EVOLUTION OP THE SOLAR SYSTEM. 
525 
present surface of the earth. The preceding numerical comparisons suggest that the 
contraction of the planetary masses has elsewhere been the more important factor, and 
that the genesis of satellites occurred elsewhere earlier in the evolution. 
It has been shown that the case of the earth and moon does actually differ widely 
from that of the other planets, and we may therefore reasonably suppose that the 
history has also differed considerably. 
Although we might perhaps leave the subject at this point, yet, after arriving at 
the above conclusions, it seems natural to inquire in what manner the simultaneous 
action of the contraction of a planetary mass and of tidal friction is likely to have 
operated. 
The subject is necessarily speculative, but the conclusions at which I arrive are, I 
think, worthy of notice, for although they involve much of mere conjectural assumption 
in respect to the quantities and amounts assumed, yet they are deduced from the 
rigorous dynamical principles of angular momentum and of energy. 
§ 8 . On the part played by tidal friction in the evolution of planetary masses. 
To consider the subject of this section, we require— 
(a) Some measure of the relative efficiency of solar tidal friction in reducing the 
rotational momentum and the rotation of the several planets. 
(/3) We have to consider the manner in which the simultaneous action of the 
contraction of the planetary mass and of solar tidal friction co-operate. 
(y) We have to discuss how the separation of a satellite from the contracting mass 
is likely to affect the course of evolution. 
It is not possible to treat these questions rigorously, but without some guidance on 
these points further discussion would be fruitless. 
The probable influence of the heterogeneity of the planetary mass on tidal friction 
has been already discussed, and it has been shown that the case of homogeneity will 
probably give good indications of the result in the true case. I therefore adhere here 
also to the hypothesis of homogeneity. 
I will begin with (a) and consider— 
The relative efficiency of solar tidal faction. 
The rate at which the rotation of any one of the planets is being reduced is 
T~{n — T2) (Jjp, where n, 5 , |] refer to the planet, and are the quantities which were 
previously indicated by the same symbols accented. 
r is Ulffi, and therefore varies as fT. With all the planets (excepting, perhaps, 
Mercury and Venus, according to Herschel) f2 is small compared with n, and we may 
write n for n—I2. 
3 y 2 
