92 Mr Dunlop's observations on the Comet of 1825. 



gives sixteen revolutions ; and the time of the revolution 19 11 

 3(X. 



From October 20th, at 10 h 36', to November 2d, at 10 h 50' 

 gives sixteen revolutions ; and the time of the revolution 19 h 31'. 



From October 7th, at ll h Q', to November 8th, at 9 h 46' 

 gives thirty-nine revolutions ; and the time of the revolution 

 19 h and 39'. 



By taking a mean of all these, it gives 19 h and S& for the 

 approximate time of the rotation. 



General Remarks. 



The tail has been subject not only to continual, but (from 

 the observations) to periodic changes of appearance, and also 

 the changes about to take place first made their appearance at 

 the head of the comet, sometimes shooting out from one side, 

 and sometimes from both, but generally made their appearance 

 first on the following side of the head. 



If these changes are occasioned by a rotation on its axis, 

 the tail of the comet must be dependent on the nature of the 

 materials of which the surface of the comet is composed, which 

 must be very irregularly scattered on some parts of its surface 

 to produce the different appearances which the tail has pre- 

 sented ; and if such be the cause, the change of appearance in 

 the tail may be more remarkable in some comets than in others 

 of equal magnitude, from the position of its poles with respect 

 to the earth, and the irregularity of its surface. 



But what may be the cause of these periodic changes in the 

 figure of the tail I do not pretend to say. The observations 

 are given in candour as they were made, and I leave it to those 

 who may have made similar observations on the tail, and are 

 better qualified to investigate the subject than myself. 



Similar appearances were observed by Le P. Cysat, in the 

 tail of the comet of 1618 ; by Hevelius, in the tails of the 

 comets of 1652 and 1661 ; and by Pingre, in the tail of the 

 comet of 1769. As this will again appear about the latter end 

 of 1835, or beginning of 1836, it will afford an opportunity, at 

 no very distant period, of investigating more minutely the 

 changes which take place in the tails of comets, which may 

 lead, not only to suspect the cause of the change which takes 



