iO 



KNOWLEDGE. 



[February 1, 1896. 



Star ( • ). D.M. No. 1822. Zone + 0°. R.A.. 

 6h. 83ra. 4'2-Os. Decl., N. 9° 44-5'. Mag., 8'1. 



Star (••). D.M. No. 121.-.. Zone + 10 . R.A., 

 Gh. 85m. 8-8s. Decl, N. 10 28-0'. Mag., 8'8. 



Star (•/). D.M. No. 1844. Zone + J)". K.A., 

 6h. 35m. 40-53. Decl., N. 0". 340. Mag., 8-1. 



Star ( : : ). D.M. No. 1350. Zone + n. R.A., 

 6h. 85m. 57'4s. Decl., N. n° 50-7'. Mag., 8-3. 



The photograph was taken with the 20-incb retloctor on 

 February 13th, 1895, between sidereal time 4h. 20m. and 

 7h. 23m., with an exposure of the plate during three hours. 



References. 



N. G. C, No. 22G4 ; G. C, No. 1440 ; h 401 ; Lord 

 Rosse, Ohs. of Xeb. and CI. of Stnrs, p. 53 ; Prof. Barnard, 

 Astronomy and Astro-physics, Vol. XIII., pp. 178, 182. 



Sir J. Herschel and Lord Rosse suspected 15 Monocemtis 

 to be a nebulous star, or else involved in nebulosity : but 

 they could not detect any nebulosity in the region surround- 

 ing the star. 



Prof. Barnard took three photographs of the region 

 in .January and February, 1894, and upon them found 

 nebulosity covering a diameter of about three degrees .■ 

 but the photo-instrument which he used had only an 

 aperture of six inches and focal distance of thirty-one 

 inches. The scale of the photographs is, therefore, too 

 small for showing the structural details of the nebulosity, 

 which are partly shown on the print herewith presented, 

 and more clearly visible on the original negative. 



The star (15 Moiiocerolis) is involved in the centre of 

 the light-circle at the centre of the print ; and the rays of 

 light which are seen in the north, south, east, and west 

 directions, from the star, are not objective, but due to 

 instrumental causes. 



COMETS OF SHORT PERIOD. 



By W. E. Plummek, M.A., F.R.A.S., Dbeetor oi 

 Bidston ( ibserratory. 



WITHIN the last few years some of the comets 

 of short period have shown a tendency to 

 behave in an irregular and unexpected 

 manner. They no longer present the well- 

 regulated class of objects that they did in, 

 say, the middle of the century, when, less numerous, their 

 return could be counted upon with confidence, and their 

 appearance predicted with an approach to certainty. 

 While some have probably disappeared as distinct comets, 

 fresh discoveries have added others — sometimes, indeed, 

 suggesting reappearances of old friends — till the very 

 nomenclature has become confusing ; so that it seems 

 desirable to review the entire history, and endeavour to 

 straighten out some of the difficulties that the addition of 

 fresh members to the solar system has introduced. 

 Evidently and conveniently, the entire family of periodic 

 comets, or rather of those whose aphelia fall near the orbit 

 of -Jupiter, can be divided into two classes : those which 

 have been observed at more than one apparition, and those 

 which have only been seen once. The first list can be 

 again subdivided with advantage, since it contams speci- 

 mens of two very different classes, namely, those which 

 remain with us after one or more revolutions, and those 

 which, after pursuing a regular course, have subsequently 

 been lost. The following table contains those, and those 

 only, whose reappearance can be counted upon with 

 tolerable certainty. The order of arrangement is that ol 

 the respective mean distances from the suu. 



The first fact that strikes one on glancing through the 

 table as a whole, and before entering into any details, is 

 the small number of regularly returning comets that 

 have remained permanent members of our system. The 

 facility with which comets can be lost will be further 

 accentuated when we enter upon the second list, and show 

 the comparatively large number that have been added 

 by recent discoveries and whose return is still doubtful. 

 These recent additions to our catalogues within the last 

 few years seem to suggest that the proportion of elliptical 

 to parabolic comets has greatly increased. This can, 

 in some measure, be attributed, possibly, to the discovery 

 of fainter objects, due to the employment of larger tele- 

 scopes in " sweeping." Not that this explanation is 

 altogether satisfactory, because it ought to follow that the 

 total number of cometary discoveries of all kinds is 

 strikingly greater than was the case thirty or forty years 

 ago, and this is not borne out by the facts. Another point, 

 not without its interest, is that the increase in the 

 aphelion distance has not kept pace with the increase in 

 the period. From Encke to Faye, this increase in period 

 implies by Kepler's law an increase of the mean distance 

 from 2-218 to 3-854, expressed (here as in all other places 

 where distances have to be mentioned) in terms of the 

 earth's mean distance from the sun. The aphelion 

 distances, however, do not exhibit the same tendency to 

 spread. The controlling influence of Jupiter is here 

 suggested. 



It will not be necessary to consider all the comets in the 

 list in detail. For instance, the history of Encke's comet 

 has been many times written, and it is quite unnecessary 

 to refer to the numerous interesting points connected with 

 its motion. It is true that the last chapters, and not the 

 least interesting, in its history have not been given with 

 complete fulness in some recent test-books ; and the care 

 that has been bestowed on the difficult problem of 

 separating the effects due to a so-called " resisting 

 medium " from others arising from the use of an erroneous 

 or questionable value of the mass of Mercury, has not yet 

 met with a sufficient notice in popular works. 



It is otherwise with the comet described as Tempel — or 

 Tempel 1873 — because the comet is very liable to be 

 confounded with another of short period which the same 

 astronomer first saw six years earlier in 1807. The first 

 return of this last-mentioned comet occm-red in 1873, and, 

 consequently, in that year ttro periodic comets, both 

 credited to Tempel, were under observation. This comet 

 of 1867 has now probably disappeared, and it will be 

 referred to again under the head of the more doubtful 

 visitants. Number two in our list was seen in 1873 and 

 again in 1878. In 1883, and in 1889, when, according to 

 its period, it must have returned to the sun, the chances 

 favourable for observation were very slight, and it passed 

 through perihelion without being seen. In 1894, however, 

 Mr. Finlay, of the Cape Observatory, was fortunate enough 

 to find it, and so carefully had the calculations been made 

 by M. Scuulhof, that, notwithstanding the long interval 



