COMETS. 749 



Son, like another sun, lying near its path, arrests its flight. The possibility of 

 such an occurrence is by no means imaginary. At least one comet (Lexell's, 

 I 770,) is supposed with good reason to have undergone that fate. There is every 

 reason to believe that the same thing may have happened in other cases. 



STELLAR COMETS. 



All arguments drawn from observation and reflection prove that the stars 

 which surround us on all sides art remarkably like our own sun. Some of them 

 are even larger and more powerful than he. Reasoning from analogy, we must 

 suppose that these suns are also attended by comets. Hence, we are led to the 

 conclusion that uncounted myriads of comets, projected forth from millions of 

 suns, during countless ages past, are now flying through space in every direction 

 — restless messengers from star to star. By mere chance some of these bodies 

 must come under the sun's far-reaching power and be drawn into our planetary 

 system. 



PHYSICAL HISTORY OF COMETS. 



§ 5. The mass (quantity of matter) of comets is conceded to be very small 

 in comparison with that of the earth. How small it is, we cannot say. No 

 comet has been found large enough to exert a sensible attraction upon any celes- 

 tial body found in its vicinity. This fact confirms the conclusion derived from 

 telescopic examination, that the real, solid nucleus, if it exists, must be extreme- 

 ly small. 



It is certain that no body entirely gaseous could exist in space. The condi- 

 tions for the stability of liquid bodies in their practical application to the explana- 

 tion of cometary phenomena, are extremely complicated; since they are closely 

 associated with the unknown elements — mass of the comet, solar radiation, and 

 absolute temperature of space. It would also be extremely difficult to show how 

 a swarm of small bodies could be preserved in a state of equilibrium, or resist the 

 tremendous tidal action to which it would be subjected in the vicinity of the sun. 

 In fact, we must view the conversion of a comet through some unusual catastro- 

 phe, into such a swarm, as the sure precursor of approaching dissolution. On 

 the whole, it is probable that there is a solid or partly liquid body near the cen- 

 ter of the comet. This body is more Hkely to consist of an aggregation of loosely 

 cohering pieces or particles, than of a single firmly united mass. 



Owing to the smallness of their attractive force, comets cannot retain a sen- 

 sible atmosphere. This conclusion is confirmed by telescopic observation, as we 

 have seen. 



If, now, we suppose the nucleus to be approaching the sun, it will eventually 

 reach a point where the Hquid or other volatile matter on the "sunny" side 

 commences to evaporate and be difl'used about the comet. Without following the 

 consequences of this evaporation into details, one can imagine for himself how the 

 appearance of central condensation, of the streaming jets, and of the nucleus 

 heavily obscured by vapors, might be produced. 



