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 reer 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 (L 
should think) have been less than would be due to that cause, ac- 
cording to that hypothesis. 
Nueleus 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 aériform 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 cil 
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 ease of the ordinary tail, for it passes 
1 peta 
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 
xactness, yet the declination was usually greater than that which would be likely 
Ks arise from an error of that kind, which error, moreover, would not have a 
always on the same side. 
