Astronomy. 161 
The rupture of the primeval planet into two parts is a matter 
of speculation, but if a planet and satellite be given in the initial 
configuration above described, then a system bearing a close 
resemblance to our own, would necessarily be evolved under the 
influence of tidal friction. 
theory postulates that there is not sufficient diffused matter 
to materially resist the motions of the moon and earth through 
pace. Sufficient lapse of time is also required. In a previous 
paper I showed that the minimum time in which the system could 
have degraded from one initial state, just after the rapture into 
two bodies, down to the present state, if fifty-four millions years. 
The time actually occupied by the changes would certainly be 
It was stated that the periodic times of revolution and rotation 
back to a common period 
of from two to four hours. In a previous paper the common 
: . P ; 
period was found to be a little over five hours in length; but that 
> 
The period of from two to four 
s mechanically impossible for 
the moon to revolve about the earth in less than two hours, an 
it 1s uncertain how the rupture of the primeval planet took place. 
But if tidal friction has heen the agent by which the earth and 
moon have been brought into their present configuration, then 
similar changes must have been going on in the other ies 
hich make up the solar system. I will therefore make a few 
the configuration of a pl and satellite is a destruction of 
energy (or rather its partial conversion into heat within the planet, 
and partial redistribution), and a transference of an m 
mentum from that of lanetary rotation to that of orbital revolu- 
ion of the two bodies about their common center of inertia, 
