EVOLUTION OF THE SOLAR SYSTEM. 
535 
fate of all satellites, because the solar tidal friction retards the planetary rotation 
without directly affecting the satellite’s orbital motion. 
The numerical comparison in Table III. shows that the efficiency of solar tidal 
friction in retarding the terrestrial and Martian rotations is of about the same degree 
of importance, notwithstanding the much greater distance of the planet Mars. 
From the discussion in this paper it will have been apparent that the earth and 
moon do actually differ from the other planets in such a way as to permit tidal friction 
to have been the most important factor in their history. 
By an examination of the probable effects of solar tidal friction on a contracting 
planetary mass, we have been led to assign a cause for the observed distribution of 
satellites in the solar system, and this again has itself afforded an explanation of how 
it happened that the moon so originated that the tidal friction of the lunar tides in 
the earth should have been able to exercise so large an influence. 
In this summary I have endeavoured not only to set forth the influence which tidal 
friction may, and probably has had in the history of the system, but also to point out 
what effects it cannot have produced. 
The present investigations afford no grounds for the rejection of the nebular hypo¬ 
thesis, but while they present evidence in favour of the main outlines of that theory, 
they introduce modifications of considerable importance. 
Tidal friction is a cause of change of which Laplace’s theory took no account/''' and 
although the activity of that cause is to be regarded as mainly belonging to a later 
period than the events described in the nebular hypothesis, yet its influence has been 
of great, and in one instance of even paramount importance in determining the present 
condition of the planets and their satellites. 
* Note added on July 28, 1881. 
Dr. T. R. Mater appears to Lave been amongst tbe first, if not quite the first, to draw attention to the 
effects of tidal friction. I have recently had my attention called to his paper on “ Celestial Dynamics” 
[Translation, ‘Phil. Mag.,’ 1863, vol. 25, pp. 241, 387, 417], in which he has preceded me in some of the 
remarks made above. He points out that, as the joint result of contraction and tidal friction, “ the whole 
life of the earth therefore may be divided into three periods—youth with increasing, middle age with 
uniform, and old age with decreasing velocity of rotation.” 
