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LXVIII. On Comets and their Tails. 

 To the Editors of the Philosophical Magazine and Journal. 



Gentlemen, 



THE observations of the comets' orbits have, I suppose, 

 abundantly proved that the apparent nucleus is the centre 

 of gravity, and therefore that there is an equal quantity of mat- 

 ter on all sides of the nucleus, and not, as appears to the eye, 

 that the luminous tail is the only matter which radiates from 

 the centre. A comet, then, is a spherical mass of nebulous mat- 

 ter, of which the radius is the length of the visible tail, or even 

 longer. If the matter is compressible, the force of gravitation 

 will make it more dense towards the middle ; the middle will 

 thus have more power of reflecting the sun's light, and be more 

 luminous than the parts at a distance from the middle. This 

 will explain the nucleus and the nebulous appearance around it. 

 It remains for us to inquire what are the circumstances which 

 can be supposed to make the less dense portion of the ball lumi- 

 nous on one side only, and give to it the appearance of a tail on 

 the side furthest from the sun, as in our fig. 1. 



Fig. 1. Fig. 2. 



AB BB JS 



Let A in fig. 2 be a ray of light passing from the sun direct 

 towards the centre of a ball of nebulous matter. It will not be 

 deflected on either side, but will mark the central axis of our 

 ball. 



Let B be any other ray parallel to the former until it enters 

 the ball. This will be refracted towards the central axis as it 

 passes from each rarer stratum of our ball into one more dense. 

 Let us suppose it to be refracted only so much as to approach 

 the central axis at a place beyond the nucleus. It will then quit 



