AO4 Seientific Intelligence. 
in contact with the present surfaces of their planets, as was shown 
in previous papers to be probably the case with the moon and 
rth. 
The numerical values spoken of above exhibit a very striking 
difference between the condition of the earth and the moon and 
that of these bet planets, and it may therefore be admitted that 
their modes of evolution have also differed considera 
The part played by tidal friction in the evolution of planetary 
masses is then discusse 
A numerical comparison is made of the relative efficiency of 
solar tidal friction in reducing the rotational momentum and the 
rotation of the ik es a ea It is found that the efficiency as 
regards the rotation is nearly the same for Mars and for the 
earth, notwithstanding the greater distance of the former from 
the earth. is point is important with reference to the rapid 
revolution of the inner nel wi of Mars, and confirms the explana- 
tion of ee fact, which has been offered in a previous paper. 
ers expressive ice the relative efficiency of solax tidal 
existed much longer than the interior ones. Nevertheless the 
disproportion between the numbers is so great that it must be 
held that the influence of solar tidal friction on J eo and Sat- 
urn has been considerably less than on the nearer plan 
The manner in which tidal friction and the Scatition of a 
planetary mass would work together is then considered, and it is 
found to be pr sobabie that tidal friction was a more important 
cause of change when the masses were less condensed than it 1s 
at _mpei thus the present rate of action of solar tidal friction 
is not to be taken as a measure of what has existed in all past 
ime. 
This diseassion leads the author to assign a cause for the ob- 
served distribution of satellites in the solar system For if, as the 
nebular hypothesis supposes, satellites are formed when instability 
is produced by the acceleration of rotation accompanying con- 
traction, then the epochs of instability would recur more rarely 
if tidal friction were operative than without it; and if tidal 
friction were sufficiently powerful, an epoch of instability would 
neve 
The efficiency of solar tidal —— apres. as we recede 
from the sun, and —— re planets near the sun should have no 
ciseiilaae, and the number of s atelliane should increase for th 
remoter planets. This is the Soeeaces condition of the solar 
stem 
This theoretical otha is also shown to explain how the earth 
and moon came to di Lapa the other planets in such a manner 
as to permit tidal fric tion o be the principal feature in their evo- 
lution, while its effects are ae a in the other planets. 
