iable even by our most delicate observations. In one thousand
years the alteration in the length of a day is only a small fraction
of a second; but what may be a very small matter in one thousand years
can become a very large one in many millions of years. Thus it is that
when we stretch our view through immense vistas of time past, or when
we look forward through immeasurable ages of time to come, the
alteration in the length of the day will assume the most startling
proportions, and involve the most momentous consequences.

Let us first look back. There was a time when the day, instead of
being the twenty-four hours we now have, must have been only
twenty-three hours, How many millions of years ago that was I do not
pretend to say, nor is the point material for our argument; suffice it
to say, that assuming, as geology assures us we may assume, the
existence of these aeons of millions of years, there was once a time
when the day was not only one hour shorter, but was even several hours
less than it is at present. Nor need we stop our retrospect at a day
of even twenty, or fifteen, or ten hours long; we shall at once
project our glance back to an immeasurably remote epoch, at which the
earth was spinning round in a time only one sixth or even less of the
length of the present day. There is here a reason for our retrospect
to halt, for at some eventful period, when the day was about three or
four hours long, the earth must have been in a condition of a very
critical kind.

It is well known that fearful accidents occasionally happen where
large grindstones are being driven at a high speed. The velocity of
rotation becomes too great for the tenacity of the stone to withstand
the stress; a rupture takes place, the stone flies in pieces, and huge
fragments are hurled around. For each particular grindstone there is
a certain special velocity depending upon its actual materials and
character, at which it would inevitably fly in pieces. I have once
before likened our earth to a wheel; now let me liken it to a
grindstone. There is therefore a certain critical velocity of rotation
for the earth at which it would be on the brink of rupture. We cannot
exactly say, in our ignorance of the internal constitution of the
earth, what length of day would be the shortest possible for our earth
to have consistently with the preservation of its integrity; we may,
however, assume that it will be about three or four hours, or perhaps
a litt