Dee. 1, 1881] 
NATURE 
105 
been detached. No doubt it was somewhere near the 
equator, but the distinction of land and water had not 
then arisen. Around the primeval earth the moon 
revolved in three hours; the earth also revolved in 
three hours, so that the moon constantly remained over 
the red region. This I can illustrate by holding the 
small globe which represents the moon in one hand, and 
making the large globe which represents the earth revolve 
by the other. 
This state of things formed what is known as unstable 
dynamical equilibrium. It could not last. Either the 
moon must fall back again on the earth, and be reab- 
sorbed into its mass, or the moon must commence to 
move away from the earth. Which of these two courses 
was the moon to take? The case is analogous to that of 
a needle balanced on its point. The needle must fall 
some way, but what is to decide whether it shall fall to 
the right or to the left? I do not know what decided the 
moon, but what the decision was is perfectly plain. The 
fact that the moon exists shows that it did not return to 
the earth, but that the moon adopted the other course, 
and commenced its outward journey. 
As the moon recedes, the period which it requires for a 
journey round the earth increases also. Initially that 
period was but three hours, and it has increased up 
until our present month of 656 hours. 
The rotation of the earth has been modified by the 
retreat of the moon. Directly the moon began to retreat 
the earth was no longer under an obligation to keep the 
same face thereto. When the moon was at a certain dis- 
tance the earth made two rotations for every revolution that 
the moon made. Thus as I carry the smull globe round 
the large globe the latter makes two revolutions for 
one revolution of the small globe. Still the moon gets 
further and further away, until the earth performs three, 
four, or more rotations for each of the moon’s revolu- 
tions. Do not infer that the rate of the earth’s rotation is 
increasing ; the contrary is the fact. The earth’s rotation 
is getting slower, and so is that of the moon; but the re- 
tardation of the moon is much greater than that of the 
earth. Even thouzh the rotation of the earth is much 
more than the primitive three hours, yet that of the moon 
has increased to several times the rotation of the earth. 
The moon recedes still further and further, and at 
length a noticeable epoch is reached, to which I must 
call attention. At that epoch the moon is so far out that 
its revolution takes twenty-nine times as long as the rota- 
tion of the earth. The month was then twenty-nine times 
the day. The duration of the day was less than the 
present twenty-four hours, but I do not believe it was 
very much less. The time we are speaking of is not very 
remote, perhaps only a very few million years ago. The 
month was then in the zenith of its glory. The month 
was never twenty-nine times as long as the day before. 
It has never been twenty-nine times as long as the day 
since. It will never be twenty-nine times as long as the 
day again. 
Resuming our history, we find the moon still continuing 
to revolve in an ever-widening circle the length of the 
month and of the day both increasing. The ratio of 
the day to the month was still undergoing a change. 
When the moon was a little further off the earth only 
revolved twenty-eight times instead of twenty-nine times 
in one revolution of the moon. Still the velocity of the 
earth abates until it only makes twenty-seven revolutions 
in one revolution of the moon. This is an epoch of especial 
interest, for it is the present time. In the present order of 
things the moon revolves round the earth once while the 
earth rotates twenty-seven times. This has remained 
sensibly true for thousands of years, and no doubt will 
remain sensibly true for thousands of years to come, 
but it will not remain true indefinitely. Wondrous as 
are the changes which have occurred in times past, not 
less wondrous are the changes which are to occur ia 
time to come. The tides have guided our gropings into 
the past; they will continue to guide our researches to 
make a forecast of the future. 
Further and further will the moon retreat, and more 
and more slowly will the earth revolve. But we shall not 
pause at intervening stages; we shall try to sketch the 
ultimate type to which our system tends. In the dim 
future, many millions of years distant, the final stage will 
be approached. As this stage draws nigh, the rotation of 
the earth will again approach to equality with the revo- 
lution of the moon, From the present month of twenty- 
seven days we shall pass to a month of twenty-six days, 
of twenty-five days, and so on, until eventually we shall 
reach a month of two days, and lastly a month of one 
day. When this state has been attained the earth will 
constantly turn the same region towards the moon, I do 
not know what is the locality on the earth which is 
destined for this distinction. 
Here you see that the first state and the last state of 
the earth-moon history are in one sense identical. In 
each case the same face of the earth is constantly directed 
towardsthe moon. In another way, how different are the 
first stage and the last. At the beginning the day and the 
month were both equal, and they were each three hours. 
At the end the day and the month will be again equal, 
but they will each be 1400 hours. The moon will then go 
round the earth in 1400 hours, while the earth will rotate 
on its axis in tie sametime. In other words, the day is 
destined in the very remote future to become as long as 
fifty-seven of our days. This epoch will assuredly come 
if the universe lasts long enough. When it has come 
it will endure for countless ages. It would endure for 
ever if the earth and the moon could be isolated from all 
external interference. 
We heard a great deal a few years ago about the 
necessity of shortening tie hours of labour. I wish to 
point out that the social reformers who are striving to 
shorten the hours of labour are pulling one way, while 
the moon is pulling the other. The moon is increasing 
the length of the day. The change will be very gradual, 
but none the less is it inevitable. Where will the 
nine-hours’ movement be when the day has increased 
to 1400 hours? This will be a very serious matter, and 
there is only one way by which it can be avoided. The 
question is one rather for engineers than for astronomers ; 
but I cannot help throwing out a suggestion. My advice 
is: Anchor the moon, and keep it from going out. If you 
can do this, and if you can also provide a brake by which 
the speed of the moon can be controlled, then you will 
be able for ever to revel in the enjoyment of a twenty- 
four-hour day. 
Should this engineering feat never be accomplished, 
then we have only the 1400-hour day to look forward to. 
Nor is there anything untoward in the prospect, when we 
take natural selection as our comforter. By natural 
selection man has become exactly harmonised with his 
present environment. No doubt natural selection moves 
at a dignified pace, but so in all truth does tidal evolution. 
Natural selection and tidal evolution have advanced paré 
passu through all the past millions of geological time. They 
will advance favi passu through all the ages yet to come. 
As the day lengthens, so will man’s nature gradually 
change too, without any hardship or inconvenience. All that 
is necessary is plenty of time. Should we think it a hard- 
ship that our children should have a day of twenty-four 
hours and one second instead of twenty-four hours? That 
the day enjoyed by our grandchildren should be a second 
longer than the day of our children? That the day of 
our great-grandchildren should be a second longer still, 
and so on continually? This would be no inconvenience 
whatever. No one except the astronomers would be able 
to detect the change, and daily life would be unaltered. 
Yet, carry on this process for only 150 million years, and 
we shall find that the whole change of the day from 
