Nov. 4, 1881.] 



KNOWLEDGE 



philosopher preferred to look for a theory of the universe 

 in the conceptions of his own brilliant and imacrinative 

 mind. As if to show future ages how little was likely to 

 be achieved by the highest mental powers without the 

 habit of patient obscn'ation, he endeavoured to educe a 

 system of philosophy from fancies, and to found it upon 

 syllogisms. Aristotle — who may be considered the typical 

 philosopher of the Greek school — included comets iu the 

 wide range of phenomena which he claimed the privilege 



firming Newton's views by results founded on actual obser- 

 vation, he collected all tlic records of comets which seemed 

 entitled to contidence, and attempted — as well as his meagre 

 materials would allow him — to calculate the elements of 

 their orbits. In this way he computed the paths of no 

 less than twenty-four. Among these, three presented a 

 remarkable similarity. One appeared in 1531, and was 

 described by Appian ; another appeared in 1607, and was 

 oliserved l>y Kepler ; the third was traced by Halley him- 



of e.xplaining. To him was due the opinion men- i self in 1G82. The equality of the intervals between these 



tioned above — an opinion confidently maintained during 

 the many centuries in which the philosophy of 

 Aristotle held sway over men's minds. To him, 

 also, was due a yet more remarkable opinion, the 

 view, namely, that the !Milky Way is a vast comet 

 which continually reproduces itself ! Xenophanes and 

 Theon, in the fifth century, adojited a rather singular view 

 of the Aristotelian theory of comets, when they spoke of 

 these objects as " travelling light-clouds." 



To these fancies the ancients added the idea that the 

 shapes of comets indicated their character as portents. 

 Thus in Fig. 1 five views of comets are shown, as an arrow- 

 head, a sea monster, a sword, a lance, and in flames. 



-Various fanciful views ■ L'o.i.ets, ajcorJing to Pliny. 

 From the CometojraiMa of Hevelins. 



Tycho Brahe was the first to express doubts respecting 

 the ^-iews of Aristotle. From a careful series of observa- 

 tions, he demonstrated that the orbits of comets are cer- 

 tainly situated beyond the moon's orbit. He thought the 

 orbits must be circular, for he lived at a time when none 

 but circular orbits were conceded to the celestial bodies. 

 Diirfel, a native of Upper Saxony, proved that the orbits 

 of comets are either very elongated ovals, or parabolas, and 

 that the sun occupies a focus of the curve. It happens, 

 singularly enough, that this discovery was effected Viut a 

 year or two before Newton propounded the theory of gravi- 

 tation. Newton himself examined the orbit of the great 

 comet of 1680 (known as " Newton's comet") and others : 

 and he found that they all accord with the law of gravity. 



But before long, Ne\six)n's friend and pupil, Halley, 

 effected a yet more remarkable discovery. In hopes of con- 



epochs led to the suspicion that the same comet had 

 apjieared three times. And Halley found, on searching 

 historical [records, that a comet appeared in 1305, another 

 in 1380, and a third in 1456. Combining these appearances 

 with those mentioned before, he tliought he had satisfactory 

 evidence of identity. For he was sufficiently familiar 

 with the results which might be expected to flow from the 

 law of gravity, to be aware tliat absolute regularity of 

 motion was not to be expected in a body traversing 

 the solar system in an eccentric orbit, and swayed from 

 its proper path by the attraction of such giant planets 

 as Jupiter and Saturn. Indeed it happens, singularly 

 enough — one out of many remarkable coincidences in the 

 history of comets — that the comet 

 of 1830 was not Halley s comet, 

 which really appeared in 1378, a 

 date bringing in a yet greater dis- 

 cordance in tiie intervals than Halley 

 had suspected and accounted for 

 With remarkable acumen — since no 

 means existed in his day for anything 

 like accurate computation — he not 

 only pointed out the possible influ- 

 ence of the great planets in disturb- 

 ing the comet in past revolutions, 

 but he made a rough approach to an 

 estimate of the eflect that they would 

 have on the period of its next visit. 

 " Instead of appearing in August, 

 1757, as it would if its period re- 

 mained unaltered, it will not appear," 

 he said, "until the end of 1758, or 

 the beginning of 1759, for it will be 

 retarded by the action of Jupiter. 

 Wherefore," he adds, with a pardon- 

 alile anxiety to secure the credit of 

 his ingenious investigations, " if it 

 should return, according to our pre- 

 diction, impartial posterity will not 

 refuse to acknowledge that this was 

 discovered by an Englishman." 



As the time for the fulfilment 

 of the prediction approached, an 

 intense interest was excited in the minds of astronomers. 

 In 1757, Clairut, Lalande, and Madame Lepaute under- 

 took the comi)utation of the epoch at whicli the comet 

 might be expected to"" return. They applied methods 

 of investigation invented by Clairaut himself. It resulted 

 fi'om their laliorious computations that April 13, 1759, 

 was fixed on for the epoch at which the comet should 

 attain its closest approach to the sun, or, as it is teclmically 

 e.xpressed, should pass its perilielion. But Clairaut was 

 careful to allow a month either way, on account of un- 

 avoidaVile omissions in tlie calculation, and for the efiects 

 of unknown forces, "such as the action of some planet 

 too far oS" to be seen " (a happy anticipation of modem 

 discoveries). 



And now the heavens were swept diligently by all the 

 telescopists of Europe, each eager to be the first to 



