360 
POPULAR SCIENCE REVIEW. 
and to suppose that comets, whether of hyperbolic or elliptic orbit, 
came to ns originally from the domain of another sun, is merely 
to suppose that that happened to such comets millions of years 
ago which we know to be happening to other comets at this 
present day, but not by any means to explain the nature of 
comets or their origin. We know that many comets leaving our 
system to visit others had not their origin within our system ; 
and we cannot assume as possible or even probable that any 
comet had its origin within the domain of another sun than 
ours, unless we assume as possible or probable that some among 
the comets leaving our own sun had their origin within our sun’s 
domain. 
Thus, then, we have been led to the conclusion that whether 
we adopt, with Schiaparelli and others, the theory that comets 
with meteoric systems can be drawn into the solar domain, or 
regard such an event as of very infrequent occurrence, we still 
find that the origin of comets must be looked for within solar 
systems ; or rather, since we cannot claim to trace back comets 
any more than planets or suns, to their actual origin, we may 
say that at an early period of their existence comets belonged to 
the solar system. The system has had no more occasion, so to 
speak, to borrow comets from other systems — that is, from other 
suns — than these have had to borrow comets from it and from 
each other. 
We decide, then, that comets may certainly be classified into 
those which belong to our solar system from the earliest period 
of their history, those which visit it from without, and pass 
away to other suns, and an intermediate class consisting of 
those which having visited it from without have been con- 
strained, by perturbations affecting them within it, to become 
attached permanently to its domain. We may note also that 
as there are comets now belonging to our solar system which 
originally belonged to other solar systems, so probably many 
comets originally belonging to our solar system are now either 
attending on other suns or wandering through the star-depths 
from sun to sun. 
It has been from viewing the matter in this way, recognising 
the almost decisive evidence that comets have from earliest 
times been members of our solar system, that I have been led 
to inquire into the possibility that some comets may have been 
expelled from the sun, and that others — those, namely, which 
seem attached to the orbits of the giant planets — may have been 
expelled from those planets when in their former sun-like con- 
dition. The evidence to show that there is an adequate ex- 
pulsive power in the sun is striking, and we may reasonably 
infer that the small suns formerly dependent upon him had a 
similar power. The motions of the members of the comet 
