4 Prof. G. H. Darwin. On the Mechanical [Nov. 1 5, 



But 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. Now, 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 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 adoption 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 interpre- 

 tation 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 mole- 

 cules 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 were forced to start from the beginning' and to consider 

 the cannonade of molecules. But when once satisfied that the kinetic 

 theory will give us a gas, which, in a space 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 way the difficulty 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 

 ba obtained by the consideration 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. The paper, of which this 

 is an abstract, is accordingly devoted to a consideration of the 



* 'Sur 1'Origine du Mor.de/ Paris, Gauthier-Villars, 1884. 'Annuaire pour 1'an 

 18S5, Bureau des Longitudes,' p. 757. 



