ASTRONOMY. 119 



family of meteors, a shower from nearly the same point might continue 

 for a considerable time, giving rise to stationary radiants to which Mr. 

 BenniDg has called attention. 



It may not be out of j)lace here to point out the value of physical 

 observations of cometary phenomena — accurate observations of jets, 

 tails, brightness, etc. — which may furnish data for testing any theories 

 of their origin and constitution that may be put forward. 



Enc'ke's comet. — The progress of investigations upon Encke's comet 

 may be briefly stated thus : The comet which has now been observed at 

 twenty-four apparitions since its first discovery in 1786 " was shown 

 by Encke to be subject to a remarkable decrease in the length of its 

 period, a decrease which could not be accounted for by the attract- 

 ive force of the sun and planets. Encke surmised that this was pro- 

 duced by the effects of a res<isting medium. His calculations, which 

 extended up to 1848, were continued by von Asten, who in a great 

 measure confirmed Encke's couclusions, but found the curious anomaly 

 that between the apparitions of 1805 and 1871, the acceleration of the 

 mean motion which had been exhibited until the former of these years 

 ceased to appear. Since the death of von Asten the work has been con- 

 tinued by Dr. Backlund, who has succeeded in showing that the appar- 

 ent anomaly in question was due to an error in the formulae of pertur- 

 bations employed, and vanished when this was corrected. He was led 

 however to the remarkable and interesting result that the acceleration 

 of the mean motion of the comet is subject to a progressive diminution, 

 and amounted between 1871 and 1885 to scarcely one-half of what it 

 was between 1819 and 18G5." It was reduced from 0".104 to 0".0G2. 

 It seems very probable that about the year 1868 the acceleration under- 

 went a change, due no doubt to some unknown modification in the 

 physical condition of the comet. 



Dr. Backlund has recently resumed his labors, which were inter- 

 rupted by illness, aud the first memoir, relating to the return in 1885, 

 has just been printed; the second, treating of the comet's motion since 

 1865, will soon be presented to the St. Petersburg Academy of Sciences; 

 while the third, which is in preparation, will comprise the x)eriod 1819- 

 1868. For these researches the author has been awarded the Lalande 

 prize of the Paris Academy. 



Comet Tempel-Swift. — Bossert has given in the Bulletin astronomique 

 an elaborate discussion of the orbit of the comet discovered by Tempel 

 in<1869, but not recognized as periodic till its rediscovery by Swift in 

 1880. The period is about five aud one-half years, but the comet es- 

 caped notice in 1875 and again in 1886. 



Comet 1873 VII.— !Vr. Schulhof has published (Bull, astron., 3 : 125 et 

 seq.) a discussion of the orbit of this comet, and has gone into the ques- 

 tion of its possible identity with 1818 I and 1457 I (the observations of 

 which byToscanelli haverecently been discussed by Professor Oeloria). 



