558 Proceedings of Royal Society of Edinburgh. [sess. 
former must gradually overtake the latter. If, for instance, one 
of the bodies possesses the mass of Jupiter, while the other body 
has only half this mass, their periods would be in the ratio 
1 : 1 ’000237 ; and hence if the bodies had been 180° apart in the 
beginning, they would be at identical points of the common orbit 
in about 14,000 years. This reasoning shows clearly that the 
planet, after having attained a portion of its mass through 
accretion, must gradually bring under its gravitational influence 
the smaller masses revolving in its orbit. The planet would 
therefore evacuate its own ring. But if we accept Professor 
Darwin’s conception of fluid-pressure, the idea of a vacuum cannot 
be maintained. The gap round the planet would be constantly 
filled up by meteors rushing into the planet’s orbit from the 
outer and inner parts of the ring. The planet would act some- 
what like a powerful air-pump, sucking in the meteoric molecules 
thrown into its sphere of gravitational attraction by the outside 
collisions. We may also gather from the mode in which the 
planet acts on the particles in its front and rear that the motions 
of those meteors which escape amalgamation with the attracting 
nucleus are deflected either towards or from the sun. This, no 
doubt, must increase the chance of collisions with the inner and 
outer portions of the ring. It is also understood that the in- 
creasing diversity of motions of the smaller' meteors may assist 
the planets in their function of incorporating the small fragments 
thrown into their paths. There should be a gradual approach 
towards conditions such as we notice at the present moment when 
we find meteors crossing the earth’s orbit in all possible directions. 
I am far from saying, however, that all the present meteors should 
be considered in this way as the last remnant of the original ring. 
A point of extreme difficulty in Laplace’s hypothesis is the 
explanation of the present slow rotation of the sun. Mr Moulton * 
has demonstrated that if the solar nebula had contracted in the way 
Laplace assumed, the moment of momentum of the solar system 
should be more than two hundred times what it actually is ; but,, 
on the other hand, if the nebula had always possessed its present 
*F. R. Moulton, “An Attempt to Test the Nebular Hypothesis by an 
Appeal to the Laws of Dynamics,” Astrophysical Journal , vol. xi., 1900, 
p. 103. 
