THE SIDEREAL UNIVERSE. 725 



comets in the sky than fish in the ocean." The worst was 

 to be feared. 1 



But modern science has swept away a part of the danger. 

 At the same time that it has shown the immense size of 

 these stars, it has also demonstrated their inoffensive nature. 

 The tail of a comet, which the Chinese fancifully call its 

 broom, because it seems to sweep the azure of the sky, and 

 which to our eye appears only like a luminous fan, some- 

 times exceeds 2,000,000 leagues in length. This luminous 

 cone may even attain much more prodigious dimensions, 

 and has been known to* equal the distance which separates 

 the earth from the sun. 



But notwithstanding these frightful proportions, comets 

 ought to produce scarcely any fear for the earth, as they are, 

 of all stars, those of which the material particles show the 

 greatest looseness. Their mass sometimes does not reach 

 TTsWth of that of the earth, which induced Theon of Alexan- 

 dria to give them the picturesque name of wandering clouds. 

 Some observers have looked upon them as even much 

 lighter than this, so light, indeed, as to surpass every- 

 thing one can imagine. Comets, says M. Flammarion, 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. 2 



1 Arago adopts the hypothesis of an equal distribution of comets in all parts of 

 the solar system, and, founding his calculations on the number of comets observed 

 between the sun and Mercury, computes the number of these stars which circu- 

 late within the known limits of the solar system, that is to say, the orbit of Nep- 

 tune, at 17,500,000. Guillemin, Le del, Paris, 1865, p. 348. 



2 Mr. Huggins, who has examined the subject very carefully, has come to the 

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



