COMETS. 87 



and considered in this manner, they furnish a simple expla- 

 nation of many of the remarkable optical phenomena already 

 spoken of. 



Comets are not only characteristically different in form, 

 some being entirely without a visible tail, whilst 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 

 a2curately 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 towards the Sun ; and the rays 

 being bent backwards, 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 

 measurements, 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 en anated, 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 vibra- 

 tions, and is of opinion that they indicate a polar force, which 

 turns one semi-diameter of the comet towards the Sun, and 

 strives to turn the opposite side away from that luminary. 

 The magnetic polarity 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 equinoxes." This is not the place to enter 



* Arago, Des changemente physiques de la Com&te de Hatley du 

 15-23 Oct., 1835. Annuaire, 1836, pp. 218, 221. The ordinary 

 direction of the emanations was noticed even in Nero's time. " Con a 

 radios so'.is effugiunt." Seneca, Nat. Qucest., vii. 20. 



