102 * COSMOS. 



ness, and, considered in this manner, they furnish a simple 

 explanation of many of the remarkable optical phenomena al- 

 ready spoken of. 



Comets are not only characteristically different in form, 

 some being entirely without a visible tail, while others have 

 a tail of immense length (as in the instance of the comet of 

 1618, whose tail measured 104°), but we also see the same 

 comets undergoing successive and rapidly-changing processes 

 of configuration. These variations of form have been most 

 accurately and admirably described in the comet of 1744, by 

 Hensius, at St. Petersburg, and in Halley's comet, on its last 

 reappearance in 1835, by Bessel, at Konigsberg. A more or 

 less well-defined tuft of rays emanated from that part of the 

 nucleus which was turned toward the Sun ; and the rays be- 

 ing bent backward, formed a part of the tail. The nucleus 

 of Halley's comet, with its emanations, presented the appear- 

 ance of a burning rocket, the end of which was turned side- 

 ways by the force of the wind. The rays issuing from the 

 head were seen by Arago and myself, at the Observatory at 

 Paris, to assume very different forms on successive nights.* 

 The great Konigsberg astronomer concluded from many meas- 

 urements, and from theoretical considerations, " that the cone 

 of light issuing from the comet deviated considerably both to 

 the right and the left of the true direction of the Sun, but 

 that it always returned to that direction, and passed over to 

 the opposite side, so that both the cone of light and the body 

 of the comet from whence it emanated experienced a rotatory, 

 or, rather, a vibratory motion in the plane of the orbit." He 

 finds that " the attractive force exercised by the Sun on heavy 

 bodies is inadequate to explain such vibrations, and is of opin- 

 ion that they indicate a polar force, which turns one semi-di- 

 ameter of the comet toward the Sun, and strives to turn the 

 opposite side away from that luminary. The magnetic polar- 

 ity possessed by the Earth may present some analogy to this ; 

 and, should the Sun have an opposite polarity, an influence 

 might be manifested, resulting in the precession of the equi- 

 noxes." This is not the place to enter more fully upon the 

 grounds on which explanations of this subject have been bas- 

 ed ; but observations so remarkable,! and views of so exalted 



* Arago, Des Changemenis physiques de la Comete de Halley du 15- 

 23 Oct., 1835. Annnaire, 1836, p. 218, 221. The ordinary direction 

 of the emanations was noticed even in Nero's time. " Comce radios sO' 

 lis effugiunt.^^ — Seneca, Nat. Qucest., vii., 20. 



t Bessel, in Schumacher, Astr. Nachr., 1836, No. 300-302, s. 188, 192, 



