I9I0.] ABOUT THE FIXED STARS. 225 



If a very flat disc could exist as a figure of equilibrium, it was 

 natural to imagine that such figures might have had a part in start- 

 ing the planets in their round orbits. The great roundness of the 

 orbits of the major planets and of the satellites then known, and 

 their uniform direction of motion near a common plane suggested 

 to Laplace that these orbits had once been nearly circular, and that 

 the bodies had developed from rings like those of Saturn. It 

 appeared to him that they had resulted from the condensation of 

 rings of vapor gently detached from the equators of the bodies which 

 now govern their motions. This reasoning of Laplace was logical, 

 and necessarily resulted from the researches of Newton on the fig- 

 ures of equilibrium of rotating masses of fluid, and the subsequent 

 extension of these researches by Maclaurin, D'Alembert and other 

 mathematicians ; and the procedure seemed so plausible that its cor- 

 rectness was assumed by all subsequent investigators. 



Thus Lord Kelvin, Newcomb, Darwin, Tisserand, Poincare and 

 others accepted the principles of Laplace as laid down in his 

 formulation of the nebular hypothesis, and proceeded to work out 

 the details of planetary development. It is true that Kirkwood, 

 Pierce and others had made objections to the Laplacian hypothesis, 

 based on the inability of a medium so rare as the postulated nebula 

 to transmit hydrostatic pressure from the center outwards, but such 

 destructive criticism was of little avail so long as the roundness 

 of the orbits could be explained only by Laplace's hypothesis of 

 detachment. The persistence of Laplace's classic nebular hypothe- 

 sis, in spite of negative criticism, was therefore inevitable. But 

 as the greatest mathematicians were unable to make out the process 

 of planetary formation, on the detachment theory, the whole subject 

 remained one of contradiction and obscurity. In his address to 

 the British Association at Capetown, in 1905, Professor Sir G. H. 

 Darwin said : 



The telescope seems to confirm the general correctness of Laplace's 

 hypothesis. . . . Nevertheless it is hardly too much to say that every stage in 

 the supposed process presents to us some difficulty or impossibility. Thus we 

 ask whether a mass of gas of almost inconceivable tenuity can really rotate 

 all in one piece, and whether it is not more probable that there would be a 

 central whirlpool surrounded by more slowly moving parts. Again, is there 

 any sufficient reason to suppose that a series of intermittent efforts would 



PROC. AMER. PHIL. SOC.., XLIX, I95 O, PRINTED JULY 29, I9IO. 



