556 Proceedings of Royal Society of Edinburgh. [sess. 
nebular hypothesis is in fact the resultant of countless impacts of 
meteorites.” 
In applying this idea of a kinetic theory of meteors to the 
present problem, we have to consider the conditions prevailing 
in a system which consists of a non-rotating nucleus surrounded 
by a gyrating ring of meteoric substance. The conception of 
fluid-pressure, as proposed by Professor Darwin, involves the 
assumption of friction between star and ring. The star’s surface 
being continually bombarded by neighbouring ring-particles, rotary 
momentum is imparted to the star, and is consequently lost by 
the ring. The motion of the inner ring is thus gradually reduced, 
in much the same way as that of an air-current passing along 
the earth’s surface. The friction being propagated throughout the 
whole ring in accordance with laws similar to those of the internal 
friction in gaseous media, the materials of the ring will be con- 
stantly submitted to resisting forces acting in the direction of their 
motion. Hence, in course of time, the eccentricity of the ring, 
as a whole, must be lessened, and the system will tend towards 
a figure of equilibrium consistent with fluid-pressure. 
This reasoning has brought us to the state of matters from 
which Laplace started his hypothesis. We see now some possi- 
bility, at least, how, by accepting this new hypothesis, we may ex- 
plain the introduction of rotation into our system without abandon- 
ing any of the Laplaceian conclusions. So far the present view 
may therefore be considered merely as an extension of Laplace’s 
cosmogonic conceptions. But in consideration of the grave objec- 
tions raised against the nebular hypothesis in its present form, it 
may seem advisable to trace also the further development of the 
rotating fluid, and to see whether the difficulties expressed by 
Mr Moulton and others are indeed so insurmountable as they 
appear. One of the most serious objections refers to the formation 
of the planetary rings. The intermittent shedding-off of annular 
aggregates, which Laplace assumes, is a process not easily adapt- 
able to our conception of the physical properties of meteoric 
matter. But I think this hiatus may be avoided. Little doubt 
can be felt regarding the assumption that the original ring must 
have been heterogeneous. Granting this, we must admit the 
existence of nuclei of condensation within the ring attracting the 
