Observations on the Tails of Halley^s Comet. 155 



in a kind of parabolic form around the telescopic envelope, to form 

 the outer envelope, which is visible to the naked eye, and which 

 appears to be contiguous to the tail, and similar to it in brightness, 

 but not contiguous or similar to the envelope within. I have en- 

 deavored to describe these objects as they appeared with our teles- 

 cope. To those who enjoyed the advantage of more powerful in- 

 struments, other minute and faint parts must have been detected, 

 whose existence may have affected the general form ; but this cir- 

 cumstance may not destroy the value of the above observations, as 

 the descriptions might still be found tolerably correct, so far as re- 

 gards parts within certain limits of magnitude and brightness. 



Those vi^ho are familiar with the history of astronomy, know that 

 double tails are not unprecedented. 



Several tails have been seen attached to a single comet. That of 

 1744 presented sis streams of nearly 30° in length, widely diver- 

 gent, like the rays of an expanded fan. 



In January, 1823, Prof. Biela, at Prague, and President Day, at 

 New Haven, saw a second tail at an angle of about 178° with the 

 first. It was seen but for a few days, and not many particulars in 

 regard to it are stated. But it would seem that this tail, like most 

 of those seen previous to 1835, proceeded rather from the envelope 

 than from the nucleus. 



In July, 1825, Mr. James Dunlop, astronomer, at Paramatta, 

 New South Wales, observed the changes of position and form which 

 occurred in the tails of a comet, and afforded evidence of the exist- 

 ence of a rotatory motion in the comet, the approximate period be- 

 ing 19 h. 36 m. His observations, which were exact and minute, 

 were published by Sir Thomas Brisbane. 



The preceding observations on Halley's comet, will perhaps be 

 thought to justify the suspicion of a rotation, but its period cannot 

 be determined by them with exactness, nor its existence with cer- 

 tainty, without comparing them with others made in a clearer sky, 

 with more powerful instruments, and on nights, and at hours, differ- 

 ing from those of my observations. 



As this is a phenomenon which is usually seen but for a few days, 

 and as the sky may be clear at one place, and cloudy at another, at 

 times when it exists, the foregoing observations may afford data of 

 some use in the investigation of this interesting subject, although 

 they cannot be put in competition with simultaneous observations, 

 made under more advantageous circumstances. 



