S26 Remarks on the Trails of Halley^s Comet. 



There is also a great resemblance between the results here and 

 those at Paris on the 17th. There, " on the 17th, the sector of the 

 preceding evening still existed. Its 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 evening." 



At Paris on the 18th, " I'afFaiblissement 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 oiF 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. That it did undergo real changes can 

 scarcely be doubted. But can any exact estimate be made of these 

 till w-e 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 circumstan- 

 ces enumerated in the preliminary remarks. I include physiological, 

 as it is not stated by M. Arago what was the position of the optic 

 axis at the time of the observations 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 



