748 KANSAS CITY REVIEW OF SCIENCE. 



THE PHENOMENA OF THE TAIL. 



The strange appearance of the tail and the gigantic dimensions it sometimesv 

 attains, are well calculated to arrest the attention of mankind. It is not wonder- 

 ful that the ancients should have regarded it with trembling apprehension, nor is- 

 it surprising that even yet, it excites an absorbing curiosity among the educated, 

 and the superstitious terrors of the ignorant. The matter of which it is composed 

 must be expanded to an almost inconceivable degree; for even when it is millions 

 of miles in diameter, the light of the faintest stars is seen through it with scarcely^ 

 diminished brightness. 



THE BLACK STREAK IN THE TAIL. 



Near thh head, it often appears to consist of two streams of matter issuing 

 from either side of the coma with a dark channel of separation between. The 

 tail at this point generally appears to have che same diameter, from whatever di- 

 rection in space it is viewed. We must, therefore, conclude that its interior is 

 nearly or quite free from matter, and like a hollow cylinder, or portion of a cone,, 

 as far as the dark channel extends. Beyond that point we may suppose that the 

 interior fills up by gradual diffusion from the circumference : in some cases, and 

 especially with small comets, the dark channel is wholly wanting, or but faintly 

 indicated. 



CURVED DIRECTION OF THE TAIL. 



In order to give an idea of the situation of the tail in space, let us imagine a 

 line from the sun continually prolonged through the moving head of a comet into- 

 space beyond. We shall always find the tail extending nearly opposite the sun, 

 in the general direction of this prolongation, but curved more or less backward from 

 it, in the direction from which the comet is moving. Sometimes we find more 

 than one tail — each distinguished by the degree of its backward inclination. 

 They have, indeed, been classified on this ground and found conformable to 

 three general types. 



ORIGIN OF COMETS. 



§ 4. The facts thus far presented, prove nothing as to the origin of comets. 

 That question demands for its solution mathematical reasoning based on the cal- 

 culated paths of all comets which have been observed. That discussion is beset 

 with great difficulties, and as yet points to no absolutely certain conclusion. The 

 balance of testimony seems to favor the supposition, that comets originate outside- 

 the solar system. The planets move in nearly circular orbits about the sun; and 

 no one has been able to show why comets, if they have the same origin, should 

 move in elongated orbits, entirely differing from those of planets. 



Let us suppose, however, that all comets must have taken their origin in 

 some primeval nebula from which a solar system has been evolved. It has been 

 shown that the velocity of a comet may be so much increased by the disturbing 

 action of a large planet, that it may escape from the control of the sun, and be 

 projected into the illimitable regions of space. Thus freed, it will go on in a 

 nearly straight line forever ; unless, perchance, some powerful source of attrac- 



