Evening star., 1937-12-14

Back in 1885 the Washington Monu-
ment was struck by lightning.
The event caused much speculation at
the time, because nobody heard an &C
companying clap of thunder. The
phenomenon was duly recorded and the
mystery has just now been solved after
a half century by DR.. K. B. McEachron,
investigating high voltage and arti-
fetal lightning for the General Elec-
trio Co.
The monument, it appears. was the
target of slow lightning which makes
no noise.
Thundred. DR.. McEachron explains, is
the result of pressure wave due to
sudden expansion of air created by a
quick lightning discharge. But. he con-
tinues. all flashes do not release energy
at the same speed Our studies during
the past three years have revealed that
in some cases the electrical current is
built up and released slowly; that is. in
one or two tenths of second compared
to millionths of second in other dis
charges. This so-called slow lightning
produces no thunder. To the human eye
it looks the same, and during general
storm the fact that one has heard no
thunder in connection with any given
stroke is generally overlooked.
1T seems strange to me that during
this 50-year period no one has en-
deavored to find out why we can have
lightning without thunder. People have
the conception that lightning moves very
swiftly and this is true but the rate at
which the current builds up, once the
path is established, may be relatively
small, thus giving rise to the phenom-
enon of lightning without audible thun-
der. The strokes of lightning which are
most destructive are of the type in which
the current builds up very rapidly, not
in terms of fractional part of Sec
ond, but in terms of millionths of
second, and this very rapid rise OF cur
rent produces correspondingly rapid
rate of pressure rise in the air. This,
when transmitted to the ear, gives the
sound of thunder"
He illustrates the difference by q
crude analogy. When a firecracker is
exploded in closed wooden box it
makes lot of noise. The same amount
Of powder scattered over the surface of
table and ignited makes hardly any
noise. In the case Of the powder burn
ing freely in the air, considerable time
is consumed in the release of the energy.
Inside the flrecracker the pressure is
built up to higher and higher value
without making any audible sound until
it suddenly "lets go" and an extremely
rapid rate Of rise of pressure reaches
the ear
Most thunder, DR.. McEachron ex-
plains, does not reach the ear as single
crack. Sound travels at approximately
1,l00 feet second. One portion of a
lightning discharge in the clouds may be
six seconds away from the earth, where
as that portion Of the discharge nearest
the earth may be only one second away.
His investigations show, he says, that
there are often multiple discharges ap-
pearing as single stroke, in which as
many as 10 discharges have been re-
corded In one second.
This type Of lightning. he says, gives
rise to q ripping or tearing sound be-
cause of the rapid succession Of dis
charges, each producing sound waves
which reach the ear at different inter-
walls. Added to this there is the reflection
of sound from clouds, hills and buildings,
all tending to increase the echo effect.
80 \
A fire which quenches itself as it grows
hotter has just been reported to the