F. W. Very— A Cosmic Cycle. 109 



There is not a single example of a nebula with the requisite 

 difference of velocities in the line of sight. The lines in the 

 spectrum of a nebula are comparatively narrow. On the con- 

 trary, there are stars with all of the spectral lines broad and 

 hazy, indicating large rotary velocities. 



The small eccentricity of the planetary orbits forbids the 

 supposition that the planets have been thrown off from the sun 

 by an explosion pure and simple. Excessive rotary velocity 

 of the parent body is necessary to account for the zodiacal and 

 nearly circular paths of the planets. Extreme velocity of rota- 

 tion would not be expected in an immensely extended and 

 excessively attenuated nebula, but might increase from a feeble 

 beginning after concentration into a condensed nucleus. The 

 requisite rotary velocities, as we have just seen, are found to 

 be associated with the stellar stage of development. Moreover 

 the densities of the most rapid Algol binaries, which, by the 

 present hypothesis, have only recently passed the epoch of 

 stellar subdivision, and may be verging upon the condition 

 requisite for planetary formation, while smaller than that of 

 the sun, are not so small as to indicate any approach to the 

 nebular state. 



We have already seen that associated nebulae may be 

 explained as resulting from the dispersals accompaning explo- 

 sive disruption, and, besides, none of them indicate any orbital 

 motion such as we should expect if they had been separated by 

 centrifugal force. Consequently the planets seem to have 

 developed from the sun after it had reached the stellar condi- 

 tion which is not likely to occur with a density much less than 

 -gL- that of the present solar stage, if we may judge from the 

 probable densities of the Algol binaries. But this requires a 

 body of much larger mass than is now possessed .by the sun, 

 if its volume is to reach dimensions comparable with the orbits 

 of the outer planets. A certain amount of recession of a 

 planet from the sun may result from tidal interaction while 

 the planet is fluid and near the central body, but this cannot 

 account for the actual distances. 



"A numerical evaluation of the angular momentum of the 

 various parts of the solar system will afford the means of 

 forming some idea of the amount of change in the orbits of 

 the several planets and satellites, which may have been pro- 

 duced by tidal friction." "From the numerical values so 

 found, it is concluded that the orbits of the planets round the 

 sun can hardly have undergone a sensible enlargement from 

 the effects of tidal friction since those bodies first attained a 

 separate existence."* 



*G. H. Darwin, Nature, vol. xxiii, p. 389, 1881. See also Trans. R. S. London, 

 vol. clxxii, p. 527, 1881. 



