THE SIDEREAL UNIVERSE. 757 



TuVoth of that of the earth, which induced Theoii of 

 Alexandria to give them the picturesque name of wander- 

 ing clouds. Some observers have looked upon them as 

 ■ even much lighter than this — so light, indeed, as to sur- 

 pass everything one can imagine. Comets, says M. Flam- 

 marion, have been seen several million leagues long, the 

 weight of which was yet so trifling that one could have 

 carried them on one's shoulders without fatigue.^ 



We have, therefore, no reason to dread their contact, 

 and may sleep securely. In 1770 astronomers saw a 

 comet bar the path of Jupiter's system, 'and envelop the 

 planet on every side, without the slightest perturbation 

 to the course either of the great star or that of its satel- 

 lites. On the contrary, it was the nebulous star that 

 suffered from the contact. Besides, it seems that during 

 the passage of certain comets in our vicinity, their tails 

 may have penetrated into our atmosphere.^ 



However, according to Maupertuis, though there are 

 some comets so small that their collision with the earth 

 would only destroy a few kingdoms, without shattering its 

 mass, there are others the contact of which might be fatal 

 to every living thing on the globe. 



In his Lettres Cosmologiqiies, Lambert leads us to dread 



' Mr. Hiiggins, who has examined the subject very carefully, has come to 

 the conclusion that the nucleus of comet 1, 1866, was self-luminous, that it con- 

 sisted of matter in the shape of ignited gas, and that this matter is similar in 

 constitution to the gaseous material of some of the nebuhe. The coma was found 

 to shine by reflected light, and as from its extreme diffusion it cannot be supposed 

 to contain solid or liquid matter at the high temperature necessary for incandes- 

 cence, it seems almost certain that it reflects the light of the sun. The nucleus 

 of meteors is probably a fragment of mineral matter, f)f which sodium is one of 

 the chemical ingredients. Their spectra are often highly coloured and continuous 

 like those from solid matter at a white-heat. — Tr. 



2 According to Humboldt, the tails of the comets in 1819 and 1823 must have 



reached our atmosphere. The same thing is supposed to have happened with the 



last great comet observed in our latitudes. 



96 



