¢ 
aI 
some remarkable results. When the moon was near the earth 
it was making a revolution in three to five hours. When the 
moon was in this position the earth must have been rotating also 
in about three to five hours—a striking coincidence in itself, which 
pointed to the moon having parted from the earth when she was 
molten and rotating at this rapid rate. Calculations in other 
directions resulted, however, in further coincidences. It was 
found that the critical speed of rotation of a molten body the size 
and mass of the earth, at which it would be likely to fly to pieces, 
‘was somewhere between three and five hours. Then, again, tides 
would be formed in this molten earth by the sun, distorting it 
twice in each revolution ; but the natural period of vibration of 
such a body was found to be about two hours. The solar tides 
therefore were acting as a carefully timed impulse would act on a 
free pendulum, and were assisting the tendency to fly to pieces. 
The following, then, can be said to have been the history of the 
earth and moon. As theearthcooled and contracted its velocity of 
rotation increased until it became instable and a portion separated. 
The day and month were then equal and of about three to five 
hours long. By tidal action of the infant moon on the earth the day 
was lengthened, the moon travelled away, and the month became 
as it is at present. At least 90,000,000 years (and probably much 
more) must have been occupied in this process. This will go on 
until the day and month are again equal, and of about fifty-five 
of our present days in length. After this solar tides will further 
lengthen the day and the moon will again approach the earth, a 
condition of things that seems to obtain at present with Mars and 
his satellite. The lecturer gave reasons for thinking that in the 
case of no other planet of our system had the same process of 
formation of moons been followed. 
$04 
