10 



KNOWLEDGE • 



[Nov. 1, 1881. 



announcu the discovery of an o1)j<M;t whose appearance 

 or non-uppeai-anee wus to I'ouKrni or to (li.s])rove the New- 

 tonian theory. It wa.s actually di.scovered, however, witli- 

 iiut tflr.Koo|>ii- aiil, by u i^uxun fiirnuT, Geor;;e I'alitscli, on 

 Cliri.stiiiius iliiy, 17r>8 It reuched its pl^rih(■iion on March Hi, 

 IT.'i'J, eontiniiin;,' at once the accuracy of (Jlaimut's coni- 

 putution.s and tiie justice of hi.s caution in assigning rather 

 wide limits of error. 



It was now evident tliut comets travel, like the planets, in 

 iletermined patlis ; and also, tliat the investij^ation of their 

 motions is a suliject worthy the study of tiie alilcst niathi? 

 maticians, and sufficient to tax tli«r highest powers. An 

 account of their labours would be out of place in such an 

 article as the present ; but we recommend the subject to 

 the notice of the agricultural student, as one of the most 

 interesting chapters in the history of modern science. 



One comet, however, discovered not long after astronomy 

 had achieved this triumpli, seemed at first to teach a 

 difl'erent lesson. In 1770 a comet appeared whose path 

 turned out to Ijc — not a long oval or parabola, as had been 

 the case with all the orbits yet examined — but an ellipse of 

 moderate extent, and not very eccentric. The orbit lay also 

 much closer than usual to that thin sUce of space (so to speak) 

 within which the planets are observed to move. Lexell, who 

 computed the path, found that the period of the comet was 

 about fiveand-a-half years. Its return was carefully watched 

 for, Iml no oiie has ever seen the comet since. The cause of 

 its disappearance, and also of its sudden appearance— for 

 tliis was equally remarkable, when we remember that so 

 conspicuous a comet could not have been circidating long in 

 its small orbit without discovery — was carefully inquired 

 into. The result was singxilar. On tracing back the path 

 of the comet, it was found that it must have passed very 

 near to the gi-eat planet Jupiter. " It had intruded," saj's 

 Herschel, '• an uninvited guest into his family circle — 

 actually nearer to him than his fourth satellite." Accord- 

 ingly, the comet's path, originally a long oval, had been 

 bent into a cirrve of less extent. Having once entered on 

 this new path, the comet was free to follow it — always 

 returning, be it noticed, to the point at wliich it had started 

 on it — so long as Jupiter was not interfered with. But it 

 happened, unfortunately for the stability of the comet's 

 motions, that, after going twice round the new path, it 

 again presented itself near Jupiter's track, when the planet 

 (which had meanwhile gone once round his orbit) was not 

 very far from the scene of his former encounter. He 

 accordingly again exei-ted his influence upon the luifortunate 

 comet, and this time dismissed it on a path wliich will not 

 admit of its approaching the earth near enough to be seen.* 



Let us return, however, to Halley's comet. 



It so chances that the comet which was the first to show 

 full obedience to the law of gravitation, was one which 

 exhibited in a very remarkable and significant manner 

 the characteristics which distinguish comets from other 

 heavenly bodies, and make them so mysterious to the 

 student of science. At the return of Halley's comet, in 

 1836, all that had signali.sed the return in 1759 was 

 repeated, but the mathematical triumph was far greater. 

 Damoiseau, Rosenberger, and Pontecoulant calculated the 

 comet's return to perihelion within two or three days, 

 instead of a month, and the time when it passed this point 

 of its orbit corresponded, within a few hours, to the mean 



• It must be noticed, however, that Lovorrier, who very carefully 

 rc-cxamincd the question, was led to <iue.stion the accuracy of the 

 results recorded above. Admitting that Jupiter had twice disturbed 

 the comet, he thinks there is no certiiinty (for want of snfificiently 

 accurate observations) respecting either the original imth of the 

 comet, or that in which it is at present circulating unobserved — if, 

 indeed, it has not been absorbed by Jupiter. 



of tlieir several estimates. On the northen heavens 

 where it was lirst seen, the comet present^-d a remarkable 

 appearance, with a long and brilliant tail stretching over 

 an arc of many degr<>es upon tlie sky. When it had pa.ss<-d 

 from our northern skies, it was carriwl (aft'-r a short interval, 

 during which it was lo.st to view in the sun's rays) to the 

 soutliem lieavens. Sir John Herschel, and Maclear (A.s- 

 tronomer Royal at th<; Caj)e), were prepared to receive it ; 

 but when first observed l>y them it showed none of the 

 features which made it .so remarkable in our skii^ It had 

 no tail and scarely any head. In fact. Sir John Herschel, 

 in one account, says, that as first s(«n it could only l>e dis- 

 tingui.shed from a fixed star liy its motion. The study of 

 its gradual change of aspect from that time threw so much 

 light on the nature of comets' tails and other appendages 

 (or at any rate of that particular comet's tail) that Sir 

 John Herscliel, not accustomed to be over confident, said 

 there could be no doubt as to the true interpretation of the 

 observed phenomena. What these phenomena were shall 

 be considered further on. 



ILLUSIONS. 



By Tuomas Foster. 



THE senses are the means by which, directly or 

 indirecth', all observations are made, and science can 

 oidy make real advance in so far as it is based on observi- 

 tion and experiment. It is most important, therefoi-e, that 

 either our senses should be trustworthy in their action, that 

 is should give us true information, or (if they neither ai!e 

 absolutely trustworthy originally nor can be so trained 

 as to become so) that we should be able to test and to 

 correct their indications. 



Now it very soon appears, when we put the matter to the 

 test, that the direct evidence of the senses is not to be 

 accepted without careful cross-examination. The science 

 of our day may be regarded as having been established in 

 opposition to the apparently obvious evidence of the sensefe. 

 Take, for in.stance, astronomy. Nearly everything that the 

 eyes tell us about the heavenly bodies, and nearly all that 

 the sight and touch tell us about the eartli (so far as 

 astronomy has to deal with the earth as one of the planets) 

 is false. Not one of all the stars we see in the skies is 

 really where we see it. The earth seems flat, large, and 

 fixed ; it is really a globe, small compared -n-ith the 

 seemingly small stars, and it is moving in many ways, 

 not one of which the senses correctly appreciate. It is 

 the same with otlier sciences. 



W'e are not concerned, however, to discuss here how far 

 the apparent teaching of the senses has to be analysed 

 before its real meaning can be understood. The examples 

 illustrating this would cover the whole range of science. 

 For instance, to sliow how the real place of a star can be 

 determined — more or less exactly — from its apparent 

 place in the sky, we require to discuss the laws of refrac- 

 tion, aberration, the proper motion of stars, and a number 

 of other matters. In sucli cases as this, though what the 

 eye tells us is in a sense incorrect, the eye is supposed to 

 do its work correctly. The eye tells us truly that the rays 

 received from the star by it have come in such and such a 

 direction, and what science has really to do is to determine 

 in what direction those rays must have set out in order 

 after various changes of direction, due to the various 

 media through which they passed, to reach the eye situate 

 on a moving and rotating body like the earth, in the direc- 

 tion which they had, or at least seemed to have — or, more 



