The news and herald. (Winnsboro, S.C.) 1877-1900, 1880-12-07
The Sound or Thunder.

A remarkable feature Of the storm is the
thunder, corresponding, of course, on the
large scale, to the snap of all electric spark.
Here we are On comparatively sure ground,
for sound ls very much more thoroughly
understood than is electricity. We speak
habitually and without exaggcration of the
crash Of thunder, the rolling of thunder
and Of peal of thunder; and various
other terms will suggest themselves to you
as being aptly employed in different casos
| All of these arc easily explained by known
properties of sound The origin of the
| sound is, in all cases to be looked for in the
| instantaneous and violent dilatation of the
air along the track of the lightning flash,
partly, no doubt, due to the disruptive ef-
fects Of electricity, but mainly due to the
excessive rise of temperature which renders
the air for a moment so brilliantly lncan-
descent. There ls thus al extremely sud
den compression Of the mir all round the
track Of the spark, and a less sudden, but
still rapid, rush Of the air into the partial
vacuum which it produces. Thus the
sound wave produced must ut first be Of
the nature OF bore or breaker. But as
such state of motion is unstable, after
proceeding 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 tho air of nearly uniform density,
and the flash short onc this would COM
pletely describe the phenomenon, and we
should have thunder crash or thunder
clap, according to the greater or less prox-
Unity OF the seat of discharge. But as has
long been we'll known not merely clouds,
but surfaces of separation of masses of
air OF different density, such as constantly
occur III 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 a long one, all its parts being nearly
cqui-distant from the observer, he hears
the sound from all these parts sImultane
ous)y; but if its parts be at very different
distances from hlm, he hears successfully
the sounds from portions farther and farther
distant from him. Ii the flash be much
fn az Sa liza l2aaF FRQ RQSS2Rsd SSaRes 8S y
and the sound from these arrive slmultanc
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 propagatcd. The usual
rough estimate of five seconds to q milo is
near enough to 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 i'll general more than about nine or ten
miles, though there arc cases in which it is
possible 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 n siege or naval engage
ment has certainly been head at distance
Of much more than 100 miles. There are
two reasons for this-thc first depends upon
the extreme suddenncss of the production
of thunder; the second, and perhaps the
more effective, on the excessive variations
OF density in the atmosphere, which arc
invariably associated with Il Thunderstorm
In certain cases thunder has been propa-
gated, for moderate distances from its ap-
parent source, with velocity fur exceeding
that Of ordinary sounds. This used to be
attributed to tho extreme suddenncss of its
productions; 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 tho
air between the ground and the cloud from
which the lightning Is discharged.