196 MEMOIR OF ENCKE. 



genius" wliicl) Encke had lavished on the calcuLations. Bessel, who had heard 

 of the comet during an absence from Konigsberg, wrote to one of his scholars : 

 " It becomes clearer and clearer that this comet is the most important scientific 

 discovery of the present century." Olbers also expressed himself in the same 

 terms in a letter to Bessel. 



Although the short period was in itself of the most pregnant consequence as 

 affording the prospect of a more certain determination of the masses of the planets 

 which exert an influence on the comet, yet further investigation soon led the way 

 to another wholly unexpected result. Encke found, in effect, that this comet 

 had been observed, also, in 1786, and hence four times in all, Avhile no less than 

 seven times in the interval its return had not been noticed. From a comparison 

 of the three intervals -between the observed transits it resulted, with all due 

 allowance for the planetary disturbances, that each revolution, as regards the 

 next preceding one, had been shortened by about three hours. 



Olbers was the first to conjecture that the comet encountered a certain resist- 

 ance whereby its approximation to the sun, and, consequently, the shortening of 

 its period of revolution might be accounted for. Encke concurred in this view, 

 while Bessel dissented from it. In the correspondence between the two, the 

 reasons for and against the hypothesis were, for many years, fully discussed. 

 In 1830 Bessel writes : " What admirable results are yielded by careful labor 

 is now again seen in the conformableness of that unknown disturbance which 

 you call resistance. Of the existence of such disturbance there can ]>e no doubt, 

 nor could there be long ago, but that it is a real resistance becomes more proble- 

 matical to me the more I reflect uj^on it." 



Encke continued assiduously to observe this comet, wdiich he always called 

 the comet of Pons, though his own naiiie was, with perfect justice, commonly 

 applied to it. Before each of its returns he made known its ephemeris in order 

 to facilitate observation, and as the shortening of the period of revolution con- 

 stantly recurred, in which fact he saw a confirmation of the above hypothesis, he 

 developed, in 1831, his theory of the movement of heavenly bodies in the resisting 

 medium. For the constants introduced, the values admitted of determination 

 from foregoing observations. 



Halley's comet, which re-appeared in 1835 after a period of 76 j^ears, occupied 

 very exactly the same positions which Rosenberger had previously calculated 

 from the earlier observations. By this body, therefore, the hypothesis of the 

 resisting medium was not confirmed, though it was by no means decidedly con- 

 tradicted, because neither the earlier measurements nor the masses of the planets 

 relied upon as a ground of calculation could be regarded as altogether certain ; 

 perhaps, also, this comet might have a denser mass, and the effects of resistance 

 be on that account not so conspicuous. 



The comet discovered still later by Faye seemed at last to remove the doubt 

 previously existing. The period of this body, 7^ years, was, according to Hol- 

 ler's computation, shortened at each revolution by about 17 hours, and Encke 

 showed {Berliner AsirononiiscJics Jahrhnchfilr 1864) that this acceleration could, 

 with very close approximation, be explained by the resistance Avhich the comet 

 of shorter circuit had undergone. In the meantime Mciller communicated the 

 results of a more rigorous calculation, [Astronomisclie Nachriclden^ vol. 64, p. 

 145,) in which were considered those quantities of the second order in the 

 co-ordinates of the disturbances which arise from the changes sustained by the 

 elements through the addition of new fundamental places at the more recent 

 retm-ns. By this it was found possible to bring all the three phenomena into 

 harmony -without the assumption of the resisting medium. The errors remaining 

 over were, through this procedure, it is true, considerably greater than after the 

 first calculation, but Muller entertained the hope of being able, by a renewed 

 and stricter calculation, to attain a still closer conformity. Whether, therefore, 

 Faye's comet does or does not confirm the hypothesis of the resisting medium, 



