$2 
NATURE 
[Wov. 24, 1881 
gold ornaments which were jfound there. Most people 
think that these tombs, whosever they were, date from at 
least 1000 B.c. On the other hand, some very high 
authorities regard the monuments as the tombs of 
northern invaders who came into Greece 500-600 A.D. 
Here then we have a range of some 1500 years for the 
date of the tombs, and no dates between these two are 
possible. I am sure I do not pretend to decide between 
them, or even to have an opinion on the subject; but I 
cannot help saying that in one respect the astronomers 
are better off than the historians. The historians cannot 
even agree whether Schliemann’s gold ornaments are B.C. 
or A.D. Astronomers are, at all events, certain that the 
date of the moon’s birth was before the present era. 
At the critical epoch to which our retrospect extends, 
the length of the day was only a very few hours, I can- 
not tell you exactly how many hours. It seems, however, 
to have been more than two and less than four. If we 
call it three hours we shall not be far from the truth. 
Perhaps you may think that if we looked back to a still 
earlier epoch, the day would become still less and finally 
disappear altogether! This is however not the case. 
The day can never have been much less than three hours 
in the present order of things. Everybody knows that 
the earth is not a sphere, but that there is a protuberance 
at the equator, so that, as our school books tell us, the 
earth is shaped like an orange. It is well known that 
this protuberance is due to the rotation of the earth 
en its axis, by which the equatorial parts bulge out by 
centrifugal force. The quicker the earth rotates the 
greater is the protuberance. If, however, the rate of 
rotation exceeds a certain limit the equatorial portions 
of the earth could no longer cling together. The at- 
traction which unites them would be overcome by cen- 
trifugal force, and a general break up would occur. 
It can be shown that the rotation of the earth when on 
the point of rupture corresponds to a length of the day 
somewhere about the critical value of three hours, which 
we have already adopted. It is therefore impossible for 
us to suppose a day much shorter than three hours. 
What occurred prior to this I do not here discuss. 
Let us leave the earth for a few minutes, and ex- 
amine the past history of the moon. We have seen 
the moon revolves around the earth in an ever-widening 
orbit, and consequently the moon must in ancient times 
have been nearer the earth than it is now. No doubt 
the change is slow. There is not much difference be- 
tween the orbit of the moon a thousand years ago and 
the orbit in which the moon is now moving. 
But when we rise to millions of years the difference 
becomes very appreciable. Thirty or forty millions of 
years ago the moon was much closer to the earth than it 
is at present ; very possibly the moon was then only half 
its present distance. We must however look still earlier, 
to a certain epoch not less than fifty millions of years ago. 
At that epoch the moon must have been so close to the 
earth that the two bodies were almost touching. I dare- 
say this striking result will come upon many with surprise 
when they hear it for the first time. It was, I know, with 
great surprise that I myself read of it not many months 
ago. But the evidence is unimpeachable, and it is indeed 
wonderful to see how such information has been gained 
by merely looking at the ripples of the tide. 
Everybody knows that the moon revolves now around 
the earth in a period of twenty-seven days. The period 
depends upon the distance between the earth and the moon. 
The time and the distance are connected together by one 
of Kepler's celebrated laws, so that, as the distance 
shortens, so must the time of revolution shorten. In earlier 
times the month must have been shorter than our present 
month. Some millions of years ago the moon completed its 
journey in a week instead of taking twenty-eight days as at 
present. Looking back earlier still, we find the month has 
dwindled down to a day, then down to a few hours, until 
at that wondrous epoch when the moon was almost 
touching the earth, the moon spun round the earth once 
every three hours, 
It would require the combined powers of a poet and a 
mathematician to pourtray the scene with becoming dig- 
nity. I have only promised to give you that glimpse 
along the Corridors of Time which I have myself been 
able to obtain. The scene is laid in the abyss of space ; 
the time is more than 50,000,000 years ago ; the dramatis 
persone are the earth and the moon. 
In those ancient times I see our earth to be a noble 
globe, as it is at present. Yet it is not partly covered 
with oceans and partly clothed with verdure. The prime- 
val earth seems rather a fiery and half-molten mass, where 
no organic life can dwell. Instead of the atmosphere 
which we now have, I see a dense mass of vapours in 
which perhaps all the oceans of the earth are suspended 
as clouds. I see that the sun still rises and sets to give the 
succession of day and of night, but the day and the night 
together only amount to three hours instead of twenty-four. 
Almost touching the chaotic mass of the earth is another 
much smaller and equally chaotic body. Around the 
earth I see this small body rapidly rotating. The two 
revolve together as if they were bound by invisible bands. 
This smaller body is the moon. Such is the picture 
which I wish to present to you as a glimpse through the 
Corridors of Time. 
I have hitherto refrained from introducing any merely 
speculative matters. If we can believe anything ot 
mathematics, anything of dynamics, we must admit that 
the picture I have attempted to outline is a faithful por- 
trait. The only uncertain elements are the date and the 
periodic time. I do however now propose to venture on 
one speculation in which Mr. Darwin has indulged. I 
propose to offer a suggestion as to how a small body 
came into this most remarkable position close by the 
earth, and how its motion was produced. 
We have hitherto been guided by the unerring light of 
dynamics, but at this momentous epoch dynamics deserts 
us, and we have only probability to guide our faltering 
steps. One hint, however, dynamics does give. It re- 
minds us that a rotation once in three hours is very close 
to the quickest rotation which the earth could have with- 
out falling to pieces. As the earth was thus predisposed 
to rupture, it is of extreme interest to observe that a cause 
tending to precipitate such a rupture was then ready to 
hand. It seems not unlikely that we are indebted to the 
sun as the occasion by which the moon was fractured off 
from the earth and assumed the dignity of an independent 
body. It must be remembered that the sun produces 
tides in the earth as well as the moon, but the solar tides 
are so small compared with the lunar tides, that we have 
hitherto been enabled to neglect them. There could 
however have been no lunar tides before the moon existed, 
and consequently in the early ages before the moon was 
detached the earth was disturbed by the solar tides, and 
by the solar tides alone. 
The primeval earth thus rose and fell under the tidal 
action of the sun. Probably there were no oceans then 
on the earth; but tides do not require oceans, or even 
water, for their operation. The primitive tides were 
manifested as throbs in the actual body of the earth 
itself, which was then in a more or less fluid condition. 
Even at this moment bodily tides are disturbing the solid 
earth beneath our feet; but these tides are now so small as 
to be imperceptible when compared with the oceanic tides. 
(To be continued.) 
SOME “GUESSES AT TRUTH” OF THE 
EMPEROR KHANG-HI 
HANG-HI or Khang-hsi, one of the greatest of 
Chinese emperors, and indeed of Asiatic rulers, 
was born in 1653 and ascended the throne in 1661. For 
