THE NEBULAR HYPOTHESIS: ITS PRESENT CONDITION. 23 
their true volume ; and secondly, to the magnitude, or rather 
the mass, of the generality of comets. As regards the former, 
it has clearly arisen from the unfortunate association of the true 
gaseous nebulae with clusters of stars. Of the enormous dis- 
tances of the latter from us there can be no doubt ; but now 
that an entirely different class of objects has been proved to 
exist in the heavens, similar only in appearance to the most 
remote clusters, there can be no reason to suppose that these 
are at the like distances. Indeed, it would seem more probable 
that they are actually nearer to us, at least in some instances, 
than the nearest fixed stars themselves, for the enormous mag- 
nitude which must be attributed to such a mass as the great 
nebula in Orion, if it be supposed to be at a very much greater 
distance, must act as a bar to such an assumption. Indeed, 
the mind experiences a sense of relief in believing that the 
nebulae are our nearest neighbours, and the uniformity of na- 
ture, which does not offer to our contemplation masses of 
matter incomparably greater than those we have to deal with 
in the solar system, seems to require that the nebulae should 
not be conceived as of such surpassing magnitude. If this be 
Admitted, it brings these objects almost within the range of 
comets visiting the solar system ; but there are, doubtless, in 
the heavens numerous groups or congeries of comets similar to 
these, not to speak of those wanderers which move in parabolic 
or hyperbolic orbits, and which only require infinite time to 
travel infinite distance. Thus the wide gulfs separating star 
from star, and which appear only to exist to allow of the free 
revolution of stellar systems, may be the theatre not only of the 
movements of comets, but also of the evolution of new worlds. 
The belief in the insignificant mass of most comets, which is 
also, I think, open to question, is grounded on more substantial 
reasons. One comet (Encke’s) has actually been weighed 
against the smallest of our planets, Mercury, and has kicked the 
beam, but perhaps no more unfortunate instance for the experi- 
ment could have been selected than this. It is a comet without 
stellar nucleus, and one that has made so many revolutions 
round the sun, that supposing, as we have reason for doing, it 
loses some portion of its matter at each visit, it must clearly 
have been a very much wasted body at the time (1842) when 
Encke made its perturbations by the planet Mercury the sub- 
ject of his able researches. The only other comet that has 
given us a favourable opportunity of weighing it against one of 
the planets is Lexell’s, which in 1767 and 1779 must have ap- 
proached very near to Jupiter, without deranging to any extent, 
so far as we are aware, his system of satellites. Laplace has 
proved that this comet had certainly less than 3 -^ o^h part of 
the earth’s mass, but there is a very considerable difference be- 
