ENCKE'S COMET. 



between 1795 and 180,3 two, and, in fine, between 1805 and 1819 

 three, unobserved returns to perihelion. 



It appears, therefore, that, excepting the oval form of the orbit, 

 the motion of this body differs in nothing from that of a planet 

 whose mean distance from the sun is that of the nearest of the 

 planetoids. Its eccentricity is such, however, that when in 

 perihelion it is within the orbit of Mercury, and when in aphelion 

 it is outside the most distant of the planetoids, and at a distance 

 from the sun equal to four-fifths of that of Jupiter. 



21. A fact altogether anomalous in the motions of the bodies of 

 the solar system, and indicating a consequence of the highest 

 physical importance, has been disclosed in the observation of the 

 motion of this comet. It has been found that its periodic time, 

 and consequently its mean distance, undergoes a slow, gradual, 

 and apparently regular decrease. The decrease is small, but not 

 at all uncertain. It amounted to about a day in ten revolutions, 

 a quantity which could not by any means be placed to the account 

 either of errors of observation or of calculation ; and, besides, 

 this increase is incessant, whereas errors would affect the result 

 sometimes one way and sometimes the other. The period of the 

 comet between 1786 and 1795 was 12081 days; between 1795 and 

 1805 it was 1207& days ; between 1805 and 1819 it was 1207 T 4 S 

 days ; in 1845 it was 1205^ days ; and, in fine, in 1852 it was 

 1204 days. 



The magnitude of the orbit thus constantly decreasing (for the 

 cube of its greater axis must decrease in the same proportion as 

 the square of the period), the actual path followed by the comet 

 must be a sort of elliptic spiral, the successive coils of which are 

 very close together, every successive revolution bringing the 

 comet nearer and nearer to the sun. 



Such a motion could not arise from the disturbing action of the 

 planets. These forces have been taken strictly into account in 

 the computation of the ephemerides of the comet, and there is 

 still found this residual phenomenon, which cannot be placed to 

 their account, but which is exactly the effect which would arise 

 from any physical agency by which the tangential motion of the 

 comet would be feebly but constantly resisted. Such an agency, 

 by diminishing the tangential velocity, would give increased 

 efficacy to the solar attraction, and, consequently, increased cur- 

 vature to the comet's path ; so that, after each revolution, it 

 would revolve at a less distance from the centre of attraction. 



22. It is evident that a resisting medium, such as the lumini- 

 ferous ether * is assumed to be in the hypothesis which forms 



* See "Hand-Book of Astronomy," 1225. 



159 



