it passes to
the underlying moisture, where it finds a convenient place to take on
a quiet form. Almost all these hurried movements of electrical energy
which intensely heat and light the air which they traverse fly from
one part of a cloud to another, or cross from cloud sphere to cloud
sphere; of those which start toward the earth, many are exhausted
before they reach its surface, and even those that strike convey but a
portion of their original impulse to the ground.

The wearing-out effect of lightning in its journey along the air
conductors in its flaming passages is well illustrated by what happens
when the charge strikes a wire which is not large enough freely to
convey it. The wire is heated, generally made white hot, often melted,
and perhaps scattered in the form of vapour. In doing this work the
electricity may, and often is, utterly dissipated--that is, changed
into heat. It has been proposed to take advantage of this principle in
protecting buildings from lightning by placing in them many thin
wires, along which the current will try to make its way, being
exhausted in melting or vaporizing the metal through which it passes.

There are certain other forms of lightning, or at least of electrical
discharges, which produce light and which may best be described in
this connection. It occasionally happens that the earth becomes so
charged that the current proceeds from its surface to the clouds. More
rarely, and under conditions which we do not understand, the electric
energy is gathered into a ball-like form, which may move slowly along
the surface until it suddenly explodes. It is a common feature of all
these forms of lightning which we have noted that they ordinarily make
in their movement considerable noise. This is due to the sudden
displacement of the air which they traverse--displacement due to the
action of heat in separating the particles. It is in all essential
regards similar to the sounds made by projectiles, such as meteors or
swift cannon shots, as they fly through the air. It is even more
comparable to the sound produced by exploding gunpowder. The first
sound effect from the lightning stroke is a single rending note, which
endures no longer--indeed, not as long--as the explosion of a cannon.
Heard near by, this note is very sharp, reminding one of the sound
made by the breaking of glass. The rolling, continuous sound which we
commonly hear in thunder is, as in the case of the noise produced by