756 THE UNIVERSE. 



subjugated by the superstition of his epoch that he saw 

 in comets a kind of monsters, similar to those produced 

 by the sea, and wandering vaguely in the heavenly regions. 



Although in its jjrogress science has eradicated these 

 absurdities, still on the other hand it has given rise to some 

 fears. It Avas dreaded every instant that the shock of 

 one of these wandering stars would shatter the earth into 

 fragments. The theory of Buff'on and the assertions of 

 Kepler in no way reassured men. The former, it may 

 be remembered, had put forth the view that our globe 

 Avas only a fragment struck off from the sun by the shock 

 of a comet; and the danger seemed only the more imminent 

 when Kepler, in his picturesque language, said that "there 

 are more comets in the sky than fish in the ocean." The 

 worst Avas to be feared.^ 



But modern science has SAvept aAvay a part of the 

 danger. At the same time that it has shoAvn the immense 

 size of these stars, it has also demonstrated their inoffen- 

 sive nature. The tail of a comet, Avhich the Chinese 

 fancifully call its broom, because it seems to SAveep the 

 azure of the sky, and Avhich to our eye appears only like 

 a luminous fan, sometimes exceeds 2,000,000 leagues 

 in length. This luminous cone may even attain much 

 more prodigious dimensions, and has been knoAvn to 

 equal the distance Avhich se2)arates the earth from the sun. 



But notAvithstanding these frightful proportions, comets 

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

 of all stars those of Avhicli the material particles shoAV the 

 greatest looseness. Their mass sometimes does not reach 



' Arago adopts the hypothesis of an equal distribution of comets in all ]5aits 

 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 

 whicli circulate within the known limits of the solar system, that is to say the 

 orbit of Neptune, at 17,.500,000.— Guillemiu, Ze del. Paris, 1865, p. 348. 



