Evening star. [volume] (Washington, D.C.) 1854-1972, 1880-10-16
The Sound of Thunder.

The next remarkable feature Of the storm is
the thunder Sda5 corresponding OF course on the
Here We are On comparatively sure ground. for
sound is very much more thoroughly understood
than is electricity. We speak habltually and
without eXaggerati0n OF the crash OF thunder,
the rolling OF thunder, and of peal OF thunder;
and various other terms VIII 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, Ia all cases, to be looked for in the
instantaneous and violent dilatatlo1 OF the
air along. the track of the lightning
rupt1ve effects OF electricity OF which /
have already spoken, but mainly due to the
excessive rise OF temperature which renders
the air for moment SO brilliantly incandescent.
There is thus an extremely sudden compression
Of the air all round the track OF the spark and
a less sudden, but Still rapid, rush or the air
into the partial vacuum which it produces. Thus
The sound-wave produced must at first be Of the
nature OF bore or breaker. But as such a state
Of motion Is unstable, after proceeding q mod
crate distance the sound becomes analogous
to other loud but less violent sounds suc1 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 phenomenon, ana 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, reject vibrations fn the air:
and thus we may nave many successive echoes.
there Is another cause often more efficient
than these. When the flash is q long one. all
its parts being nearly eqItdIstant from the
observer, he hears the sound from all these
parts simultaneously; but If its parts be at
very different distances from him he hears sue
cess1vely the sounds from portions further and
further distant from him. If the fl ash be much
zg-zagged, long portions OF its course may run
at one anC the same distance from him, and
The sounds from these arrive simultaneously
at his ear. Thus we have no difficulty in
;Eunder. 1t is, in fact, mere consequence,
sorlet lines 81 the reelection of sound, s1me-
times OF the flute velocity With which It is
propagated. The usual rough estimate OF five
seconds 10 the mile is near enough TO the truth
for all ordinary calculations Of the distance OF
a flash from the observer. The extreme dis.
lance a. which thunder is heard Is not great
when we consider the freql great intensity
OF the sound. No trustworthy observation
gives In general more than about or 10 miles
hough there are cases In which it is possible
hat it may have been heard IA miles Off. But
the discharge Of single cannon is often heard
at SO miles, and the noise Of siege or naval
engagement has certainly been heard at a dis
lance Of much more than 100 miles There are
two reasons for this: the first depends upon the
extreme suddennesS OF the production of thun-
der; the second, and perhaps the more effect
i've, on the excessive variations OF density in
the atmosphere, which are invariably associated
with a thunder storm. In certain cases thunder
has been propagated, for moderate distances
rom its apparent source, with a velocity far
exceeding that OF ordinary sounds. This used
10 be attributed to the extreme suddenness OF
its production: but it is not easy, II we adopt
this hypothesis, to see why It should not occur
IN all cases. Sir ". Thomson has supplied a
very different explanation, which requires no
unusual velocity OF sound, because 1t 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
Prof. Tat IN Nature.