146 



♦ KNOWLEDGE ♦ 



[Feb. 20, 1885. 



by rapid changes of position and of form attracts the 

 admiring attention of all men. 



I propose to make tLe leturn of Eacke's comet — the 

 comet of shoitest period known — the text for some 

 remarks about the theories of comets more or less in vogue 

 among astronomers, and especially those theories ^\hich 

 relate to the origin of these bodies, or at least their intro- 

 duction iuto our solar sj-stem. 



It may be remembered thnt at the last meeting of the 

 American Association for the Advancement of Science, 

 Professor Young touched on what he called the receivtd 

 theory of the origin of comets, and what he admitted was 

 a valid objection of mine against that theory. 



What Professor Young calls the received theory is, I 

 take it, neither a theory nor generally received — it only 

 comes in company with a received theory. Schiapaielli, 

 the ingenious chief of the Observatory of Milan, threw 

 out, in 18GG, tie idea that the bodies which produce the 

 star-showera of Aug. 10 and 11 (the Tears of St. Lawrence 

 they were fancifully called in old time.'-) are attendants on 

 the Comet of 1802. When he had .shown, which was an 

 easy thing to do, that the apparent movements of those 

 falling stars on the stellar heavens accord with the theory 

 that they are mo'.ing in parallel tracks, touching (at any 

 rate) the orbit of that comet at the place where it crosses 

 the earth's track (a point passed by the earth on or about 

 Aug. 10 11), it was felt that he had done something in 

 support of his hypothesis. But when Professor Adams 

 had shown, which was by no means an ea^y thing to Jo, 

 that the bcdie.^ producing the display of November meteors 

 travel in the very track of the coiret of 1866 (known as 

 Tempels), astronomers saw that .Schiaparelii's ca>e was 

 proved. It passed thenceforward from the condition of a 

 mere speculation to that of a received theory. 



This is the received tlieory about comets and meteors 

 which eveiy a^tror.omer who can understand the evidence 

 accepts without hesitation — Meteors are bodies ir/iich iravtl 

 on the tracks of comets. More than that has not yet been 

 shown, and more thnn that is certainly not received by 

 abtronomers as a body. 



But SLliiapirelli suggested more. He threw out a 

 speculation couotrning the origin of comets based on his 

 established theory as to the connection between comets and 

 meteors. This speculation would explain, if e.stablished, 

 the way iu which meteors travel on the tracks of comets. 

 It ran as follows : — Amidst the interstellar depths are 

 flights and clouds of meteoiic bodies, which from time to 

 time are drawn out of those depths by the attractive 

 influence of the sun. Weie the sun alone in the universe 

 they would be drawn towards him, sweep around him in 

 greater or less proximity to his surface according to the 

 course on which they chanced to be drawn, and so pass out 

 again to the de pths from which they came. But as the sun 

 has a family of attendant planets and some of these are 

 somewhat stalwart fellows, many of the meteor-flights 

 drawn suowaids are so acted upon by the disturbing 

 influence of Jupiter, or Saturn, or Uranus, or Neptuue, or 

 mayhap tome outer and still unknown members of the 

 family of giant planets, that they are deflected from their 

 course and thenceforwaid travel on a closed path— elliptic, 

 of course — around the sun. The place where the deflection 

 took place remains thenceforward a part of the comet's 

 path, which therefore seems to a.ssociiite itself with the 

 path of the deflecting planet, in such sort that, though the 

 sun is the chief ruler of the comet, the planet which intro- 

 duced it into the solar system retains a sort of secondary 

 iufluence over the comet's movements. Should the comet 

 chance to revisit the scene of deflection when the planet is 

 passing the same place, potent disturbing influences may be 



exerted on the comet, which may even senel it wandering 

 yet once more iuto the domain of interstellar space whence, 

 according to this e-peculation, it was drawn. 



Now I pointed out more than eleven years bince that this 

 part of Schiajjarelli's imaginings is entirc-ly without feiunda- 

 tion in known facts. We may guess that the interstellar 

 depths are a sort of breeding-place for comets and meteor 

 systems, — though why they shuuld be so not even Schia- 

 parelli has ventured a suggestion. We may imagine that 

 in the iuterstellar depths there still remain the scattered 

 fragments of such materials as, when gathered in, had 

 formed our solar system with all its worlds ; though why 

 any such fragments should remain there, instead of respond- 

 ing to the influences which brought their fellows to the 

 neighbourhood of our system, would remain still unex- 

 plained. Only five or six millions of years would be 

 required to draw in matter to the sun from half the 

 distance separating liim from his nearest neighbour among 

 the stars, and our earth's crust tells us of tens of millions 

 of years already passed since the sun had gathered in his 

 mass so as to shine as a sun upon the earth. But we may 

 concede for a moment the possibility of the wandering 

 meteor flights of interstellar space imagined by Schia- 

 parelli. How are we thereby helped to an interpretation 

 of the origin of meteor systems now in attendance on the 

 sunl Not a whit, seeing that we have only succeeded in 

 replacing one difficulty by another still greater. 



If we suppose the meteor streams to have come into the 

 interstellar depths from beyond, that is from the domain of 

 some star, we have removed our difficulty only a step, and 

 not a step bringing us any nearer a solution. That other 

 star is a sun like ours, aiiel if a meteor system came from it 

 to u», we have the same difficulty in understanding how 

 .the meteor .system came to be in the neighbourhood of that 

 sun as we have in understanding it as belonging for awhile 

 to our own sun. One may compare this attempt at a solu- 

 tion of a really seiioua difliculty to Sir William Thomson's 

 well known attempt to explain the origin of life in our 

 planet. This he did by surmisirg that millions of yeara 

 ago anotber planet was the abode of life, that that planet 

 came unfortunately into collision with another or burst, 

 and that some of tLe fragments after flitting from sun to 

 sun a few times chanced in their passage tijiough our solar 

 system to encounter our earth, where, falling on good soil, 

 tlie germs brought forth abundant life : development did 

 all tbe rest. That planet may have inherited the germs of 

 life from another which had burst or collided a few millions 

 of years before, and so on : we may in fact adopt a theory 

 of planetary life akin to the theory of individual life — 

 omne vivian ex ova — and say every live j'Uinet received its 

 life from a planet which was full of life, but burst up. 

 Schiaparelii's cometic speculation asserts in like manner 

 that every meteoiic system or comet now associated with 

 the sun came here athwart the star depths from another 

 sun with which, mdlious of years ago, it was iu like manner 

 associated. 



All thi.s however, is not scientific theorisiog but specula- 

 tion. There is no evidence in support of Schiaparelii's 

 supposition. If it were established we should be as far off 

 as ever from knowing the real origin of comets. But 

 lastly, there happens to be demonstrative evidence against 

 the theory : — 



Take the November meteors, whose path crosses that of 

 Uranus so closely as to show that Uranus was the planet 

 which introduced this particular meteor system, if the 

 theory has in it any truth at all. The November meteors, 

 and of course Tempel's comet, in whose track they travel, 

 cross the path ef Uranus now with a velocity of li miles 

 per second. A meteor coming to our sun from interstellax 



