On the Formation of the Tails of Comets. Ill 



velocity would carry the body to a perceptible distance from the 

 parabolic orbit. These calculations cannot, however, be made 

 with any pretension to accuracy, for want of the requisite data. 

 Still we can make such suppositions as will almost infallibly give 

 a result less than the truth, which will serve our present purpose 

 equally well. Let us suppose, then, the comet to be at its peri- 

 helion, and with the view of making the computation above 

 referred to, seek for some safe conclusion as to the length of the 

 tail at that point. 



In solving this inquiry, the first consideration that I would 

 present, is that the apparent length of the tail gives no certain in- 

 dication of its actual length. This is manifestly true when the 

 comet is seen in the day time, as was the recent comet on the 

 28th of February, for, we know that the part of the tail near the 

 head, when a comet is seen at night, is in general brighter than 

 the other parts. That we never see all of the actual tail, even at 

 night, may be inferred from the fact that the apparent length is 

 very different as seen from different places. For example, it is 

 said that " the tail of the comet of 1759, appeared at Paris to be 

 only 2° or 3° in length ; but at Montpelier 25°. The comet of 

 1769 at Paris seemed to have a tail 60° long ; but at Boulogne 

 70° ; and at the Isle of Bourbon 97°." The same inference 

 may be drawn from the circumstance that the light diminishes 

 gradually, from the head to the extremity of the apparent tail. 

 In fact, certain observations made upon the recent comet seem to 

 show, that the actual quantity of matter in a section perpendicu- 

 lar to the axis, was about the same at the extreme end of the tail 

 as in the vicinity of the head. Thus, on the evening of the 11th 

 of March, this comet was barely discernible at this place, ten 

 minutes after the three stars in Orion's belt came distinctly into 

 view, which would make its brightness about equal to that of a 

 star of the third magnitude. That it was not greater than this 

 is evident from the fact that the tail was less bright than the nu- 

 cleus, which had about the same lustre as the star Zeta Ceti, of 

 the third magnitude. It was observed also that the extremity of 

 the tail was about twice as broad as the most luminous part of it, 

 which was not far from one sixth of the length distant from the 

 nucleus : and I find that the ratio of the distances of these parts 

 from the earth, was very nearly as 3 to 2. From which it appears 

 that the tail was three times broader at its extremity than at the 



