incalculable number of centuries before the earth became a separate planet.
Until quite recently, it was generally conceded that two comets of short period have revealed the existence of a resisting 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 diminishing.
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 furnish important evidence on this question. These are Faye's and Winnecke'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 Möller, of Lund, Sweden. At first his calculations 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 Möller revised his calculations, and found that it was possible to harmonize all of the facts without 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 motions. It thus appears that, up to the present time, Encke's comet stands alone in demanding the existence of a resisting medium to explain its motions. Nevertheless, it must be recollected that such investigations 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 category, in relation to a resisting medium, as that of Encke.
In the mean time, divers physical considerations press upon us the inherent probability of the existence of a resisting medium in the celestial spaces. The connection between our organs of sense and remote bodies necessarily implies the existence of some intervening medium; and, moreover, to convey physical impression to the organ of sense, this medium must be material. Whatever theory of light we
