RemarJcs on the Tails of Halhy^s Comet. 329 



tected a curvature on one side. They will, however, scarcely dis- 

 tinguish this emanation from some of those to which the name tail 

 has been generally appropriated. 



If we call this cone a tail, it would be desirable to classify these 

 appendages. Were there others, like the cone of Halley, traceable 

 directly to the nucleus by observation alone, we might denominate 

 them caudae nuclei, in contradistinction to the ordinary appendages 

 of the envelope. But let us see whether there are not physical 

 considerations which should induce us to make this class more ex- 

 tensive. As some luminous emanations are nearly opposite to the 

 sun, and retain a position nearly constant in relation to the radius 

 vector, and as others, which are usually much shorter, have different 

 and variable positions in relation to the radius vector of the comet, 

 but in all probability positions nearly or exactly constant in relation 

 to the radius of its nucleus at the point of emanation, we might 

 still distinguish the latter as the tails of the nucleus, (although in 

 some instances, as in the comet of 1825, they are not directly trace- 

 able to it by observation,) and the former as the caudce solares of 

 comets, on account of their characteristic position and peculiar mode 

 of development. In the former ailicle, I hazarded a conjecture as 

 to the fact of a rotation of thy nucleus of Halley, suggested by ob- 

 served differences in the position of the cone on different days. 

 This interesting fact, (or what for our present purpose is equivalent 

 to it,) may now be regarded as established by the observations of 

 M. Bessel, not only on different days from the 2d to the 25th of 

 October, but during 8 or 9 successive hours on the 12th, during 

 vphich there was a rapid progressive motion from right to left. It is 

 an interesting circumstance, that the angular opening of the cone, 

 (" about 90°"*) observed at Konigsberg on the 2d of October, was 

 the same as that observed at Schenectady on the 16th. This tends 

 strongly to confirm the identity of the cone at distant epochs, and 

 affords some data for determining the period of rotation, as there 

 are reasons for believing that most of the angular changes of the 

 intermediate periods were not real, but resulted from variations in 

 the obliquity of the axis of the cone in relation to the visual ray. 

 If we assume three complete rotations with an equable motion be- 

 tween the 2d and 16th of October, the period of each must have 

 been 4f days. Now M. Bessel (from data which have not yet 



* Bib. Univ. Avril, 1836, p. 357, 



Vol. XXXI.— No. 2. 42 , 



