404 



COMETS. 



one revolution about the sun in the short period of 

 about live years and a bait'. Coming near to Jupiter 

 ;i -I'i'ond time, in the remote part of its course, its 

 orbit is again shifted to one requiring a period of 

 about twenty years. In one of these cases the 

 comet is proved to have passed so near to Jupiter, 

 that its attraction to this planet was two hundred 

 times greater than its attraction to the sun. Still, 

 it not only had no sensible effect upon the planet 

 itself, but it did not even disturb the motion of its 

 satellites, although it passed between them and the 

 body of Jupiter. 



This comet came nearer to the earth than any 

 other whose path is known. Its least distance was 

 about six times that of the moon. It was retarded 

 in consequence, about two days, by the earth's at- 

 traction. If the comet had the same quantity of 

 matter with the earth, it must have affected the 

 length of our year to the amount of nearly three 

 hours ; but it did not exert the least perceptible 

 influence. If the year had suffered an alteration 

 to the amount of two seconds, it would probably 

 have been detected. It is hence inferred that the 

 quantity of matter contained in this comet could 

 not exceed the five thousandth part of that con- 

 tained in the earth. 



What is here stated with respect to the lightness 

 of comets, agrees very well with the result of 

 actual observation. Many comets have appeared 

 that seem to be destitute of any nucleus or solid, 

 compact mass at the centre. They are apparently 

 composed of a matter resembling the thinnest 

 vapour, so that the light of the faintest star which 

 is obscured by the least perceptible fog, scarcely 

 Buffers any diminution by penetrating many thou- 

 sand miles of this substance. The comets of 

 Encke and Biela, and some others, seem to be thus 

 constituted. In those cases where there are signs 

 of a solid nucleus, it is for the most part of small 

 extent. It has been estimated in two instances at 

 less than thirty miles in diameter. The comets of 

 1807 and 1811 were supposed by Sir W. Herschel 

 to have each a diameter of about five hundred 

 miles. 



Even where there is the appearance of a com- 

 pact mass at the centre, there is still surrounding 

 it the same nebulous appearance of vast extent, 

 which seems to constitute, in other cases, the 

 whole matter of the comet. This nebulous ap- 

 pearance, instead of being of a uniform or a pro- 

 gressively increasing brightness from the circumfer- 

 ence inward, is often apparently composed of one 

 or more rings or strata, many thousand miles in 

 thickness, alternating with fainter portions, an 

 appearance not unlike what would be presented to 

 a spectator viewing from a distance our earth with 

 its different layers of clouds, one above another 

 separated by portions of transparent atmosphere. 



A remarkable circumstance has been observed 

 in two or three instances, which consists in the 

 great contraction of the body of the comet as it 

 approaches the sun. Encke's comet was carefully 

 examined at different distances from thesun, varying 

 from about one hundred and fifty to fifty millions of 

 miles. In the former case, its diameter was twenty- 

 five times as great as in the latter, and its bulk, of 

 course, was reduced to about one sixteen thousandth 

 part by a reduction of its distance to one-third 

 part. This enormous change in its dimensions, 

 has been ascribed by some to the varying density 

 of the sun's atmosphere, the gaseous matter of the 

 comet, being, as is supposed, more and more con- 



densed as it penetrates the dcnsi-r strata of this 

 supposed medium, just as a balloon is enlarged or 

 contracted, according as it is situated in a denser 

 or rarer part of the earth's atmosphere. Others 

 suppose, that as the parts of the nebulous matter 

 of the comet are distant, and held together by so 

 feeble a power, they revolve in a degree independ- 

 ently of one another, each having its own perihe- 

 lion, whereby they are brought nearer to each 

 other, as they approach the sun, and separate again 

 farther and farther as they depart from it. Another 

 hypothesis is that the nebulous matter ot tin- 

 comet, like fog or common vapour, which disap- 

 pears before the rising sun, ceases to be visible, in 

 like manner, as the comet approaches its perihe- 

 lion, in consequence of the heat to which it is ex- 

 posed, and that it is restored to its sensible form, 

 when it is sufficiently withdrawn from the influ- 

 ence of the solar rays. 



But the most obvious and striking characteristic 

 of comets, especially to the majority of observers, 

 is the luminous train called the tail, with which 

 they are generally attended. We say generally, 

 since this is not by any means a constant append- 

 age. Many comets have appeared, as those of 

 Encke and Biela, that seemed to be as round as the 

 planets. Others again, as the comet of 183G for 

 instance, have exhibited a train only in certain parts 

 of their course and not in others : sometimes it is 

 visible only in certain parts of the earth, and in 

 certain states of the atmosphere. Where it exists 

 at all it seems to increase as the comet approaches 

 the sun, and to be more conspicuous, and of greater 

 extent, after the comet has passed its perihelion 

 than before. It is also remarked that the direction 

 of this luminous train is, for the most part, in a 

 line opposite to the sun. There are remarkable 

 exceptions, however, to this rule. It is found in 

 some instances, to be right angles to a line joining 

 the comet and sun. In its common position the 

 train is often curved more or less, the extreme parts 

 being left as it were a little behind, while the pre- 

 ceding or convex portion is observed to be somewhat 

 brighter than the rest, as though there were some 

 rare medium which condensed the luminous matter 

 and slightly impeded its motion. Where the train 

 is not curved, it is often remarked that the light is 

 faintest in the middle and strongest towards the 

 edges, as though it were a hollow cylinder, or 

 truncated cone. The tail is sometimes divided 

 into two or more branches, lying in some cases the 

 same way, but often more or less inclined to each 

 other. The comet of 1744 had six tails, opening 

 and diverging like the sticks of a fan, but curved 

 at the same time like a sickle. They were each 

 about 4 broad, and from 30 to 44 long. The 

 light was so strong, that they were distinctly per- 

 ceived after the sun rose. Single trains have been 

 observed of all lengths less than 100. The abso- 

 lute lengths have been computed in many instances, 

 and found to vary from a few millions to one hun- 

 dred millions of miles. 



Nothing that deserves the name of an explana- 

 tion has yet been offered, respecting the constitu- 

 tion and manner of production of this curious phe- 

 nomenon. The hypothesis most in vogue, is that 

 which ascribes it to the mechanical impulse of the 

 solar rays, penetrating such an extent of vaporous 

 matter very feebly held together by the small cen- 

 tral attraction ; just as a discharge of shot through 

 a mass of cotton or eider down may be supposed to 

 carry a portion of this light substance with it, to 



