\ 
6 Dr. L. N. G. Filon, on Comets : 
a comet is found to be constantly directed away from the sun, so 
that as the comet moves away from the latter, the tail precedes it. 
Thus the analogy of the locomotive smoke would be improved if 
we suppose a wind, directed from the sun, always blowing the 
smoke (tail) outwards. 
This hypothesis is confirmed by the fact that the tail is usually 
not straight, but somewhat curved, because the nucleus of the 
comet, which is nearer the sun, moves faster than the tail, which 
1s More remote. 
The formation of this tail is explained by the supposition that 
the nucleus throws out matter towards the sun (as we have seen 
to be the case with the jets and envelopes), and that this matter 
is somehow repelled by the sun. 
_ The tail of a comet is thus analogous to a vertical jet of water 
in a garden, the drops curving outwards and falling under the 
Earth's attraction. This is illustrated by Fig. 4, which is also 
Fic. 4.—ForMATION OF A Comet’s TatL By MATTER EXPELLED 
FROM THE HEAD. 
taken from Professor Young’s very instructive treatise. When 
viewed sideways, the drops will appear to be more numerous on 
either side, even though they really are distributed symmetrically, 
and thus we have the explanation of the dark streak behind the 
nucleus. 
Cause of the Repulsion—The question, how is this repulsion © 
produced ? remains a very vexed one. 
One theory brought forward is that the sun and the particles 
of the comet’s tail are both electrified. Bodies charged with 
electricity of the same kind repel one another. 
Another cause suggested is the pressure of light. According 
