106 
twenty-four hours to 1400 hours has been accomplished. 
The actual rate of change is indeed much less than 
this, and is at present so small that astronomers can 
hardly even detect it. 
Our remote posterity will have a night 700 hours long, 
and when the sun rises in the morning 700 hours more 
will elapse before he can set. This they will find a most 
suitable and agreeable arrangement. They will look back | 
on our short periods of rest and short periods of work 
with mingled curiosity and pity. Perhaps they will even 
have exhibitions of eccentric individuals able to sleep for 
eight hours, work for eight hours, and play for eight 
hours. They will look on such curiosities in the same 
way as we look on the man who undertakes to walk a 
thousand miles in a thousand hours. 
I am beyond all things anxious to give you the impres- 
sion that I am not indulging in any mere romance. No 
doubt the various figures I have mentioned are but esti- 
mates. They may be found to require correction—per- 
haps large correction; but the general outline of the theory 
must be true. Should any traces of doubt still linger in 
the mind of some prejudiced person, let me finally dissi- 
pate them. Perhaps some caviller may say, Where 
are the proofs of all this action of the tides? How do 
you know that the tides are sufficiently powerful to pro- 
duce such changes? I believe I have shown this abun- 
dantly, but some people require a great deal of conviction. 
I have therefore kept my best argument for the end. 
For an overwhelming proof of tidal efficiency I shall 
summon the heavens themselves to witness, and I shall 
point to the stupendous task which tides have already 
accomplished. As the moon has made and is making 
tides on the earth, so the earth once raised tides on the 
moon. These tides have ceased for ages; their work 
is done ; but they have raised a monument in the moon 
to testify to the tidal sufferings which the moon has un- 
dergone. To that monument I now confidently appeal. 
The moon being much smaller than the earth, the tides 
on the moon produced by the earth must have been many 
times as great as the tides on our earth produced by the 
moon. It matters not that the moon now contains no 
liquid ocean. Nor does it matter whether the moon ever 
had a liquid ocean. In very ancient days the moon was 
not the hard, rigid mass which it now appears. Time 
was when the volcanoes raged on the moon with a fury 
which nothing on our earth at present can parallel. The 
moon was then in a soft or a more or less fluid condition, 
and in this viscous mass the earth produced great tides. 
Great tides in truth they were, for the earth is eighty 
times as heavy as the moon. On the other hand, the 
moon is only one-fourth the diameter of the earth; so 
that the actual height of the tides on the moon would 
be still many times as great as the tides on the earth. When 
the moon was nearer to us, as it was in early ages, those 
tides were still greater. Think for one moment of what 
a lunar tidal wave of such magnitude would be capable. 
This wave is perhaps of molten lava; it would tear 
over the surface with terrific power, and anything that 
friction could accomplish that great current would do. 
That tidal current has done its work ; even if the moon 
were fluid at the present day it could no longer be dis- 
tracted by tides. Remember, it is not the mere presence 
of the tide which produces friction. It is the action of 
the tide in rising and in falling which accomplishes the 
work. If, therefore, the moon moved so that it was 
always high tide at the same place, the tides could pro- 
duce no further effect. The spot where the tide is high 
on the moon is the spot which is towards the earth. It 
hence follows that the action of the tides will cease when 
the moon constantly directs the same face to the earth. 
The moon has thus at length gained a haven of rest 
from a tidal point of view. No doubt the moon has a 
high tide and it has a low tide, but those tides no longer 
ebb and flow: the moon has succumbed to the incessant 
NATURE 
~ . ail" a Oy, s at 
[ Dec. 1, 1881 
action of friction, and has assumed the only attitude 
which can relieve it from incessant disturbance. 
For many centuries it had been an enigma to astrono- 
mers why the moon should always turn the same face to 
the earth. It could be shown that there were many 
million chances to one in favour of this being due to 
some physical cause. The ordinary theory of gravitation 
failed to explain the cause. Every one had noticed this 
phenomenon. Yet the explanation was never given till 
lately. It was Helmholtz who showed that this was a 
consequence of ancient tides, and this simple and most 
satisfactory explanation has been universally accepted. 
The constant face of the moon is a living testimony to the 
power of the tides. What tides have accomplished on 
the moon is an earnest of what tides will accomplish on 
the earth. 
In the great conflict of the tides the earth has 
already conquered the moon, and forced the moon 
to render perpetual homage as a token of submis- 
sion. Remember, however, that the earth is large, 
and the moon is small. Yet small though the moon is, 
it gallantly struggles on. “You have forced me,” cries 
the moon to the earth, ‘‘to abandon the rotation with 
which I was originally endowed; you have compelled 
me to rotate in the manner you have dictated. I will 
have my revenge. It is true I am weak, but I am unre- 
lenting ; day by day I am exhausting you by the tides 
with which I make you throb. The time will assuredly 
come, though it may not be for millions of years, when 
you shall be forced: to make a compromise. When that 
compromise is made the turmoils of the tides will cease ; 
our mutual movements will be adjusted. With equal 
dignity we shall each rotate around the other ; with equal 
dignity we shall each constantly bend the same face to 
the other.”’ 
There is another point to be considered. We must 
not forget that there is a sun in the heavens as well as 
a moon. The sun also produces tides in the earth. 
Those tides were much smaller than the lunar tides, so 
that we could afford to neglect them. But we have seen 
that the lunar tides will gradually decrease to nothing. It 
behoves us then to consider what the solar tides can 
effect which shall be worthy of our attention. In a lecture 
which I gave here some years ago, I made allusion to the 
discovery of the satellites of Mars. I mentioned that one 
of the satellites of Mars presented a phenomenon un- 
paralleled in the solar system. The satellite revolved 
around Mars in a period of seven hours, while Mars him- 
self rotated on his axis in a period of twenty-four hours. 
We here actually find the moon of Mars rotating around 
Mars in much less than one of Mars’ own days. This 
was a most curious and unexpected circumstance, but 
the observations of the discoverer, Asaph Hall, placed 
the great fact beyond any doubt. The mystery has now 
been explained. It is due to the action of the solar 
tides on Mars. Nay more, we can actually foresee that 
at some incredibly remote future time our earth and moon 
are destined to present the same movements which have 
seemed so anomalous in Mars. 
Left to themselves, the earth and the moon would 
have remained for ever in the condition of compromise. 
The moon would have revolved round the earth in 1400 
hours. The earth would have rotated on its axis in 1400 
hours also. But now the solar tides intervene. They have 
little effect upon the moon ; it revolves as before, but the 
solar tides begin to retard the earth still further. Instead 
of a period of 1400 hours, the earth will have a still longer 
day, so that finally the moon revolves more rapidly around 
the earth than the earth rotates on its axis. 
It seems to me that the episode I have mentioned is one 
of the most interesting in the whole of modern astronomy. 
We have first a most delicate telescopic discovery of 
the tiny satellite of Mars and of its anomalous move- 
ments. We then have a beautiful explanation of how 
