all that I shall say in these lectures. I must be
myself accountable for the way in which the subject is being treated,
as well as for many of the illustrations used, and some of the
deductions I have drawn from the subject.

It is almost unavoidable for us to make a surmise as to the cause by
which the moon had come into this remarkable position close to the
earth at the most critical epoch of earth-moon history.

With reference to this Professor Darwin has offered an explanation,
which seems so exceedingly plausible that it is impossible to resist
the notion that it must be correct. I will ask you to think of the
earth not as a solid body covered largely with ocean, but as a glowing
globe of molten material. In a globe of this kind it is possible for
great undulations to be set up. Here is a large vase of water, and by
displacing it I can cause the water to undulate with a period which
depends on the size of the vessel; undulations can be set up in a
bucket of water, the period of these undulations being dependent upon
the dimensions of the bucket. Similarly in a vast globe of molten
material certain undulations could be set up, and those undulations
would have a period depending upon the dimensions of this vibrating
mass. We may conjecture a mode in which such vibrations could be
originated. Imagine a thin shell of rigid material which just encases
the globe; suppose this be divided into four quarters, like the four
quarters of an orange, and that two of these opposite quarters be
rejected, leaving two quarters on the liquid. Now suppose that these
two quarters be suddenly pressed in, and then be as suddenly
removed--they will produce depressions, of course, on the two opposite
quarters, while the uncompressed quarters will become protuberant. In
virtue of the mutual attractions between the different particles of
the mass, an effort will be made to restore the globular form, but
this will of course rather overshoot the mark; and therefore a series
of undulations will be originated by which two opposite quarters of
the sphere will alternately shrink in and become protuberant. There
will be a particular period to this oscillation. For our globe it
would appear to be somewhere about an hour and a half or two hours;
but there is necessarily a good deal of uncertainty about this point.

We have seen how in those primitive days the earth was spinning around
very rapidly; and I have also stated that the earth might at th