Montgomery County sentinel. [volume] (Rockville, Md.) 1855-1974, 1880-11-19
MARYLAND. FRIDAY MORNING. NOVEMBER 49,
Thunder.

| We speak habtually and without exag-
geration of the crash of thunder, the rolling
of thunder, and of peal of thunder, and
various other terms will suggest themselves
as being APPLY employed in different cases.
All of these are 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 dilataton of the air
along the track of the lightning flash, partly,
no doubt, due to the disruptive effects OF
electricity, but mainly due to the excessive
rise of temperature which renders the air
for moment so brilliantly incandescent.
There is thus an extremely sudden compress
sion of the air all round the track of the
spark, and a less sudden but still rapid rush
of the air the partial vacuum which it pro-
duces. Thus the sound wave produced must
at 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 the air of nearly uniform density; and
the flash a short one, this would completely
describe the phenomenon, and we should
have thunder crash or thunder clap, accord
ing 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 flash is u long one, all its
parts being nearly equidistant from the OBJ
server, he learns the sound from all these
simultaneously; but if its parts be at very
different distances from him he hears sue
cessively the sounds from portions further
and further distant from him. If the flash
be much zig-zagged, long portions of its
course may run at one and the same dis
arrive simultaneously ut his ear, Thus We
have no difficulty in accounting for the roll
ing and pealing Of thunder. 1t is, in fact, a
mere consequence, sometimes of the rellec-
ton Of sound, sometimes of the finite velocity
with which it's propagated The usual rougl
estimate of five seconds to mile is neal
enough to the truth for all ordinary calcua
tons of the distance of flash from the OBJ
server. The extreme distance at which thun
der 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 or 10 miles, though
there are cases in which it is possible that il
may have been heard IA miles off. But the
discharge OF "a single cannon is often heard
at 40 miles, and the noise of siege or nava
engagement has certainly been heard at {
distance of much more than 100 miles
There are two reasons for this: the first de
ponds upon the extreme suddenness of thu
production Of thunder the second, and per
haps the more effective, on the excessive va
riations Of density in the atmosphere, which
are invariably associated with II thunder
storm. In certain cases thunder has beer
propagated for moderate distances from III
apparent source, with velocity far exceed
ing that Of ordinary sounds. This used t.
be attributed To the extreme suddenness o
It's production but it is not easy if w
adopt this hypothesis to see why it SHOULD
not occur in all cases. Sir w. Thompson
has supplied very different explanation
which requires no usual velocity of sound
because it asserts the production Of sounC
simultaneously at all parts of the air be
tween the ground and the cloud from which
the lightning is discharged.-Prgfess0r Tai
IN Nature