The National tribune., 1890-04-10
THE WEATHER,
Some Facts About Lightning.
BY LIEUT.. j. P. FINLEY, U. s. SIGNAL SERVICE.

Correspondence invited suggestions desired
and information checrfully furnished. Every ef-
fort employed to make this Bureau of practical use
10 the readers of TIE NATIONAL TRIBUNE. All
communications should be addressed Weather,
THE NATIONAL TRIBUNE Office. Washington ID. O.
Since the development of instantaneous pho
Biography much interest has arisen i'm the pres
| eryation and study of lightning flashes as re-
produced by the camera. Owing to the quick
ness of the flash the eye Is unable to take in
and form an adequate impression of the great
beauty and infinite variety of the delicate
traciugs of the light athwart the heavens.
Mr.. Henry Z. Curtis, of the Boston Scientific
Society, has the largest collection of lightning
photographs in this country and is making q
special study of the conditions of development,
intensity and distribution of electric discharges
in the atmosphere. How these discharges may
f0reshadow the occurrence of severe or
light thunderstorm, or otherwise explain the
variations of atmospheric electricity are mat
ters now receiving consideration. The various
forms Of lightning have been classified as zigs
gag, ball, sheet and heat.lghtning The first
named presents long irregular, jagged line of
light like the ordinary spark drawn from an
electric machine. This zigzag path is some
times four or five miles, and perhaps even 10
miles in length.
The irregularity of the path is ascribed TO the
compression of the air before the electricity,
thereby opposing greater resistance, and turn
ing inc Hulu aSlue TO SOCK some path upon
which the resistance is less.
Ball lightning appears like ball of fire, and
Is usually accompanied by terrific explosion.
1t probably results from an unusually intense
discharge of electricity, which forces direct
instead of Circuitous passage through the air.
Some have supposed that ball lightning was
the agglomeratiOn OF ponderabe substances in
state OF great tensity, strongly charged with
electricity, but this theory is very doubt
full. There appears to BIO no record of an OBJ
server gathering up even the fragments of such
a body, much less the entire material intact,
and preserving it for identification and exam
ination. Sheet lightning is ditfuse glare of
light, sometimes illuminating only the edges
of a cloud, and sometimes spreading over its
entire surface. This may sometimes be due to
distant lightning which ilumines cloud,
while tho direct fash is hidden from the OBJ
server by intervening clouds. Sometimes 1t
may result from movement of electricity in
the interior of cloud which is a very imper-
feet conductor, producing an illumination an
alagous to that observed OH a plate of moist
glass employed in dischargiDg an electrical ma
chine.
The expression heat lightning" does not
convey definite and proper meaning Of the
phenomena referred to- The word heat has
grown to lie used in this connection because the
peculiar electric discharges in question are most
frequently observed during the heat of Summer.
The horizon is sometimes illumined for hours
In succession by flashes of light unattended by
thunder. This illumination Is sometimes due
TO the reflection from the atmosphere Of the
lightning of clouds so distant that the thunder
cannot be heard. Some believe that it is a
manifestation of auroral light where great dis
tance causes the streamers to appear blended,
or that the display is essentially n weak one.
The best authority asserts that the so-caIled
heat lightning is due to electrical discharges
attending n storm so far distant from the OBJ
server that the noise of thunder cannot lie
heard.
Systematic observation appears 10 substan-
tiate this conclusion. Sometimes, however,
this light overspreads the entire heavens show
ing that the electricity of the clouds escapes in
flashes so feeble that they produce no audible
sound. Such cases may occur when the air is
very moist, tho air being then tolerable con-
dnctor, and Offering just sufficient resistance
to the passage of the electricity to develop a
feeble light
The color of lightning varies from white to
rose color and violet. IigZag lightning is gen.
orally white, sometimes of purplish violet
or blush tinge. Diffuse flashes of lightning
are often of an intense red, sometimes mixed
with blue or violet. These differences depend
upon the density and moisture of the strata of
air in which the clouds are formed, and also
upon its conducting power When the density
of tho medium IS slight the light becomes diff
fuse aud reddish when the density is consid-
cradle, the light is concentrated and brilliant.
Similar variations in the color of the electricity
are perceived when tho fluid is passed through
glass receiver in which the air has been rare
fled by means Of an air-pump.