1889.] of the various Species of Heavenly Bodies. 209 



but is a steady growth outwards. The tail of a comet is always 

 directed away from the sun, so that it sweeps round in a semicircle 

 as the comet passes through perihelion. The apparent repulsion of 

 the tails suggested to Olbers in 1812 the idea that the materials com- 

 posing them are subject to electrical repulsion proceeding from the 

 sun, that they consist, in fact, of small electrified particles repelled by 

 the similarly electrified sun. 



As a rule, the tail increases very quickly and considerably in 

 length after perihelion passage. Thus Borelly's Cornet of 1874 in- 

 creased from 4 to 43^ in length from July 3rd to July 19th in that 

 year, or from 4 millions to 25 millions of miles in length.* This effect 

 is precisely what we should expect if the tail be fed by vapours due 

 to collisions, for at perihelion the tidal action, and therefore the 

 interior movements, will be greatest ; besides which it is probable 

 that collisions with meteorites external to the swarm will here be 

 more frequent and more heat-producing on account of the highest 

 velocity of the comet. 



M. Bredichin, of the Moscow Observatory, has shown that there are 

 three distinct types of tails. In the first class, the tails are long and 

 straight, and the repellent energy of the sun upon the small particles 

 is about twelve times as great as the energy of his gravitational 

 attraction. The particles therefore leave the nucleus with a high 

 velocity, generally about 14,000 or 15,000 feet per second. The 

 greater this velocity in relation to the rate of travel of the comet, the 

 straighter of course will be the tail, because the particles forming it 

 do not lag behind. In the second type, the energies of the attraction 

 and repulsion balance each other, or nearly so, and the tails of this 

 class are plumy and gently curved. In this case the particles which 

 go to form the tail leave the head with a velocity of about 3000 feet 

 per second. 



Tails of the third type are short and strongly bent, the repellent 

 energy being only about one-fifth of the attractive energy of the sun, 

 and the velocity of the particles leaving the head is only about 1000 

 feet per second. 



Many comets exhibit tails of more than one type, and it was con- 

 jectured long ago that such tails were composed of different kinds of 

 matter. 



Bredichin went further, and defined the composition of the different 

 kinds of tails which he had classified, by referring to the weights of 

 the materials which would give the relative values of the repulsive 

 and attractive forces necessary for tails of the different types. He 

 thus found that the long straight tails of the first type would be 

 probably formed by hydrogen, since this substance, on account of its 

 exceeding lightness, would be little influenced by gravity, while at 

 * Hind, * Nature,' vol. 10, p. 252. 



