328 MemarJcs on the Tails of Halley^s Comet, 



afterwards unfavorable. On the 9th November the length was 

 about 2i° as seen with the portable telescope by ray friend C. H. 

 Stillman, who had often assisted me in the observations. On the 

 16th the tail of 45^ was shown to many of the senior class who had 

 been for a time studying optics and had been instructed in regard to 

 the difference between the results afforded by different modes of 

 observing. 



M. Arago with great candor combats the opinion which he lately 

 supported in regard to the perishable character of this comet. The 

 observations of the tail and other parts of this comet at its last re- 

 turn, have convinced him and most astronomers that there is no evi- 

 dence of any recent diminution. So far as my observations have a 

 bearing they will tend to confirm this last opinion. As to the chan- 

 ges which have been generally believed to have taken place between 

 former returns, but which begin to be regarded as only apparent ; 

 they may probably be explained on other principles stated in the 

 preliminary remarks to the other article.* Some of these principles 

 are applicable also to some of the daily and instantaneous variations. 

 Schenectady, Oct. 1st, 1836. 



The foregoing remarks were communicated for the October No. 

 of this Journal, but too late for insertion. I have more recently 

 seen extracts from an article in the Bibliotheque Universelle of Ge- 

 neva, relating to a memoir on the physical constitution of the com- 

 et of Halley, by M. Bessel, the illustrious astronomer of Konigs- 

 berg.f His views on this subject appear to differ in some respects 

 from those of the equally distinguished French savant above quo- 

 ted. What M. Arago " did not hesitate to name" a new sector, 

 appears to be regarded by M. Bessel as the sector in different posi- 

 tions. It appears that he never saw but one. Reference is made 

 to it also under the names of luminous emanation, emission, efflu- 

 via and eruption ; terms expressive of its nature or mode of devel- 

 opment. M. Bessel speaks of it as the cone and the sector, terms 

 which were employed in the first article. ij: By the former we in- 

 tend to express its actual form, by the latter that of its projection. 

 These terms are sufficiently exact, though M. Bessel sometimes de- 



* Vid. also the Annaaire, and the last No. of this Journal, 

 t I am indebted for these to the same correspondent who furnisiied the former 

 extracts. 

 X Last No. of the American Journal. 



