The Indianapolis journal. [volume] (Indianapolis [Ind.]) 1867-1904, 1894-05-21
WHAT Is A COMETS TAIL.
A More Transparent Substance Than
the Earth's Atmosphere.
Prof. A. C. lanyard, in knowledge Non-


Prof. A. c.. lanyard, in Knowledge Oon-
DOM
No bride was ever covered by such a
transparent gauzy veil as that which trails
to the sun or as it backs, like a courtier,
out of the solar presence, Keeping It's train
behind it. 1t has frequently. been NOTICED
that the light of small stars is not sensiDly
dimmed when the tail Of a large comet
sweeps between the earth and the star;
therefore, to compare the matter or comets
tails with a white mist or a silver FOG IN
space is too gross a comparison. A few hun
dred yards of the thinnest fog or mist We
are familiar with cuts down very materially
the light of objects seen through II, Dut
the light of a star in passing through the
tail of a large comet must nave Travelled
through hundreds of thousands or miles OF
the. nebulous material which is streaming
away from the nucleus. The thickness OF
Notwithstanding material traverSed by a ray
Of light makes a greater dinerence e1 Sa
amount of light transmitted through a FOG
or haze, or other absorbing medium, for the
light loss increases in geometrical. proper
ton as the thickness of the Notwithstanding
medium increases in arithmetfcal proper
ton; thus, If a haze ten miles thick reduced
the light of an object seen through 1t by
one-half, a similar haze twenty miles thick
would absorb three-quarters of the light
that would otherwise be nsmitt Tor
the second ten miles or haze Will naive the
light that has been transmitted through
the first ten miles. Thirty miles of such
haze VIII reduce the light to an eighth, and
TWO Hu0uIcQ IlllcS VI NV wan AO0AN
IL to about one-milliorth, for two to the
power of twenty is a little more than a mil
lion.
The tail of a comet must be far more
TRANSPARENT than the earth'S atmosphere;
indeed a mass of gas as bulky as the tail
of a large comet would, even ii 1t were
a thousand times more transparent than
air. act as an Opaque screen in space, cut
ting out the light of the stars and probably
even sncs ec ! o : the t iigh tbo of the bass sun bet ,ssel5
the earth and the solar disc. The light de-
rived from the tail of q comet is generally
found to give a bright line spectrum in the
neighborhood of the nucleus and to be more
or less polariZed at a greater distance from
the nucleus. The bright lines seem to
point TO the presence of incandescent gas,
and have frequently been taken to indi-
cate a high temperature in the region above
the nucleus; but the spectrum of the aurora
which CLOVIS in the cold upper strata Of cur
atmosphere is also characterized by mar
row gaseous lines. and many comets be-
gin to glow and throw out tails in regions
of the solar system where they can derive
but little heat from the warming effect of
the suns rays.
general rule slowly develop in size as they
approach the sun and then diminish as they
recede from him would seem TO point TO
the conclusion that the growth of comets
tails is principally due to the intensity of
the suns heat or to some other cause which
varies with the distance of the comet from
and brightness which many comets have
exhibited seem to show that the evolution
OF R1S ITOTl lle nuCltuS anu lIt -tDuiSlO1
of matter In the tail is Influenced or in
some way partly controlled by some more
i~rregularly varying conditions, such as col
lisions with meteors or dust in space or
the passage OF the cometary nucleus
through a mass or gas.
A comet when Ii is first seen as il ap-
proaches the sun, and also when last seen
as Ii recedeS from the sun, generally ap-
pears as a small roundish patch of family
brighter patch or stellar point near the GEN.
ter. As the comet brightens on approaching
the sun il generally begins to emit Jets or
streamerS or to form more or less Symmet-
rical envelopes on the side next the sun
and develops a tail on the side remote from
the sun. In the great comet of 1855. usually
known as Donatfs comet, the action was
most symmetrical, one envelope after an
other rising from the nucleus and expand
ing, as II the material forming the en-
velopes was repealed by the nucleus. and
was also repealed by the sun, till il was ul-
timately driven away within a hyperbolic
envelope or stratum which formed about
the nucleus. The tail in this and other COM
cts seemed TO be composed of hollow cones,
slightly bent ackward in the plane of the
orbit. The backward curvature is easily eX-
planned, because particles repealed from the
comets head would still retain their orig
the line drawn from the sun through the
comets nucleus. as they We're driven into
a larger and larger orbit. The amount of
backward curvature of the tail evidently
depends on the velocity with which the par
ticles are driven away from the sun. and
we find in this and other comets multiple
tails. indicating that the different branches
terials. which are repealed from the sun
NTH diffaranf ValDrfffGG