Millheim Journal. [volume] (Millheim, Pa.) 1876-1984, 1880-12-09
The sound OF Thunder.

A remarkable feature of the storm is the
thunder, corresponding, Of course, on the
large scale, 10 the snap of an electric spark.
Here we are on comparatively sure ground,
for sound Is very much more thoroughly
understood than is electricity. We speak
habitually and without exaggeration of the
crash of thunder, the rolling OF thunder,
and of a peal of thunder; and various
other terms will suggest themselves to you
as being aptly employed in different cases.
All of these are easily explained by known
properties of sound. The origin of the
sound is, in all cases 10 be looked for in the
instantaneous and violent dilatation of the
air along the track Of the lightning fash,
partly, no doubt, due 10 the disruptive ef-
fects of electricity, but mainly due 10 the
excessive rise of temperature which renders
the air for a moment so brilliantly Incan-
descent. There Is thus an extremely sud
den compression of the air all round the
track of the spark, and less sudden, but
still rapid, rush of the air into the partial
vacuum which il produces. Thus the
sound wave produced must at first be OF
the nature of q bore or breaker. But as
such a state of motion is unstable, after
proceeding a moderate distance the sound
becomes analogous TO other loud but less
violent sounds, such as those of the dis
charge of guns. Were there few clouds,
were the air of nearly uniform density,
and the flash short one, this would COM
pletely describe the phenomenon, and we
should have a thunder crash or thunder
clap, according to the greater or less prox-
unity of the seat of discharge. But as has
long been well known not merely clouds,
but surfaces of separation OF masses of
air of different density, such as constantly
occur in thunder storms, reflect vibrations
in the air; and thus we may have many
successive echoes, prolonging the original
sound. But there is another cause often
more efficient than these. When the flash
is long one, all its parts being nearly
equi-distant from the observer, he hears
the sound from all these parts slmultane-
ously; but if its parts be at very different
distances from hlm, he hears successively
the sounds from portions farther and farther
distant from him If the flash be much
zigzagged, long portions OF its course may
run al one and the same distance from him,
and the sound from these arrive slmultane-
ously at his ear, Thus we have no diff
Guilty in accounting for the rolling and
pealIng of thunder. 1t is, in fact, mere
consequence, sometimes of the reflection
of sound, sometimes of the finite velocity
with which it is propagated. The usual
rough estimate of five seconds TO a mile is
near enough 10 the truth for all ordinary
calculation of the distance of flash from
the observer. The extreme distance at
which thunder is heard is not great, when
we consider the frequent great intensity Of
the sound. No trustworthy observation
gives in general more than about nine or ten
miles, though there are cases in which ii is
possible that it may have been heard four
teen miles off. But the discharge of
single cannon is often heard at fifty miles,
and the noise of a siege or naval engage
ment has certainly been head al a distance
of much more than 100 miles. There are
two reasons for this-the first depends upon
the extreme suddenness of the production
of thunder; the second, and perhaps the
more effective, on the excessive variations
of density in the atmosphere, which are
invariably associated with q thunderstorm.
In certain cases thunder has been pTopa-
gated, for moderate distances from its ap-
parent source, with a velocity far exceeding
that of ordinary sounds. This used TO be
attributed to the extreme suddenness of its
production; but it is not easy, if we adopt
tdis hypothesis, to see why it should not
occur in all cases. Sir w. Thompson has
supplied a very different exp auation,
which requires no unusual velocity of
sound, because it asserts the production of
the sound simultaneously at all parts of the
air between the ground and the cloud from
which the lightning is discharged.