CLASSIFICATION OF COMETS. 
353 
Comets, then, must be classified in some other way. It is not 
difficult to select the proper method of classification — a method 
not only satisfactory as respects the distinctions on which it de- 
pends, but exceedingly suggestive (as, in fact, every just mode 
of classification may be expected to be). 
I would divide comets into three classes, according to the 
nature of their paths. 
First, there are the comets which have paths so moderate in 
extent that their periods of revolution belong to the same 
order as the periods in which the planets revolve around the 
sun. This class includes all the comets which have been 
described as Jupiter’s comet-family, and all those similarly 
related to Saturn, to Uranus, and to Neptune. Other comets 
of somewhat greater period than Neptune’s comet-family may 
perhaps be regarded as associated with as yet undiscovered 
planets revolving outside the path of Neptune, and therefore as 
belonging to the same family. I would not, however, attempt 
to define very narrowly the boundary of the various classes into 
which comets may be divided, and in what follows I shall limit 
my remarks to comets which are clearly members of one or 
other class, leaving out of consideration those respecting which 
(for want, perhaps, of more complete information than we at 
present possess) we may feel doubtful. 
Secondly, there are comets of long periods, but which yet 
show unmistakably, by their motions, that they are in reality 
members of the solar system — such, for instance, as Donati’s 
comet, which may be expected to return to the sun’s neighbour- 
hood in the course of about two thousand years. 
Lastly, there are the comets whose motions indicate a path 
not re-entering into itself. These are of two orders : those 
which retreat from the sun on a path tending with continual 
increase of distance to become more and more nearly parallel 
to the path by which they had approached him ; and those 
whose retreating path carries them divergingly away so that 
they retreat towards a different part of the heavens than that 
from which they arrived. Technically, the two orders are those 
of comets pursuing (i.) parabolic and (ii.) hyperbolic paths. In 
reality, however, we may dismiss the parabolic path as never 
actually followed by any comet, any more than a truly circular 
path is ever actually followed by any planet. We may take it for 
granted that any comet which seems to follow a parabolic path 
really follows either an enormously elongated oval path, and so 
belongs to our second class ; or a path carrying it for ever away 
into outer space, and nearly in the direction from which it had 
arrived, but not exactly . A comet’s path could only have the 
true parabolic form by a perfect marvel of coincidence ; and in 
point of fact if a comet could by some amazing chance approach 
VOL. XIII. NO. LIII. A A 
