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tion in the form of a very long cone of light, called the tail of the 

 comet. 



The planetary body was at no time perceptibly otherwise than cir- 

 cular. Its apparent magnitude was about three quarters of a second, 

 and its real diameter is estimated at 428 miles. The position of this 

 body never appeared to be in the centre of the head, but to be more 

 or less eccentric at different periods of observation, but always more 

 remote than the centre from the sun. 



Nevertheless, the greatest illumination of the surrounding head is 

 represented by Dr. Herschel as greatest hi the centre, and in its de- 

 crease from thence to be somewhat brighter on the side toward the 

 sun than at the part occupied by the planetary body. The apparent 

 magnitude of the head was found to measure 3|- minutes ; so that 

 its real magnitude is estimated to have been 127,000 miles. 



Between the head and the surrounding envelope there was a space 

 comparatively dark, which Dr. Herschel imagines to be filled with 

 an elastic atmosphere, and estimates its actual extent to be at least 

 507,000 miles, since its apparent diameter was nearly 15 minutes of 

 a degree. 



The train of light, to which Dr. Herschel gives the name of en- 

 velope, from its surrounding the head on one side in a semicircular 

 form, was found to measure 19 minutes of a degree in diameter, and 

 was thence inferred to be 643,000 miles in real extent. 



The two extremities' of this curve being continued beyond the head 

 in two streams of light, rather divergent from each other, form the 

 appearance which is called the tail. The distance to which this ap- 

 pears to reach from the head varied on different nights, according to 

 the state of the atmosphere, as well as from other circumstances 

 which affected its actual length. The greatest length observed by 

 Dr. Herschel was on the 6th of October, when he measured it 25 ; 

 but he thinks the measure of .23^, taken on the 15th of October, 

 more to be depended upon ; and he thence computes the actual 

 length to have been at that time 100,000,000 of miles. 



With respect to the curvature of the tail, Dr. Herschel remarks, 

 that it varied not only in degree, but in direction ; for on the 2nd of 

 December he observed that it appeared convex on the following side, 

 as if the extremity of the tail preceded the head instead of being left 

 behind. 



The author also notices many other irregular appearances of the 

 tail, the branches on each side occasionally dividing into two or three 

 parts, and sometimes one branch, sometimes the other, seemed longest. 



From the appearances observed, Dr. Herschel next infers what is 

 the real construction of the various parts. And, first, the planetary 

 body seems to be spherical, as might be expected from the common 

 laws of gravitation, and to shine by light of its own ; for if it were 

 not so, it must have appeared to change its figure hi moving as it 

 did through more than a quadrant while it remained visible. The 

 head also must, for the same reason, be spherical : and so likewise 

 that portion of the envelope which is on the side towards the sun 

 2 E 2 



