334 



ON THE THEORY OF ENCKE'S COMET, 



must have been one and the same body. He now proceeded to 

 carry his calculations forward, and again taking into consideration 

 the attraction of the planets, he predicted its re-appearance and 

 perihelion passage on the 24th of May, 1822. As it was not 

 seen in Europe on that occasion, this opportunity of verifying the 

 correctness of Encke's calculations must probably have been lost 

 but for the Parramatta Observatory, when it was observed by 

 Rumker, on the 2nd of June, and on subsequent occasions. 

 From the observations then made, it would appear that the 

 comet passed its Perihelion on May 23rd, at 23h. 7m. that is to 

 say, just 53 minutes before the beginning of May 24th. 



With the improved data now before him, Encke was enabled 

 to correct his elements, and predict with still greater accuracy, 

 the Comet's re-appearance at intervals of about 1210 days. 

 Since then, its name has been down as one of this World's 

 regular visitors, and it never forgets to leave its card at the 

 appointed time. 



I have endeavoured to give some idea of the form and 

 dimensions of this comet's orbit, by a diagram, constructed on 

 scale from the elements computed for the year 1865 ; and 

 I will just compare a few of Encke's elements for 1832 

 with the corresponding ones for the present year, merely observ- 

 ing that, for the sake of simplicity in numbers, the Earth's 

 mean distance from the Sun is taken for unity, and that the 

 quantities are set down sufficiently approximate for our purpose. 

 They are as follows : 



These results are not intended to bear a very close investi- 

 gation, but they serve to illustrate, in a general manner, the fact 

 that the axis, and, therefore also, the periodic times of the 

 elliptic orbit of Encke's Comet, are continually diminishing ; and 

 Encke attributes this phenomenon to " the existence of an 

 SBtherial medium which pervades all space." 



