$26 Remarks on the Tails of Halley’s Comet. ; 
There is also a great resemblance between the results here and 
those at Paris onthe 17th. There, ‘onthe 17th, the sector of the 
preceding evening still existed. Hts form and its direction appeared 
not remarkably changed, but its light was much less vivid.” Here, 
as quoted above from the journal, ‘the sky is much less clear than 
on last night, and the envelope, condensed on one side to form the 
short tail, is indistinct ; yet its direction and form seem not to have 
been sensibly changed since last evenin 
At Paris on the 18th, ‘ iMisiithesennsen avait fait de nouveaux 
progres.”” Schenectady, 18th, the proper tail “seen directly is 
longer and indirectly shorter than it was on the 16th. The atmos- 
phere appears to be less clear. In consequence of this and of the 
wind, the nucleus is not discernible.’ When the sky is free from 
clouds, may there not be frequently a precipitation or crystallization 
of vapor to a great horizontal extent, in the elevated regions of the 
atmosphere, which, whilst it has less effect on bright stars and the 
more brilliant part of a comet’s tail, entirely cuts off that fainter 
portion which is seen by oblique vision? It would be interesting 
to know in what degree the fading of other parts, which on some 
days made similar progress as seen at Schenectady and Paris, was 
owing to the above cause, or whether, as M. Arago assumes, it was 
solely a change in the comet. Thatit did undergo real changes can 
scarcely be doubted. But can any exact estimate be made of these 
till we can determine those in our atmosphere? But whatever may 
be the real changes in the head and tail, the difference in the length 
of the latter as seen simultaneously at Paris and Schenectady must 
depend chiefly upon the meteorological or physiological cireumstan- 
ces enumerated in the preliminary remarks. | include physiological, 
as it is not stated by M. Arago what was the position of the optic 
axis at the time of the chieieiticis from which the length of the 
tail was deduced, or whether it was ever inclined to the visual ray- 
On the 4th of October it could be seen here in no other way. I 
have seen no account of its appearance elsewhere previous to the 
10th. On the 16th, it appeared at Paris to be from 10° to 12°. 
At Schenectady on the same day, it was 7° or 8° by direct, and 45° 
by oblique vision; next evening 35°. At none of the former re- 
turns of this comet has the tail ever been represented as having 
such lengths anterior to the perihelion passage. Nor since 1456, 
has it, after the perihelion passage, been seen of such lengths as 
those given above, which were a month before, if we except a single 
