8 Dr. L. N. G. Filon on Comets : 
particles of which the comet is composed to overcome the solar 
attraction. 
Whatever may be the cause, there is no doubt that the tail is 
due in some way to the solar action, for the comet only acquires 
its tail when it gets within a certain distance of the sun, and it 
loses it as it leaves the solar neighbourhood. 
Multiple Tails. — The repulsion theory derives considerable 
support from the fact that a very large number of comets have 
several tails (see Fig. 5). Now it seems probable that the pressure 
of light is independent of the material on which it is exerted, 
so that, other things being equal, heavy matter will be less 
forcibly repelled than light matter. Hence we see why it is 
that the material of a comet's tail should sort itself into several 
streamers. : 
Bredichin classified comets’ tails into three main types (shown. 
on Fig. 5). The straight tails, which point nearly away from the 
sun, and which one would expect to be composed of the lightest 
matter, he assumed to consist chiefly of hydrogen. 
The middle tail, which is usually the brightest and is much 
more curved, he supposed to contain gases which are known as 
hydrocarbons—e. g. the gas in the blue part of an ordinary gas 
or candle flame. 
On examining the light from these tails with a spectroscope, 
the existence of some such gas, in the middle tail, has been 
established beyond qiestion. 
The hydrogen tails are usually too faint for their light to be 
analyzed, and so this hypothesis still remains doubtful. 
Finally, in certain cases a third tail can be observed—a short 
brush—still more strongly curved than the main tail. : 
Bredichin states that such a tail would be composed of iron 
and sodium; but these tails are very rare, and all the evidence 
we have on this subject is, that the great comet of 1882, when 
it approached the sun, brightened up considerably, and its light 
showed evidence of the presence of iron and sodium and other 
metals. 
The Speed of Comets.—The speed of comets is highly variable. 
When at a large distance from the sun they move very slowly, 
quickening up their pace as they approach nearer. 
The comets of 1668, 1848, 1880, and 1882 all came very close 
to the sun, and went round it at a tremendous speed. The comet 
of 1848 circled round the sun, at perihelion, at a distance from 
the centre less than the solar diameter, and went through a half- 
turn in something under three hours, corresponding to a maximum 
speed of 820 miles per second. 
Again, the comet of 1882 went round the sun so fast that its 
tail could not follow it, and parted company; so that the comet, 
like the sheep of the story, left its tail behind it, but when it re- 
