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COMETS. 



ets, their satellites, and the concentric ring 

 that belongs to at least one of the outermost 

 of them, as products of a projectile force, and 

 as connected with one another by intimate bonds 

 of mutual attractions. 



We have still to speak of Comets, an innu- 

 merable host, which revolve around the sun in 

 definite orbits, and from him derive their light. 

 When we estimate the relative lengths of the 

 orbits of these bodies, the boundaries of their 

 perihelia, and the great likelihood of their re- 

 maining invisible to the inhabitants of the earth, 

 by the rule of probabilities, we find that they 

 must amount to such myriads as makes the 

 imagination pause amazed. Kepler, with the 

 liveliness of expression that distinguished him, 

 says, that there are more comets in the depths 

 of space, than there are fishes in the bosom of 

 the ocean ; and yet we have scarcely the ac- 

 curately-computed orbits of some 150 of the 

 six or seven hundred of these bodies, upon 

 whose appearance and course through known 

 constellations we have indications more or less 

 rude. Whilst the classic nations of the west, 

 the Ancient Greeks and Romans, occasionally 

 give the place in the heavens where a comet 

 was first seen, but never say a word of its ap- 

 parent course, the ample literature of the Chi- 

 nese, those accurate observers of nature and 

 of individual things, contains circumstantial no- 

 tices of the cowBtellations through which each 

 comet passed. These notices extend to more 

 than five hundred years before the commence- 

 ment of the Christian era, and many of them 

 are used by astronomers at the present day("). 



Of all planetary bodies, comets are those 

 which, with the smallest masses, occupy the 

 largest fields of space. The particular obser- 

 vations that have hitherto been made upon 

 them, indicate masses much under the ;jo^oT^th 

 of that of the earth ; yet have these bodies tails, 

 which often extend over many millions of miles, 

 both in length and breadth. The light-reflect- 

 ing tail, or cone of vaporous matter which com- 

 «ts emit, has occasionally been observed to be 

 as long as is the distance of the earth from the 

 son, a line which intersects the orbits of two 

 of the planets, those of Mercury and Venus. 

 This was the case with the remarkable comets 

 of 1680 and 1811 ; and it is even probable that 

 our atmosphere was mingled with the vapour 

 of the comets' tails of the years 1819 and 1823. 



Comets exhibit such variety of forms or ap- 

 pearances, often appertaining to the individual 

 rather than to the kind, that a description of 

 one of these travelling light-clouds — for so 

 they were called by Xenophanes and Theon of 

 Alexandria, the contemporaries of Pappus — 

 can only be applied with certain precautions to 

 another. The feeblest telescopic comets are 

 generally without any visible tail, and resemble 

 the nebulous stars of Herschel. They appear 

 as rounded, palely-glimmering nebulae, with the 

 light stronger or more concentrated towards 

 the middle. This is the simplest type ; but it 

 is even as little a rudimentary or nascent type 

 on this account, as it is a type of a planetary 

 body grown old, and become exhausted by ex- 

 halation. In larger comets we distinguish a 

 head, or nucleus, as it is commonly called, and 

 a simple ^r compound tail, which the Chinese 

 astronomers entitle, very characteristically, the 



brush (sui). In general the nucleus has no def- 

 inite outline, although, in some cases, it has 

 the splendour of a star of the first or second 

 magnitude ; and in the great comets of 1402, 

 1532, 1577, 1744, and 1843, it had such brill- 

 iancy, that it could be seen in bright sun- 

 shine('^). This last circumstance seems to tes- 

 tify to the existence, in some members of the 

 family at least, of greater density and a highly 

 reflective faculty in the mass. But no more 

 than two comets have yet been seen, which, in 

 Herschel's great telescope, presented well-de- 

 fined discs(**); these two are the one of 1807, 

 discovered in Sicily, and the magnificent one 

 of 181 1 . The disc of the former appeared under 

 an angle of 1", that of the latter under an an- 

 gle of 0"-77, from which an actual diameter of 

 134 and 107 miles respectively is obtained. The 

 less precisely defined nuclei of the comets of 

 1798 and 1805, indicated diameters of no more 

 than 6 or 7 miles. In several comets that have 

 been accurately observed, particularly in the 

 one of 1811, mentioned above, and that was 

 seen so long, the nucleus, and the misty en- 

 velope which surrounded it, were wholly sep- 

 arated from the tail by a darker space. The 

 intensity of the light of the nucleus does not go 

 on increasing continuously towards the centre ; 

 bright zones are repeatedly separated by con- 

 centric misty envelopes. The tail, as stated, 

 has appeared now single, now double ; but rare- 

 ly, although this was the case in the comets of 

 1809 and 1843, of very different lengths in the 

 two branches; one comet, that of 1744, has 

 appeared, which had six tails. The tail, again, 

 is either straight or curved, now to both sides, 

 now outwardly (1811), or convex to the side 

 towards which the comet is tending (1618) ; 

 occasionally the tail has been waving or flame- 

 shaped. The tails of tomets are always turn- 

 ed from the sun in such wise that their axes 

 produced would pass through the centre of that 

 luminary ; a fact which Biot assures us was 

 notified by the Chinese astronomers so long 

 ago as the year 837, but which was first dis- 

 tinctly mentioned in Europe by Fracastorius 

 and Petrus Apianus in the 16th century. These 

 effusions may be regarded as conoidal enve- 

 lopes, having thicker or thinner walls — a view 

 upon which several very remarkable optical ap- 

 pearances may readily be explained. 



The several comets, however, are not so 

 characteristically distinguisUfid by their mere 

 forms or appearance — they are not in one case 

 tailless, in another provided with a tail of 104 

 degrees in length, as was the third of the yeai 

 1618; we further observe them passing through 

 a rapid succession of varying formative pro- 

 cesses. This change of form was most accu- 

 rately and ably observed by Heinsius, of St. 

 Petersburgh, in the comet of 1744, and in Hal- 

 ley's comet, on its last appearance in 1835, by 

 Bessel, of Konigsberg, by whom it has been 

 very carefully described. On the part of the 

 nucleus which was turned towards the sun 

 there was a kind of tufted emanation apparent. 

 The rays of this that bent backwards went to 

 form part of the tail. " The nucleus of Halley's 

 comet, with its emanations, presented the ap- 

 pearance of a burning rocket, the train of which 

 was deflected sideways by a current of air." 

 The rays proceeding from the head were seen 



