1904-5.] Dr J. Halm on the Nebular Hypothesis. 555 
rotation in the solar system by one-sided impacts of a meteoric 
cloud upon the solar nucleus. We must admit, on dynamical 
grounds, that the partial destruction of the orbital velocities of the 
meteors involves the generation of closed orbits round the star as 
focus, and also that one-sidedness of the impacts leads to the pre- 
ponderance of a distinct direction of rotation. It may, however, 
seem difficult at first sight to understand how a system, in which 
the outer orbits must have possessed large eccentricities, should 
have developed into one in which all the bodies move now sensibly 
in circles. But on closer examination this difficulty seems to be 
lessened. Professor Darwin, in his essay on “ The Mechanical 
Conditions of a Swarm of Meteorites and on Theories of Cos- 
mogony,” in the Transactions and Proceedings of the Royal Society 
for 1888, has proposed an ingenious thermodynamical theory of 
meteoric matter based on the laws of the kinetic theory as 
ordinarily applied to gases. One point of his investigation refers 
to the viscosity of such an agglomeration of meteoric substance, 
which he finds to be remarkably great. His conclusion suggests 
that friction must have largely influenced the orbital motions of 
the ring-particles. The passage in Professor Darwin’s paper 
which has a direct bearing on this point may here be quoted : — 
“The very essence of the nebular hypothesis is the conception 
of fluid-pressure, since without it the idea of a figure of equilibrium 
becomes inapplicable. How, at first sight, the meteoric condition 
of matter seems absolutely inconsistent with a fluid-pressure 
exercised by one part of the system on another. We thus seem 
driven either to the absolute rejection of the nebular hypothesis, 
or to deny that the meteoric condition was the immediate ante- 
cedent of the sun and the planets. The object of this paper [ Proc . 
Roy. Soc., vol. 45, p. 4] is to point out that by a certain interpreta- 
tion of the meteoric theory we may obtain a reconciliation of these 
two orders of ideas, and may hold that the origin of stellar and 
planetary systems is meteoric, whilst retaining the conception of 
fluid-pressure. According to the kinetic theory of gases, fluid- 
pressure is the average result of the impacts of molecules. If we 
imagine the molecules magnified until of the size of meteorites, 
their impacts will still, on a coarser scale, give a quasi-fluid- 
pressure. I suggest, then, that the fluid-pressure essential to the 
