May 1, 1894.] 



KNOWLEDGE. 



lis 



WHAT IS A COMET'S TAIL? 



By A. C. Ranyard. 



NO bride was ever covered by such a transparent 

 gauzy veil as that which trails behind a comet as 

 it comes to do obeisance to the sun, or as it backs 

 like a courtier out of the solar presence, keeping 

 its train behind it. It has frequently been noticed 

 that the light of small stars is not sensibly dimmed when 

 the tail of a large comet sweeps between the earth and 

 the star ; therefore, to compare the matter of comets' 

 tails with a white mist, or a silver fog in space, is too 

 gross a comparison. A few hundred yards of the thinnest 

 fog or mist we are familiar with, cuts down very materially 

 the light of objects seen through it, but the light of a star 

 in passing through the tail of a large comet must have 

 travelled through hundreds of thousands of miles of the 

 nebulous material which is streaming away from the 

 nucleus. The thickness of light-absorbing material 

 traversed by a ray of light makes a greater difference than 

 would at first sight be suspected in the amount of light 

 transmitted through a fog or haze, or other absorbing 

 medium, for the light lost increases in geometrical propor- 

 tion as the thickness of the light-absorbing medium 

 mcreases in arithmetical proportion ; thus, if a haze ten 

 miles thick reduced the light of an object seen through it 

 by one half, a similar haze twenty miles thick would 

 absorb three-quarters of the light that would otherwise be 

 transmitted, for the second ten miles of haze will halve 

 the light that has been transmitted through the first ten 

 miles. Thirty miles of such haze will reduce the light to 

 an eighth, and two hundred miles of such haze will 

 reduce it to about one millionth, for two to the power of 

 twenty is a little more than a million. 



On the clearest summer day the brightness of objects at 

 a distance of ten miles on the horizon is reduced to less 

 than one half by the absorption of the intervening atmos- 

 phere — a fact that becomes very evident to photographers 

 who attempt to photograph distant objects, and who find 

 that long exposures are necessary to obtain upon their 

 plates any trace of the blue distance which so charms the 

 eye. The tail of a comet must consequently 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 if 

 it were a thousand times more transparent than air — act 

 as an opaque screen in space, cutting out the light of the 

 stars, and probably even eclipsing the light of the sun itself, 

 if such a cometary tail should pass between the earth and 

 the solar disc. 



The light derived from the tail of a comet is generally 

 found to give a bright line spectrum in the neighbourhood 

 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 indicate a high temperature in 

 the region about the nucleus ; but the spectrum of the 

 aurora which glows in the cold upper strata of our atmos- 

 phere is also characterized by narrow gaseous lines, and 

 many comets begin 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 sun's rays. 



Thus, according to Mr. Marth, comet Brooks, at the 

 time of the photograph reproduced in our plate 

 (October 21st), was at a distance 1-02 times the earth's 

 mean distance from the sun — that is, it was at a little 

 greater distance from the sun than the earth ever is.* 



• During our summer in the Northern Hemisphere the earth 

 attains a distance of l'0i677 times its mean distance from the sun. 



We may therefore compare the temperature of the comet 

 as derived from the sun's rays, at the time that its photo- 

 graph was taken by Prof. Barnard, with the temperature 

 of the moon, and it was shown in the article in the April 

 number that the temperature of the moon's equatorial 

 regions during the lunar day probably does not exceed the 

 temperature of melting ice. Consequently, if the ebullition 

 which was evidently going on in the nucleus of Brooks' 

 comet at the time the photograph was taken was due to 

 the rapid driving into vapour of matter by the sun's heat, 

 the material that was being vaporized must have had a 

 very low melting point, such as is possessed by carbonic 

 acid or substances which freeze at a still lower temperature, 

 such as hydrogen, nitrogen, and other forms of matter 

 which in terrestrial laboratories we are, under ordinary 

 circumstances, only familiar with in their gaseous state. 



The way in which comets' tails, as a general rule, 

 slowly develop in size as they approach the sun, and again 

 diminish as they recede from him, would seem to point to 

 the conclusion that the growth of comets' tails is princi- 

 pally due to the intensity of the sun's heat, or to some 

 other cause which varies with the distance of the comet 

 from the sun. But the rapid variations in form and 

 brightness which many comets have exhibited seem to 

 show that the evolution of gas from the nucleus, and the 

 repulsion of matter in the tail, is influenced or in some way 

 partly controlled by some more irregularly varying con- 

 ditions, such as collisions with meteors, or dust in space, 

 or the passage of the cometary nucleus through a mass of 

 gas. The rapid variations in form and brightness which 

 a comet sometimes undergoes are well illustrated by the 

 four photographs of Swift's comet, taken by Dr. Max Wolf, 

 which are reproduced in our second plate. A still more 

 striking instance of a rapid increase of brightness was 

 afforded by Holmes' comet, which, at more than double 

 the earth's distance from the sun, seems to have suddenly 

 brightened up. 



Before attempting to speculate on the causes of these 

 irregular variations in brightness, it may be well to give an 

 account of the phenomena which are usually observable. 

 A comet when it is first seen as it approaches the sun, and 

 also when last seen as it recedes from the sun, generally 

 appears as a small roundish patch of faintly luminous 

 nebulosity, sometimes with a brighter patch or stellar 

 point near the centre. As the comet brightens on 

 approaching the sun it generally begins to emit jets or 

 streamers, or to form more or less symmetrical envelopes 

 on the side next the sun, and develops a tail on the side 

 remote from the sun. In the great comet of 1858, usually 

 known as Donati's comet, the action was most symmetrical, 

 one envelope after another rising from the nucleus and 

 expanding, as if the material forming the envelopes was 

 repelled by the nucleus, and was also repelled by the sun, 

 till it was ultimately driven away within a hyperbolic 

 envelope or stratum which formed about the nucleus. 



The tail in this and other comets seemed to be composed 

 of hollow cones slightly bent backwards in the plane of the 

 orbit. The backward curvature is easily explained, because 

 particles repelled from the comet's head would still retain 

 their original orbital motion, and would fall behind the 

 line drawn from the sun through the comet's nucleus, 

 as they were driven into a larger and larger orbit. 

 The amount of backward curvature of the tail evidently 

 depends on the velocity with which the particles are 

 driven away from the sun, and we find in this and 

 other comets multiple tails, indicating that the different 

 branches of the tail are composed of difl'erent materials 

 which are repelled from the sun with diff'erent velocities. 



But in Dr. Max Wolf's photographs of Swift's comet we 



