With the present experiences I am satisfied that the phenomenon of the fireball is
produced by the sudden heating. to high incandescence, of a mass of air or other gas as
the case may be, by the passage of a powerful discharge. There are many ways or less
plausible in which a mass of air might be thus affected by the spark discharge, kut I
hold the following explanation of the mode of production of the ball as being, most
likely of all others which I have considered, the true one. When sudden and very powerful
discharges pass through the air, the tremendous expansion of some portions of the latter
and subsequent rapid cooling and condensation gives rise to the creation of partial vacua
in the places of greatest development of heat. These vacuous spaces, owing to the
properties of the gas, are most likely to assume the shape of hollow spheres when, upon
cooling, the air from all around rushes in to fill the cavity created by the explosive
dilatation and subsequent contraction. Suppose now that this result would have been
produced by one spark or streamer discharge and that now a second discharge, and
possible many more, follows in the path of the first. What will happen? Before answering
the question we must remember that, contrary to existing popular notions, the currents
passing through the air have the strength of many hundreds and even thousands of amperes