The Manitowoc pilot. [volume] (Manitowoc, Wis.) 1859-1932, 1880-12-02
THUNDER-ITS ILLUSTRATION
OF KNOWN PROPERTIES
OF SOUND


"We speak habituslly and without e1ag
Grafton of the crash of thunder, the roll
ing of thunder, and of a peal of thunder,
aud various other terms will suggest
themselves 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 ali cases,
to be looked for in the instantaneous and
violent dilatation of the air along the
track of the lightning fash, partly, no
doubt, due to the disruptive effects of
electricity, but mainly due to the excess
i've rise Of temperature which renders the
air for moment so brilliantly incande
scent. There Is thus an extremely sud-
den compression of the air ali round the
track of the spark, and less sudden,
but still rapid, rush OF the air into the
partial vacuum which it produces. Thus
the souud-wave produced must at first
be Of the nature of a bore or breaker,
but Is such state of motion isunstsble,
after proceeding moderate distance the
sound become analogous to other loud
but less violent sounds such as those of
the discharge of guns. Were there few
clouds, were the air of nearly uniform
density, and the flash short one, this
would completely describe the phenome-
non, and we should have thunder crash
or thunder clap, according TO the greater
or less proximity 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, pro.
longing the original sound.
But there is another cause, often more
efficient than these. When the fash is
a long one, all its parts being nearly equi.
distant from the observer, he hears the
sound from all those parts simultaneous.
ly; but if its parts be at very different
distances from him, he hears successively
the sounds from portions further distant
from him, If the flash be much zigzag-
ged, long portions of its course NAV run
at one and the same distance from him,
and the sounds from these arrive simul-
taneously at his ear Thus we have no
difficulty in accounting for the rolling
and pealing of thunder. 1t is, in fact, a
mere consequence, sometimes, of the
finite velocity with which it is propagat-
ed. The usual rough estimate of five
seconds to a mile is near enough to the
truth fer all ordinary calculations OF the
distance of fash 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 gen.
eral more than about nine or ten miles,
though there are cases in which it is pos
sible that it may have been heard four
teen miles off. But the discharge of a
single cannon is often heard at fifty miles,
and the noise of siege or naval engage
ment has certainly been heard at dis
tance of much more than 100 miles,
There are two reasons for this: the first
depends upon the extreme suddenncss of
the production Of thunder; the second,
and perhaps the most effective, on the
excessive variations of density in the at
mosphere, which are invariably associat-
ed with thunderstorm. La certain cas-
es thunder has been propagated, for mod
crate distances from its apparent source,
with a velocity far exceeding that of or
dinary sounds. This used to be attribut-
ed to the extreme sGddenness of its pro-
duction; but it is not easy, if We adopt
this hypothesis, to see why it should not
occur in all cases. Sir w. Thompson has
supplied very different explanation,
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 dis
charged