154 Observations on the Tails of Halley^s Comet. 



evenings, from the 11th to the 25th Oct. inclusive.* It was for- 

 merly believed that the tail of a comet was in most cases directly 

 opposite to the sun ; but modern observers have discovered that it 

 is generally inclined a little backwards. This has been attributed 

 by some, to the resistance of an ethereal medium ; but that curva- 

 ture which would naturally be produced by this cause, and which 

 has been often observed in these luminous trains, was not in this in- 

 stance detected. The straightness of a tail of such immense length 

 and levity, appears to be rather unfavorable to that hypothesis. If 

 there was any curvature, (which was once suspected,) it must (I 

 should think) have been less than would be due to that cause, ac- 

 cording to that hypothesis. 



jSfucleus and Envelopes. — What I remarked in my journal in re- 

 lation to apparent nuclei of different orders, is equivalent to the ex- 

 pression, that there are real envelopes of different orders, the less 

 brilliant one being at the greater distance. This is analogous to the 

 results of former observations, which have detected, in the case of 

 other comets, two or more concentric envelopes, as though there 

 were different strata of luminous clouds at different heights in the 

 comet's atmosphere, with intermediate aeriform matter, uncondensed 

 and transparent. 



The difference in this instance was, that the envelopes were not 

 always concentric ; in other words, that there was a kind of 



Second Tail. — This was a tail to the nucleus, but not to the exte- 

 rior envelope : in this respect, it differed from the ordinary train. 

 In being equal in width to the nucleus at the part where it joined it, 

 and in diverging from it, it sustained about the same relation to the 

 nucleus, as was sustained to the whole head by that part of the lon- 

 ger tail which joined it, although there was no neck or contraction 

 of the shorter tail near the nucleus. When the sky was clear, it 

 was not circular or elliptical, but presented the appearance of a cir- 

 cular sector, with straight divergent sides; so that analogy justifies 

 us in calling it a tail. Indeed, this term might be applied to any 

 stream of diverging light, even were the analogy less complete. And 

 even though a stratum of it may have surrounded the real nucleus, 

 this would be analogous to the case of the ordinary tail, for it passes 



* This was the case according to the situation of the comet, as observed by me; 

 and although I had not the means of determining the place of the head with great 

 exactness, yet the declination was usually greater than that which would be likely 

 to arise from an error of that kind, which error, moreover, wottld not have been 

 always on the same side. 



