September 30, 1897] 



NA JURE 



519 



the same as when last seen in the Strassburg refractor of 

 20 inches aperture, on November 24, 1881. In view of 

 the doubts prevailing as to its exact period, it is question- 

 able whether it will be redetected in 1899. At its following 

 return in 1907 the comet ought to be conspicuously 

 visible for some months, as it will be comparatively near 

 to the earth, and the favourable return of 1881 will be 

 repeated, three periodic revolutions (i = 8'687 years) of 

 the comet being equal to twenty-six years. At its last 

 return in 1890 May, the position of the comet was such 

 that it never approached within 150 millions of miles of 

 the earth, and thus it entirely escaped observation. 



Tcinpel, 1866 I. — This comet is due in the spring, but 

 it will be separated from the earth by a much wider 

 inter\'al than in 1866. Its favourable returns are those 

 when perihelion occurs in about November or December. 

 If the comet has the same periodic time as its associated 

 meteor shower (the Leonids), then it is well visible at one 

 return only out of every three, and its next favourable 

 apparition will occur in 1965-6. 



Barnard, 1892 II. — The period of this faint comet (dis- 

 covered by photography) is somewhat doubtful. Hind 

 gave 6*64 years, Krueger 6'309, Porter 618, and Coniel 

 6"52. The comet will probably return to perihelion in 

 the spring of 1899, when it will, however, be invisible, 

 being obliterated in the sun's rays. When the comet 

 reaches its perihelion in the autumn it can be well 

 observed. 



Holmes. — This comet returns to perihelion in April 

 1899 according to Zwiers, the probable error being 072 

 day. His orbit was derived from 600 observations. Dr. 

 Kohlschiitter has also given a definitive orbit for this 

 comet, his periodic time for it being 2520829 days, while 

 Zwiers gives 252 1*2 days. The latter, after allowing for 

 perturbations by Jupiter and Saturn, gives .-Xpril 27*97 as 

 the date of perihelion, and his ephemeris for 1898, as 

 abridged, is as follows : — 



Greenwich Noon, 1898. 



R.A. Dec. 



The comet must therefore be looked for at southern 

 observatories, in 1898 ; it will be well placed for northern 

 observers in 1899. 



Tuttle. — This comet, first discovered by Mechain in 

 1790, and re-observed by Tuttle in 1858, was also seen in 

 1 87 1 and 1885. It belongs to Saturn's comet family, its 

 orbit, at aphelion, being just outside that of Saturn. The 

 conditions are not favourable for seeing the comet at the 

 ensuing return in the summer of 1899, as its longitude of 

 perihelion is 116^, and perihelion distance ro3, or about 

 3 millions of miles outside the earth's orbit. The comet 

 is therefore best visible when it comes to perihelion at the 

 end of January, the earth and comet being then on the 

 same side of the sun, and only a few millions of miles 

 distant from each other. 



Tempel, 1873 !• — Returns early in July 1899, and will 

 he observed under pretty good conditions, the earth and 

 comet being on the same side of the sun. The comet 

 will be visible during the whole night, and is likely to be 

 as successfully observed as in 1873, when it was first 

 discovered, for five of its periods of 5 '20 years are equal to 

 twenty-six years. 



No doubt some of our largest telescopes will be em- 

 ployed in the redetection of these objects as they sever- 

 ally return to perihelion. In recent years the diligence of 

 observers has been the means of increasing the number 

 of periodical comets at the average rate of one per year, 



NO. I457» VOL. 56] 



and this increase will probably be niaintained, if not 

 exceeded, in the future. 



It might be supposed that comets returning to the sun 

 at comparatively short intervals would soon be all de- 

 tected ; but when the circumstances are considered, it will 

 be seen that this state of things will be never realised. 

 The comets of short period are faint objects, and often 

 pass their perihelia under conditions which render them 

 totally invisible. Thus De Vico's of 1844 was computed 

 by Briinnow to have a period of 5*469 years, but it was 

 not seen again until 1894, though during the fifty years it 

 had returned unobserved on eight occasions. Pons's 

 comet of 18 19 was assigned a period of 5 '6 18 years by 

 Encke, but it was not seen at any of the six subsequent 

 returns. Winnecke, however, in 1858, at its seventh 

 return, picked it up accidentally. Mechain's comet of 

 1790, with a period of i3"8 years, must have returned in 

 1803, 18 1 7, 1830 and 1844, but it eluded re-observation 

 until Tuttle recovered it in January 1858. 



Most of the periodical comets at perihelion are outside 

 the earth's orbit, and hence it follows that they escape 

 observation unless the earth is on the same side of the 

 sun as the comet. As an instance of the favourable 

 presentation of a comet, that of 1894 I. may be alluded 

 to. Its perihelion is 14 millions of miles outside the 

 earth's orbit, and is in longitude 130^^', which it reached 

 on February 9. Now the earth was m longitude 140^ at 

 the same time, so the comet was nearly in opposition and 

 visible under the most favourable circumstances during 

 the whole night. 



Some comets, as Tempel-Swift's, are only perceptible at 

 alternate returns ; others are not well visible except at 

 intervals after two, three, or four returns. 



Another circumstance which will prevent our e.xhaust- 

 ing the discovery of these objects, is that the planet 

 Jupiter frequently introduces disturbances into their 

 motions, and possibly into the physical conformations of 

 the short period comets. He appears, also, capable of 

 effecting new captures, and thus bringing these bodies 

 into permanent membership of the solar system. The 

 Jovian family of comets is already a numerous one, and 

 is probably increasing, though some of the objects which 

 owe allegiance to Jupiter are in process of disruption and 

 gradual dispersion, and seem likely in the end to lose their 

 visible character, as compact bodies, to form meteoric 

 streams with the residue of their material. Biela's comet 

 has not been seen for forty-five years, though it ought to 

 have returned six times in that interval, and was one of 

 the earliest discovered, as well as one of the best known, 

 of the periodical comets. It will probably never be seen 

 again as a comet, though its associated meteors will be 

 displayed in November 1898, and in future years at 

 periods conforming with the time of revolution of the 

 parent comet. 



We must, however, not be too hasty in assuming the 

 collapse of known comets, for experience has taught us 

 that they may reappear when least expected. The 

 visible return of De Vico's comet of 1844, after being 

 unseen for fifty years, and of Pons's comet of 18 19, after 

 an invisibility of thirty-nine years, shows us what is 

 possible. Brorsen's comet, which escaped observation in 

 1884, 1890 and 1895, is supposed to have disappeared 

 like Bielas, but a small comet may be swept up at some 

 future time which will exhibit a similarity of elements to 

 that of the missing Brorsen sufficient to prove the actual 

 identity of the two objects. 



The distribution of the aphelia of periodical comets 

 near the orbits of the major planets is not the least 

 interesting feature connected with these bodies. But it 

 is perhaps a little remarkable that though Jupiter's family 

 has enormously increased in recent years, yet the other 

 groups have received very few, if any, additions, though 

 a large number of new comets have been discovered. 



W. F. Denning. 



