Pairclough. — On the Constitution of Comets. 483 



whole of the vast orbit is peopled by flights of meteorites in endless chase, 

 and that the comet, as the gem of the ring, moves round preceded and 

 followed by a retinue of cosmic chips. That all comets are associated with 

 meteors, though fairly probable, cannot be confidently asserted. 



I must now come to the most difficult, but also the most fascinating, 

 part of the subject — the so-called tail of the comet. When this appendage 

 is shown at all, it is only while comparatively near the sun. A comet may 

 develope a tail in approaching the sun and show none after perihelion ; or it 

 may only develope tail after perihelion ; or it may show one both before and 

 after ; or neither before nor after. 



The nucleus of a comet is separated from the coma by a dark ring, as 

 the sunlit surface of a cloud might be separated from the earth by a band 

 of invisible air. The nucleus is similarly parted from the tail, which 

 appears to be an extension of the coma in the direction opposite to the sun. 

 The nose of the comet is like the flame of a torch blown back by the wind, 

 or like the apex of an upright jet of water from a fountain, where the 

 liquid turns to fall back. The theory now very generally accepted by 

 astronomers is that for some kinds of matter, or for matter in certain con- 

 ditions, the sun has a repulsive force far more potent than his power of 

 gravity ; that, under the influence of intense heat, jets of volatilized matter 

 are thrown out, perhaps in all directions, but certainly towards the sun ; 

 that presently the repulsive force overcomes their forward motion, turns 

 them back, and sweeps them away into space until the particles are so 

 widely dispersed as to be invisible. 



It was at one time hoped that Mr. Crookes' radiometer was about to 

 show us the repulsive force of the sun's rays at work in our very hands ; 

 but the dream vanished, and the repulsive force is still theory, although 

 Sir J. Herschel declares it is proved beyond question by his own observa- 

 tions and those of others. This theory also explains some other pheno- 

 mena, as the curvature of the tail, and the fact that the convex side of the 

 tail is the brightest and least curved. The convexity of the tail is always 

 towards the direction of the comet's motion. This has led to the gross 

 idea that, like the smoke of a steamer, the tail was retarded by the medium 

 in which the comet moved. 



If we conceive of a comet being a rigid body, and that the tail is swung 

 round as a walkingstick might be brandished by the handle, it will be 

 evident that the end of the tail will have much further to travel than the 

 head. But when matter is repelled from the nucleus, and from the sun, it 

 has exactly the same forward momentum as the nucleus ; as, therefore, it 

 is driven further and further from the sun, and has a larger and larger 

 orbit to describe, it of necessity falls behind, and cannot therefore be swung 



