68 
PROFESSOR G. H. DARWIR ON THE MECHANICAL CONDITIONS OF 
The total energy of agitation in an isothermal-adiabatic sphere is half the potential 
energy lost in the concentration from a condition of infinite dispersion. 
The half of the potential energy lost, which does not reappear as kinetic energy of 
agitation, is expended in volatilising solid matter and heating the gases produced 
on the impact of meteorites. The heat so generated is gradually lost by radiation. 
The amount of heat generated per unit time and volume varies as the square of 
the quasi-hydrostatic pressure, and inversely as the mean velocity of agitation. The 
temperature of the gases volatilised probably varies by some law of the same nature. 
The path of the meteorites is approximately straight, except when abruptly 
deflected by a collision with another. This ceases to be true at the outskirts of 
the swarm, where the collisions have become rare. The meteorites here describe 
orbits, under gravity, which are approximately elliptic, parabolic, and hyperbolic. 
In this fringe to the swarm the distribution of density ceases to be that of a gas 
under gravity, and, as we recede from the centre, the density at fii’st decreases more 
rapidly, and afterwards less rapidly, than if the medium were a gas. 
Throughout all stages of the history of a swarm there is a sort of evaporation, by 
which the swarm very slowly loses in mass, but this loss is more or less counter¬ 
balanced by condensation. In the early stages, the gain by condensation outbalances 
the loss by evaporation ; they then equilibrate ; and, finally, the evaporation may be 
greater than the condensation. 
Throughout the swarm the meteorites are partially sorted, according to size. As 
we recede from the centre, the number of small ones preponderates more and more 
and, thus, the mean mass continually diminishes with increasing distance. The loss 
to the system by evaporation fails principally on the smaller meteorites. 
A. meteor-swarm is subject to gaseous viscosity, which is greater the more widely 
diffused is the swarm. In consequence of this, a widely extended swarm, if in 
rotation, wdll revolve like a rigid body, without relative movement of its parts. 
Later in its history, the viscosity will, probably, not suffice to secure uniformity of 
rotation, and the central portion will revolve more rapidly than the outside. 
[The kinetic theory of meteorites may be held to present a fair approximation to 
the truth in the earlier stages of the evolution of the system. But ultimately the 
majority of the meteors must have been absorbed by the central Sun and its attendant 
planets, and amongst the meteors which remain free the relative motion of agitation 
must have been largely diminished. These free meteorites—the dust and refuse of 
the system—probably move in clouds, but with so little remaining motion of agitation 
that (except, perhaps, near the perihelion of very eccentric orbits) it would scarcely 
be permissible to treat the cloud as in any respects possessing the mechanical 
properties of a gas.*] 
The value of this whole investigation will appear very different to different minds. 
To some it will stand condemned, as altogether too speculative ; others may think that 
* Added Nov. 23, 1888. 
