THSL OF THE NEBULAR HYVPOTHE STS gat 
tidal problem then becomes the dispersal of the planets from 
this central position to their present places. 
Concerning the competency of the solar tides to alter the 
orbits of the planets (and hence their moments of momenta), 
Darwin says:* ‘“‘It may be shown that the reaction of the tides 
raised in the sun by the planets must have had a very small 
influence in changing the dimensions of the planetary orbits 
around the sun. From a consideration of numerical data with 
regard to the solar system and the planetary subsystems, it 
appears improbable that the planetary orbits have been sensi- 
bly enlarged by tidal friction since the origin of the several 
planets.” Again, he says:* “If the whole of the momentum 
of Jupiter and his satellites were destroyed by solar tidal fric- 
tion, the mean distance of Jupiter from the sun would only be 
increased by one twenty-five hundredth part. The effect of the 
destruction of the internal momentum of any of the other planets 
would be very much less.” sAnd again:3 ‘‘The present investi- 
gation shows, in confirmation of preceding ones, that at this 
origin of the moon the earth had a period of revolution about 
the sun shorter than at the present by perhaps only a minute 
or two, and it also shows that since the terrestrial planet itself 
first had a separate existence the length of the year can have 
increased but very little, almost certainly by not so much as an 
hour, and probably by not more than five minutes.” 
Aside from the quantitative difficulties there are formidable 
qualitative ones growing out of the proportional distances of the 
planets and the enormous lapses of time involved in a tidal 
retrogression of the planets through the postulated distances. 
Conclusions —The general result of the inquiry is to show, if 
we have not somewhere fallen into error, various relationships 
of mass and momentum which are seemingly altogether incom- 
patible with an evolution of the solar system from a gaseous 
spheroid controlled by the laws of hydrodynamic equilibrium 
‘Encyclopedia Brittanica, Article “ Tides,” p. 380. 
2On the Tidal Friction, etc., p. 524. 
3 Loc. cit., p. 533. 
