iSToVEMBER 1, 1887.] 



♦ KNOW^LKDGE ♦ 



15 



However, we were all mistaken. The comet of 1882 

 retreated on such a coui'se, and with such variation of 

 velocity as to show that its real period must be measured 

 not by months, as bad been supposed, nor even by years, 

 but by centuries. Probably it will not return till 600 

 or 700 years have passed. Had this not been proved, we 

 miglit have been not a little perplexed by the return of 

 apparently the sume comet in this present year. A comet 

 was discovered in the south early in January, whose course, 

 dealt with by Professor Kruger, one of the most zealous of 

 our comet calculators, is found to be partially identical with 

 that of the four remarkable comets w-e have been con- 

 sidering. Astronomers have not been moved by this new 

 visitant on the well-worn ti-ack, as we were by tlie arrival 

 of the comet of 1882, or as we should have been if either 

 the comet of 1882 had never been seen, or its path had not 

 been shown to be so wide ranging. Whatever the comet of 

 the present year may be it was not the comet of 1882 

 returned. No one even supposes that it was the comet of 

 1880, or 1843, or 1668. Nevertheless, rightly apprehended, 

 the appearance of a comet travelling on appreciably the 

 same track as those four other comets is of extreme 

 interest, and indeed practically decisive as to the inter- 

 pretation we must place on these repeated coincidences. 



Observe, we are absolutely certain that the five comets 

 are associated together in some way ; but we are as abso- 

 lutely certain that thej- are not one and the same comet 

 which had travelled along the same track and returned after 

 a certain number of circuits. We need not trouble ourselves 

 with the q>ie.stion whether two or more of the comets may 

 not have been in reality one and the same body at different 

 returns. It sufKces that they all five were not one ; since 

 we deduce precisely the same conclusion whether we regard 

 the five as in reality but four or three or two. But it may 

 be mentioned in passing as appearing altogether more 

 probvble, when all the evidence is considered, that there 

 were no fewer than five distinct comets, all travelling on 

 what was practically the self-same track when in the neigh- 

 bourhood of the sun. 



There can be but one interpretation of this remarkable 

 fact — a fact really proved, be it noticed (as I and others have 

 maintained since the retreat of the comet of 1882), inde- 

 pendently of the evidence supplied b}' the great southern 

 comet of the present year. These comets must all originally 

 have been one comet, though now they are distinct bodies. 

 For there is no reasonable way (indeed, no possible way) of 

 imagining the separate formation of two or more comets at 

 different times, which should thereafter travel in the same 

 path. 



No theory of the origin of comets ever suggested, none 

 even which can be imagined, could account for such a 

 peculiarity. Whereas, on the other hand, we have direct 

 evidence showing how a comet, originally single, may be 

 transformed into two or more comets travelling on the same, 

 or nearly the same, track. 



The comet called Biela's, which had circuited as a single 

 comet up to the year 18-46 (during a period of unknown 

 duration in the past— probably during millions of years), 

 divided them into two, and has since broken up into so 

 many parts that each cometic fragment is separately undis- 

 cernible. The two comets into which Biela's divided, in 

 1846, were watched long enough to show that had their 

 separate existence continued (visibly) they would have been 

 found, in the fulness of time, travelling at distances very 

 far apart, though on nearly the same orbit. The distance 

 between them, which in 1846 had increased only to about a 

 quarter of a million of miles, had in 1852 increased to five 

 times that space. 



Probably a few thousands of years would have sufliced to 



set these comets so far apart (owing to some slight difference 

 of velocity, initiated at the moment of their separation) that 

 when one would have been at its neare.st to the sun the 

 other would have been at its farthest from him. If we 

 could now discern the separate fragments of the comet, we 

 should doubtless recognise a process in progress by which, 

 in the course of many centui-ies, the separate cometic bodies 

 will be di.sseminated all round the common orbit. We 

 know, further, that already such a process has been at work 

 on portions removed from the comet many centuries ago, 

 for as our earth passes through the track of this comet she 

 encounters millions of meteoric bodies which are travelling 

 in the comet's orbit, and once formed part of the substance 

 of a comet doubtle.ss much more distinguished in appearance 

 than Biela's. 



There can be little doubt that this is the true explanation 

 of the origin of that family of comets, five of whase members 

 returned to the neighbourhood of the sun (possibly their 

 parent) in the years 1668, 1843, 1880, 1882, and 1887.* 



But it is not merely as thus explaining what had been a 

 most perplexing problem that I have dealt with the evidence 

 supplied by the practical identity of these five comets' orbits. 

 When once we recognise that this, and this only, can be the 

 explanation of the associated group of five comets, we per- 

 ceive that very interesting and important light has been 

 thrown on the subject of comets generally. To begin with : 

 what an amazing comet that must have been from which 

 these five, and we know not how many more, were formed 

 by disaggregative proces.ses — probably by the divellent action 

 of repulsive forces exerted by the sun I Those who remember 

 the comets of 1843 and 1882 as they appeared when at their 

 full splendour will be able to imagine how noble an appear- 

 ance a comet would present which was formed of these com- 

 bined together in one. But the comet of 1880 was described 

 by all who saw it in the southern hemisphere as most 

 remarkable in appearance, despite the faintness of its head. 

 The great southern comet of the present year was a striking 

 object in the skies, though it showed the same weakness 

 about the head. That of 1668 was probably as remai-kable 

 in appearance as even the comet of 1882. A comet formed 

 by combining all these together would cert^nnly surpass in 

 magnificence all the comets ever observed by astronomers. 



And then, \yhat enormous periods of time must have 

 been required to distrilnite the fragments of a single comet 

 so widely that one would be found returning to its perihelion 

 more than two centuries after another I When I spoke of 

 one member of the Biela group being in aphelion when 

 another would be in perihelion, I was speaking of a difference 

 of only three and one-third years in time ; and even that 

 would require thousands of years. But the scattered 

 cometic bodies which returned to the sun's neighbourhood 

 in 1668 and 1887 speak probably of millions of years which 

 have passed since first this comet was formed. It would be 

 a matter of curious inquiry to determine what may have 

 been the condition of our sun, what even his volume, at that 

 remote epoch in history. 



* It may be interesting to compare the orbital elements of the 

 five comets above dealt with. They may be presented as follows ; 

 but it should be noticed that the determinations must be regarded 

 as rough In the case of Comets I. and V., as the observations were 

 insufficient for exact determination of the elements : — 



