CLASSIFICATION OF COMETS. 
355 
perturbations to which they have been subjected within our 
system, to become members of it ; and as comets and meteor- 
systems have been found to be associated together in some 
mysterious way, this theory of the introduction of meteor- 
systems is in reality a theory of comets. Now since some 
•certainly among the meteor- systems have periods of moderate 
length, this theory of Schiaparelli’s would regard the short- 
period comets as drawn out of the interstellar depths, while 
manifestly it would be absurd not to extend Schiaparelli’s theory 
to hyperbolic comets. In fact, we know that he himself regards 
his theory as requiring the occasional appearance of meteors of 
hyperbolic path, and therefore as not merely consistent with the 
phenomena of hyperbolic comets, but accounting for them. 
Adopting his theory, then, to its fullest extent, we should 
regard all comets and meteors as bodies coming from the 
interstellar depth ; for it is not easy to see how any comet or 
meteor-system could be so far distinguished from its fellows as 
to be regarded as originally a member of the solar system. 
But for reasons which appear to me incontrovertible, I find 
it impossible to give in my adhesion to Schiaparelli’s views, in 
the form in which he presented them. A line ought to be 
carefully drawn between what has been proved and what has 
not been proved respecting the opinions which Schiaparelli has 
advanced. His most happy conception, that meteors would be 
found to travel in the paths of comets, has been realised, and no 
possible question can be raised as to the completeness of the 
demonstration ; but it is quite otherwise with his supposition 
respecting the manner in which meteoric systems or comets 
have been introduced into the solar system. It not only has 
not been proved that comets have been compelled by the 
perturbations of the planets to become permanent members of 
the solar system, but grave doubts rest on the bare possibility 
of such an event occurring. 
Let it be remembered that the conditions of the problem 
are purely dynamical. We know that a comet’s head obeys 
the laws of gravity, and whatever peculiarities may affect the 
motions of the matter of comets’ tails are not by any means 
such as would help to render easier the captures conceived by 
Schiaparelli. Confining ourselves then to gravity, we can 
determine readily in what way a comet might be captured. 
Take the case of a particle travelling towards our solar system 
from out the interstellar depths under the influence of the sun’s 
attraction. Such a particle may be regarded as practically 
approaching the sun from an infinite distance,* and we know 
* The point considered is the velocity of the particle at given distances 
from the sun ; and the estimated velocity is appreciably the same whether 
a a 2 
