94 CELESTIAL PHENOMENA. 



comets of 1807 and 1843) the two branches were of very dif- 

 ferent length : in the comet of 1744, the tail had six branches, 

 the two exterior diverging at an angle of 60. The tails have 

 been sometimes straight, sometimes curved ; in the latter case, 

 either concave towards both sides, or, as in 1618, convex 

 towards the direction in which the comet is moving ; and 

 sometimes the tail even appears inflectedlike aflame in motion. 

 The tails are always turned from the sun, and so directed 

 that the prolongation of the axis would pass through the 

 centre of that body; this circumstance (according to Edouard 

 Biot) was remarked by the Chinese astronomers as early as 

 837, but was first clearly stated in Europe by Eracastoro and 

 Peter Apian in the sixteenth century. We may conceive that 

 these emanations form conoidal envelopes of greater or less 

 thickness, which would furnish a very simple explanation of 

 several of the remarkable optical phenomena above mentioned. 

 Not only are different comets characterised by great 

 differences of form, some being entirely without visible tails, 

 others, as the third comet of 1618, having a tail of 104 

 degrees in length, but we also see the same comets 

 pass through successive and rapid variations. These 

 changes have been well and most accurately described in the 

 comet of 1744, by Heinsius, at Petersburgh, and in Halley's 

 comet, on its last reappearance, in 1835, by Bessel, at 

 Konigsberg. A tuft of rays issued from that part of the 

 nucleus which was turned towards the sun. These rays thus 

 issuing were bent backwards, so as to form part of the 

 tail. ' ' The nucleus of Bailey's comet, with its emanations, 

 presented the appearance of a burning rocket, the train of 

 which was deflected sideways by a current of air." The 

 rays issuing from the head of the comet were seea 



