126 On the Formation of the Tails of Comets. 



this line. Nor is it necessary that there should be a rotatory mo- 

 tion in the plane of the orbit, so as to keep the first equator con- 

 stantly in the same situation with respect to the radius-vector. 

 Any change in this situation would only alter the rate of revolu- 

 tion and consequently the degree of divergence of the sides of 

 the tail, the laws of the variation of which are not known. Still, 

 if such a motion be supposed to subsist, we have an explanation 

 in the centrifugal force of the first mentioned rotation, (in case 

 its axis lies nearly in the plane of the orbits,) of the fact of the 

 closer proximity of the nucleus to the nebulosity on the side 

 towards the sun : and Bessel has, in point of fact, seen reason to 

 infer its existence in the case of Halley's comet in 1835.* 



As to the variation in the velocity with which a particle flows 

 off into space, it is evident that the tendency of the force must 

 be to make the velocity increase continually, but more rapidly at 

 first than afterwards. It will be seen too, that, whether we sup- 

 pose the nucleus to have a repulsion for the matter that is moving 

 away from it or not, the variation of the velocity at any consid- 

 erable distance must be mainly due to the repulsive power of the 

 sun. But if it be a fact that the matter which enters into the 

 constitution of the tail, is, for a considerable portion of the length 

 uniformly distributed, as we have seen reason to suppose was the 

 case, or very nearly the case with the comet of 1843, then the 

 particles must soon attain to a maximum velocity, and continue 

 afterwards to move on at the same rate. This might nearly hap- 

 pen in case of a very rapid reduction of the force ; but the more 

 probable conception would be that the limitation to the velocity 

 was an effect of the resistance of an ether in space. Such a resist- 

 ance would, however, afterwards diminish the velocity as the 

 force decreased, (though in a less rapid ratio,) unless the density 

 of the ether decreased at the same rate that the force did. 



I conceive that the tail terminates, to us, when from the in- 

 crease of its tenuity in consequence of the divergence of the sides 

 and of the augmentation of the velocity of the flowing particles, 

 (and from the increase in the distance from the sun, if this lumi- 

 nary is the source of the light of the comet,) its light becomes 

 too feeble sensibly to affect our eyes. It augments in brightness 



* See Vol. xlv, p. 206, of this Journal. The greater divergence of the sides of 

 the tail than of the lines of direction of the sun's force, and the form of the neb- 

 ulosity, are natural consequences of Olbers' theory of a repulsion from the nucleus. 



