250 Comets. 



vivid nucleus, like a star or planet. From the head, and in 

 a direction opposite to that in which the sun is situated from 

 the comet appear to diverge two streams of light, which 

 grow broader and more diffused at a distance from the head, 

 and which most commonly close in and unite at a little dis- 

 tance behind it, but sometimes continue distinct for a great 

 part of their course ; producing an effect like that of the 

 trains left by some bright meteors, or like the diverging fire 

 of a sky-rocket (only without sparks or perceptible motion) : 

 This is the tail. This magnificent appendage attains occa- 

 sionally an immense apparent length. Aristotle relates of 

 the tail of the comet of 371 B.C., that it occupied a third 

 of the hemisphere, or 60° ; that of A.D. 1618, is stated to 

 have been attended by a train no less than 104° in length. 

 The comet of 1680, the most celebrated of modern times, 

 and on many accounts, the most remarkable of all, with a 

 head not exceeding in brightness a star of the second mag- 

 nitude, covered with its tail an extent of more than 70° of 

 the heavens, or, as some accounts state, 90°; that of the 

 comet of 1769, extended 97° ; and that of the last great 

 comet (1843), was estimated at about 65° when longest. 

 The figure (Fig. 2, Plate II.)* is a representation of the comet 

 of 1819, — by no means one of the most considerable, but 

 which was, however, very conspicuous to the naked eye. 



The tail is, however, by no means an invariable appen- 

 dage of comets. Many of the brightest have been observed to 

 have short and feeble tails, and a few great comets have 

 been entirely without them. Those of 1585 and 1763 offei^ed 

 no vestige of a tail ; and Cassini describes the comets of 1665 

 and 1682 as being as roundf and as well defined as Jupiter. 

 On the other hand, instances are not wanting of comets fur- 

 nished with many tails or streams of diverging light. That 



* This refers to a P^ate iu the " Outlines,' but not copied here. 



t This description however applies to the " disc"' of the head of these 

 comets as seen in a telescope. Cassini's expressions are, " Aussi roud, aussi 

 net, et aussi clair que Jupiter" (where, it is to be observed, that the latter 

 epithet must by no means be translated briglii). To understand this passage 

 fully, the reader must refer to the description given further on, of the " disc" 

 of Halley's comet, after its perihelion passage in 1835-6. 



