124 



KNOW^LEDGE ♦ 



[Feb. 1, 1886, 



have, apart from the evidence already decisive, obtained 

 by Olmsted, Schiaparelli, and Adams, the following 

 points : — A comet appears, whole, in 182G : returns, still 

 whole, in 1832 ; returns, but is not seen, being unfavour- 

 ably situated, in 1839; returns, still whole, in 1845; 

 but early in 1846 divides into two ; returns, still double, 

 in 1852; may or may not have returned as a double, 

 or perhaps multiple, comet in 1859, but would have been 

 invisible, being unfavourably near the sun on the skies ; 

 was not seen in 1866 in any form, or as a comet in 1872, 

 1879, or 1885 ; but in each of these three last-named 

 years, when the earth passed through the comet's track, 

 evidence was given by the appearance of falling stars 

 that along that track meteors were travelling, in flights 

 of many millinns, far behind the parent comet It is not 

 merely the agreement as to the year and the day of the 

 year, which enables us to associate the meteors with 

 Biela's comet ; they were travelling in that direction, 

 and in that direction alone, out of millions of ]iossible 

 directions, which corresponded with the motion of 

 meteors travelling along the track of the comet. We 

 have to add to this that evidence already regarded as 

 overwhelmingly convincing had shown other comets to 

 be similarly followed by meteoric trains ; that, in fact, 

 so far as can be judged, it is in the nature of all comets 

 to have such trains ; and that all meteor-streams are 

 thus associated with comets (either whole or long since 

 dissipated). No doubt, then, as to the connection between 

 the Andromedes of Nov. 27 and 28 and Biela's comet can 

 be reasonably entertained. 



The occasion seems a good one for touching on the re- 

 markable nature of the problem which meteors and 

 comets, thus understood, present. The facts collected 

 together are so significant that we might fairly expect 

 them to suggest the true theory of comets and meteors 

 as clearly as — to take an appropriate illustration — the 

 meteors seen during some groat display indicate by their 

 intersecting paths the " radiant point" of the system. 

 If we examine carefully what has been proved, and sift 

 c '.refully the theories which have seemed to be established, 

 accepting such parts of theories as really have been esta- 

 blished and suspending judgment as to what still remains 

 doubtful, we shall find a singularly suggestive body of 

 evidence in favour of a general theory which, viewed 

 apart from such collected evidence, would appear sur- 

 prising — nay, even startling — in character. 

 _ We require at st a-ting only the following two assump- 

 tions — which may be regarded as altogether reasonable: — 

 First, that what has been proved about comets generally 

 may be regarded as true about meteor - systems ; and, 

 secondly, that what has been proved aboat meteors 

 may be regarded as throwing light upon the nature of 

 comets. 



Now, it has been shown by the researches of Stanislas 

 Meunier, Tschermak, and others that among the me- 

 teorites which reach our earth are bodies, ranging in 

 structure from the asiderites (with very little iron) to 

 the holosiderites (almost wholly iron), which arc prac- 

 tically identical with volcanic products, ranging from 

 the ultrabasic to the iron masses found at Ovifak, in 

 Greenland — these last being so like the holosiderito 

 meteors that they were long regarded as indubitably 

 meteoric. It has further been shown that, on a careful 

 investigation of all the evidence in regard to these 

 meteorites and a discussion of the probabilities of their 

 encounter with the earth on various theories as to their 

 origin, it becomes almost a certainty that most of them 

 not only are, as their structure shows, of volcanic 

 origin, but were ejected originally from volcanoes 



on this very earth on which we live. Mr. Ball, As- 

 tronomer Royal for Ireland, has discussed the evidence 

 on this point very soundly, though, as will be seen, 

 he does not carry the reasoning quite as far as it 

 may be fairly taken. Every mass, small or great, ejected 

 from the earth by volcanic action with a velocity ex- 

 ceeding seven miles per second would pass away on a 

 path thenceforth carrying it round the sun, but crossing 

 the earth's track at or near the point where the ejection 

 took place. But for perturbations, the crossing- place 

 would be always at that very point ; but perturbations 

 would shift the place of crossing. Thus every meteoric 

 mass so ejected would be exposed to the risk of recapture 

 by the parent earth. On the contrary, the chance that a 

 body ejected from any other planet whatsoever would 

 fall on the earth would be almost infiuitesimally small. 

 Thus, while a comparatively small amormt of terrestrial 

 ejections would serve to account for the considerable 

 number of meteorites cajitured within historic times by 

 the earth, we should have to imagine an almost infinitely 

 large number of ejections from any other planet to 

 account for so many captured meteorites (since for each 

 one captured by the earth millions of millions must have 

 been ejected by that other planet if all of them cxme 

 from it). 



So far we are on tolerably sure ground. Yet already 

 we have deduced a very surprising result. Nothing in 

 the present activity of volcanoes would suggest an 

 eruptive power capable of ejecting matter with a 

 velocity of seven miles per second, or more. Yet, re- 

 membering the evidence we have that the earth was 

 once in a 5?(a.«-.sunlike state, we can well believe that in 

 her snnlike youth she may from time to time have so 

 concentrated her volcanic energies as to expel matter 

 even with the tremendous velocities indicated by Bair.s, 

 or rather Tschermak's theory. Possibly, turning pre- 

 sently to a body which actually is in a sunlike state, we 

 may find evidence showing that this is the way with 

 sunlike orbs. 



Here Tschermak and Ball ]5ause. Content with 

 showing that probably many millions of meteoric bodies 

 were ejected from onr own planet when she was a small 

 sun, they overlook the inference that what she could do 

 the other planets could presumably do also. Mr. Ball, 

 indeed, while admitting the possibility of this in the case 

 of planets no larger than the earth, expresses the opinion 

 that the giant planets could not expel volcanic products 

 with the much greater velocities necessary to overcome 

 their own much greater attractions. He overlooks, 

 apparently, the circumstance that if much greater power 

 would be required much greater power existed. The 

 volcanic energies of a planet result from the planet's 

 internal heat, and the internal heat is now recognised as 

 a direct i^roduct of the planet's gravity. It would seem 

 highly probable, then, if not certain, on a jjriori groiinds 

 alone, that the giant planets would be able, like the earth, 

 to expel millions of millions of meteoric flights from their 

 interior during their sunlike youth — possibly not yet 

 altogether past. 



Turning next to direct evidence, we find that among 

 the mu.ltitudinous comets belonging to our solar system 

 can be recognised certain families dependent on the giant 

 planets in a somewhat peculiar manner. Long before 

 the significance of the feature had been noticed, I 

 wrote of "The Comet-families of the Giant Planets" as 

 a phenomenon needing to be inquired into. The comets 

 thus classed into families travel around the sun as their 

 ruling centre, but with paths passing near severally to 

 the track of one or other of the giant planets — Jupiter, 



