ASTRONOMY. 411 



servations, and as such, the errors exhibited by a parabolic orbit had 

 not so great a preponderance in his opinion as to enforce such necessity. 

 He concluded that the subject still required examination by a combina- 

 tion of all the observatious, and especially if the originals of those at 

 Marseilles could be found 



Lately the orbit of the comet of 1771 has formed the subject of two 

 memoirs, the first by Mr. W. Beebe, in the Transactions of the Connec- 

 ticut Academy of Arts and Sciences, Vol. V; the second by Dr. H. 

 Kreutz, published in the Proceedings of the Vienna Academy. Mr. 

 Beebe gives also a hyperbolic orbit, accompanied by the most probable 

 parabola for comparison. Dr. Kreutz is led to a parabolic orbit for the 

 closest representation of the comet's path, and though the original ob- 

 servations at Marseilles had again been sought for unsuccessfully he 

 does not think their recovery would affect the conclusion at which he 

 has arrived. The elements of the definitive parabola are as follows: 



Perihelion passage, 1771, April 19.14144 M. T. at Paris. 



O I II 



Longitude of perihelion 104 1 21.7 1 



Longitude of ascending node 27 53 11.7 > M. Eq. 1771.0. 



Inclination 11 15 53.1 ) 



Logarithm of perihelion distance, 9.955127. 



— (Nature.) 



Theory of Encke's Comet. — Nature, December 13, 1883, contains an 

 abstract of the recent results of Dr. Backlund, whose paper has not yet 

 reached this country. 



Dr. v. Asten, in August, 1878, showed that an acceleration of 0".104 

 in the mean motion would satisfy all the successive revolutions of the 

 comet between 1819 and 1858. The probable error of a normal posi- 

 tion was 9" in each co-ordinate. The appearance of 1871 presented a 

 striking exception to others, in that the acceleration had a quite differ- 

 ent value, and Dr. v. Asten was led to the belief that some one of the 

 asteroids had produced the retardation in question. In 1881 a similar 

 retardation was indicated, and Dr. Backlund, employing v. Asten's 

 methods, was able to fix on the time and place where the retardation 

 occurred, which was again in the region of the small planets. 



A complete revision of the formula} by Dr. Backlund has led to the 

 detection of a material error in the computations, which, being rectified, 

 enables the whole of the observations of all the appearances, 1808-1881, 

 to be well represented. The probable error of each co-ordinate of a nor- 

 mal position, 18G8-1881, is now not above 4".l. By introducing Sclim's 

 reduction of Bessel's mass of Jupiter, this error is finally reduced to 2 // .S. 

 and the resulting acceleration for each revolution, 1868-1881, is // .054. 

 The precision with which the normal places are satisfied is truly re- 

 markable. 



The theory for the years 1819-1868 will next be examined, and the 



