
But it was the introduction and general employment of lightning-rods
that dealt a final deathblow to the thunderbolt theory. A
lightning-conductor consists essentially of a long piece of metal,
pointed at the end whose business it is, not so much (as most people
imagine) to carry off the flash of lightning harmlessly, should it
happen to strike the house to which the conductor is attached, but
rather to prevent the occurrence of a flash at all, by gradually and
gently drawing off the electricity as fast as it gathers before it has
had time to collect in sufficient force for a destructive discharge. It
resembles in effect an overflow pipe which drains off the surplus water
of a pond as soon as it runs in, in such a manner as to prevent the
possibility of an inundation, which might occur if the water were
allowed to collect in force behind a dam or embankment.
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