July, 1904.] 



KXCnVLT-DGK c'v SCIENTIFIC NEWS. 



147 



tlien, was rarely avoided. The glass, H, is united to 

 the cell. A. liy the ela.stic ring", D, the whole beinii- held 

 tosjcther and protected by the outer casing- K. They 

 have now introduced a level made on exactly tlie same 

 lines but with the tlan§:e for attachment to the camera 

 at the upper part instead of the lower part of the body, 

 so that the level, when fixed, presents a surface almost 

 flush with the woodwork, as shown in the second 



figure. These will be useful in cases where the others 

 were impossible. There is one matter concerning- the 

 fixing^ of levels th;it is not always attended to, namely, 

 that their sole use is to show when the plate is vertical. 

 They must, therefore, be fixed to that part of the 

 camera that carries the plate. If the back of the 

 camera swings, the level must be attached to the back; 

 it would be useless on the base-board, except only when 

 the swing-back was not in use, and it was certain that 

 the plate was perpendicular to the base-board. 



Notes on the R.et\jrn. of 

 Ei\cke's Comet. 



This comet will be favourably visible this year during- 

 the autumnal montlis. I'erihelion will be reached on 

 1905, January 4, but it will probably be picked up in 

 some of the large telescopes employed in cometary work 

 in .August and September next. Its nearest ap- 

 proach to the earth will occur in, the third w-eek of 

 November, when its distance will be about 35 millions 

 of miles. 



On October 4 the position of the comet will be about 

 midway between the stars /? -Andromedae and o Trian- 

 guli. Moving- westwards it will be found 5° N.E. of /i 

 I'egasi on November i. Its appiirent motion will in- 

 crease, and in November and December be directed to 

 S.W. Early in December the comet will be close to 

 the brilliant star a -Aquilse (.Altair). At about this period 

 it will be easily visible in smaJl telescopes, and may 

 possibly be within reach of the naked eye. 



This will form the 36th return of the comet (and the 

 29th observed apparition) since it w;is first discovered 

 by Mechain in 1786. .At intervals of 33 years (including- 

 10 p>eriods) the jserihelion is reached at nearly the same 

 time of the year as before, and in 1904 the favourable 

 prasentations of 1805, 1838, and 1871 will be repeated. 

 In the three years last mentioned the comet was visible 

 to the naked eye. Drawings of its physical aspect in 

 1871 appeared in Monthly A'o/ices, XXXII, pp. 26 and 

 217, and Astronomical Register, X., p. 13. 



The comet has been seen at every return since 1819. 

 The following- is a list of its perihelion passages, ob- 



servers, and perio^ls. The average duratioii of a r<'\(ilu- 

 tion from 36 returns appears lO' be 1,206}, days. The 

 nature of the orbit was deterniincd from the observa- 

 tions in 1819. 



R^eturrvs of Encke's Comet. 



* Discovered near the star ^Aquarii. Observed on two nights (Jan, t7 and 19) 



only. 

 i Independently discovered by Pons, Bouvard. and Huth, on Oct. 20, 1805. It 



had a tail 3 ' lon^j, 

 ; Distinctly visible to naked eye at end of November. 

 •"Boguslawski picked np the comet on July 30. 

 S visible to the naited eye at Bristol in October. 

 Computed date of perihelion passage. 



There were se\en unobser\ed return,' 

 between 1786 and 1819, coinpuled by 

 occurred as under : — 



1789 



1792 



1799 



1802 



1809 



1812 



tO' perihelion 

 ■".n<-ke to have 



13 



May 19. 



September 4. 



.April II. 



.August 2. 



March 12. 



June 26. 



October 13. 

 Encke found the period of the comet in 1789 to have 

 been 1,212 days 19 hours. .Seagrave, in L'opiihir 

 Astronomy, 1904, February, gives the present period as 

 1,206 days 20^ hours, and says the time has decreased 

 \erv nearly six days. This decrease would amount to 

 about four hours per revolution. He gives an ephemeris 

 for the coming return, but it has not been corrected for 

 perturbations, and will, therefore, not accurately repre- 

 sent the path of the comet. No doubt Mr. Crommellii 

 V ill supply a reliable ephemeris in later months. The 

 comet, having: a very interesting history as the first one 

 known belonging- to the Jovian family, as having- 

 suggested the idea of a resisting medium, and as having 

 a smaller orbit than that of any other comet, should be 

 followed by everyo^ne possessing a telescope next 

 autumn as it travels from .Andrometia slowly through 

 Pegasus and .\quila southwards to Capricornus. 



W. E. DENNINti. 



