70 1s (On, Cle EISIL ION 
from the sun by tidal friction ?——-Darwin has made familiar the 
principle of the transfer of the moment of momentum of a rotat- 
ing body to its satellite by his classic investigation of the 
evolution of the earth-moon system. Applying this principle 
to the solar system, is it possible to explain the low rotatory 
momentum of the sun and the high moments of momenta of 
the planets by a transfer of momentum from the former to the 
latter ? 
The most obvious and tangible effect of solar tidal friction 
on the planets is to destroy their rotations. The patent fact 
that most of them still retain high speeds of rotation is a 
physical expression of the limitations of past tidal action. 
Darwin has computed the rotational momenta of all the 
planets that afford the requisite data and also the revolutionary 
momenta of their satellites. Making a generous allowance for 
the unknown and uncertain factors and counting in unnecessarily 
the orbital momenta of the satellites, the whole internal 
momentum of the planetary systems falls short of a thousandth 
part of the sun’s rotational momentum computed on the minimum 
basis. This means that to have reduced the sun’s rotational 
momentum from twice the present amount to the existing status, 
and to have transferred this to the planets, more than a thousand 
times the total rotatory momentum of all the planets must have 
been destroyed. But this would be only a slight step toward 
the adjustment contemplated. 
To realize what might be necessary, if the foregoing nebular 
computations are well founded, let the matter of the solar system 
be converted into a gaseous nebula in hydrodynamic equilibrium 
extending beyond the orbit of Neptune; let this nebula be 
given the moment of momentum of the present solar system, 
and then let it contract by cooling, with the development of 
accelerated rotation, as postulated in the Laplacian hypothesis. 
An inspection of the foregoing data will show that the centrif- 
ugal force would not become equal to the centripetal force until 
the nebula had shrunk far within the orbit of Mercury. The 
*On the Tidal Friction, etc., pp. 519-523. 
