MAES AND HIS MOONS. 89, 



incalculable number of centuries before the earth became a separate 

 planet. 



Until quite recently, it was generally conceded that tico comets of 

 short period have revealed the existence of a resistmg medium in the 

 celestial spaces. It is well known that the celebrated Encke inferred 

 the existence of a resisting medium from the fact that the periodic 

 times of the comet which bears his name were progressively diminish- 

 ing. 



Thus he found the following values of these times : 



1786-1795, periodic time = 1208-112 days. 

 1795-1805, " " = 1207-879 '' 



1805-1819, " " =1207-424 " 



1845-1855, " " = 1205-250 " 



In this view he was sustained by Olbers and most contemporary 

 astronomers, although Bessel and some others dissented from it. But 

 Encke continued steadfast in his theory of a resisting medium in space 

 for more than forty years ; in fact, up to the period of his death in 

 1865. 



There are two other periodical comets which were expected to fur- 

 nish important evidence on this question. These are Faye's and Win- 

 necke's comets, which have periods of seven and a half and five and a 

 half years respectively. The orbit of the former has been carefully 

 determined by Professor Axel Moller, of Lund, Sweden. At first his cal- 

 culations indicated that the period of this comet was shortened at each 

 revolution by about seventeen hours ; and Encke, in his declining 

 years, thought that this fact was a complete proof of his hypothesis of 

 a resisting medium. But, in 1865, Professor Moller revised his calcula- 

 tions, and found that it was j)ossible to harmonize all of the facts loithout 

 the assumption of the resisting medium. 



With regard to Winnecke's comet, it seems that, according to the 

 computations of Professor Oppolzer, of Vienna, it is scarcely necessary 

 to call in the assistance of a resisting medium to account for its mo- 

 tions. It thus appears that, up to the present time, Encke's comet 

 stands alone in demanding the existence of a resisting medium to ex- 

 plain its motions. Nevertheless, it must be recollected that such in- 

 vestigations involve the computing of complex planetary perturbations, 

 and that, consequently, more accurate data and better mathematical 

 methods may, in the future, place these two comets in the same cate- 

 gory, in relation to a resisting medium, as that of Encke. 



In the mean time, divers physical considerations press upon us the 

 inherent prohahility of the existence of a resisting medium in the 

 celestial spaces. The connection between our organs of sense and re- 

 mote bodies necessarily implies the existence of some intervening 

 medium ; and, moreover, to convey a p'hysical impression to the organ 

 of sense, this medium must be material. Whatever theory of light we 



