2 
PROFESSOR G. H. DARWIX OX THE MECHAXICAL COXDITIOXS OF 
the researches of Mr. Lockyer afford actual evidence in favour of the abundauce of 
meteorites in space. 
But the very essence of the nebular hypothesis is the conception of fluid pi’essure. 
since without it the idea of a figure of equilibrium becomes inapplicable. Kow, at 
first sight, the meteoric condition of matter seems absolutely inconsistent with a fluid 
pressure exercised by one part of tlie system on another. We thus seem driven either 
to the absolute rejection of the nebular hypothesis, or to deny that the meteoric con¬ 
dition was the immediate antecedent of the Sun and Planets. M. Faye has taken the 
former course, and accepts as a necessary consequence the formulation of a succession 
of events quite different from that of the nebular hypothesis.* I cannot myself find 
that his theory is an improvement on that of Laplace, except in regard to the adop¬ 
tion of meteorites, for he has lost the conception of the figure of equilibrium of a 
rotating mass of fluid. 
The object of this paper is to point out that by a certain interpretation 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 nebular hypothesis is, in fact, the 
resultant of countless impacts of meteorites. 
The problems of hydrodynamics could hardly be attacked with success, if we wmre 
forced to start from the beoirminof and to consider the cannonade of molecules. But 
when once satisfied that the kinetic theory will give us a gas, wdiich, in a sjaace 
containing some millions of molecules, obeys all the laws of an ideal non-molecular 
gas filling all space, we may put the molecules out of sight and treat the gas as a 
plenum. 
In the same wa.y, the difflculty of tracing the impacts of meteorites in detail is 
insuperable ; but, if we can find that such impacts give rise to a quasi-fluid pressure on 
a large scale, we may be able to trace out many results by treating an ideal plenum. 
Laplace’s hypothesis implies such a plenum, and it is here maintained that this 
plenum is merely the idealisation of the impacts of meteorites. 
As a bare suggestion this view is worth but little, for its acceptance or rejection 
must turn entirely on numerical values, which can only be obtained by the considera¬ 
tion of some actual system. It is obvious that the solar system is the only one about 
which we have sufficient knowledge to afford a basis for discussion. This paper is 
accordingly devoted to a consideration of the mechanics of a swarm of meteorites, with 
special numerical application to the solar system. 
The investigation has entailed a considerable amount of mathematical analysis ; 
* ‘ Sur rOrigine du Monde,’ Paris, Gauthiee-Villaes, 1884; ‘ Annnaire pour I’an 1885, Bureau des 
Longitudes,’ p. 757. 
