Nov. 24, 1881 | 
NATURE 
8I 
derived with which the tides do their work? The answer 
seems a very obvious one. If the tides are caused by the 
moon, the energy they possess must also be derived from 
the moon. This looks plain enough, but unfortunately it 
is not true. Would it be true to assert that the finger of 
the rifleman which pulls the trigger supplies the energy 
with which the rifle bullet is animated? Of course it 
would not. The energy is derived from the explosion of 
the gunpowder, and the pulling of the trigger is merely 
the means by which that energy is liberated. In a some- 
what similar manner the tidal wave produced by the moon 
is the means whereby a part of the energy stored in the 
earth is compelled to expend itself in work. I do not say 
this is an obyious result. Indeed it depends upon a re- 
fined dynamical theorem, which it would be impossible 
to enter into here. 
But what do we mean by taking energy from the earth ? 
Let me illustrate this by a comparison between the earth 
rotating on its axis and the fly-wheel of an engine. The 
fly-wheel is a sort of reservoir, into which the engine 
pours its power at each stroke of the piston, The various 
machines in the mill merely draw off the power from the 
store accumulated in the fly-wheel. The earth is like 
a gigantic fly-wheel detached from the engine, though 
still connected with the machines in the mill. In that 
mighty fly-wheel a stupendous quantity of energy is stored 
up, and a stupendous quantity of energy would be given 
out before that fly-wheel would come to rest. The earth’s 
rotation is the reservoir from whence the tides draw the 
energy they require for doing work. Hence it is that 
though the tides are caused by the moon, yet whenever 
they require energy they draw on the supply ready to 
hand in the rotation of the earth. 
The earth differs from the fly-wheel of the engine in a 
very important point. As the energy is withdrawn from 
the fly-wheel by the machines in the mill, so it is restored 
thereto by the power of the steam-engine, and the fly runs 
uniformly. But the earth is merely the fly-wheel without 
the engine. When the work done by the tides withdraws 
energy from the earth, that energy is never restored. It 
therefore follows that the energy of the earth’s rotation 
must be decreasing, This leads to a consequence cf the 
most wonderful importance. It tells us that the speed 
with which the earth retates on its axis is diminishing. 
We can state the result in a manner which has the merits 
of simplicity and brevity. 
“The tides are increasing the length of the day.”’ 
This statement is the text of the discourse which I am 
to give you this evening. From this simple fact the new 
and wondrous theory of tidal evolution is deduced. A 
great scientific theory is generally the outcome of many 
minds. Toa certain extent this is true of the theory of 
tidal evolution. It was Prof. Helmholtz who first ap- 
pealed to what tides had already done on the moon. It 
was Prof. Purser who took an important step in the 
analytical theory. It was Sir William Thomson’s mathe- 
matical genius which laid the broad and deep foundations 
of the fabric. These are the pioneers in this splendid 
research. But they were only the pioneers. The great 
theory itself is chiefly the work of one man. You are all 
familiar with the name he bears. The discoverer of 
tidal evolution is Mr. G. H. Darwin, Fellow of Trinity 
College, Cambridge. 
It would be impracticable for me now to go into the 
actual mathematical calculations. I shall rather en- 
deavour to give you an outline of: this theory, shorn of 
its technical symbols. I think this can be done even 
though we attempt to retain the accuracy of mathe- 
matical language. Nor would it be fair to throw on Mr. 
Darwin or the other mathematicians I have named the 
responsibility for all I am going to say. I must be my- 
self responsible for the way in which those theories are 
- forth,‘as well as for some of the deductions made from 
‘them, 
At present no doubt the effect of the tides in changing 
the length of the day is very small. A day now is not 
appreciably longer than a day a hundred years ago. 
Even in a thousand years the change in the length of 
the day is only a fraction of a second. But the import- 
ance arises from the fact that the change, slow though it 
is, lies always in one direction. The day is continually 
increasing. In millions of years the accumulated ,effect 
becomes not only appreciable but even of startling 
magnitude. 
The change in the length of the day must involve a 
corresponding change in the motion of the moon. This 
is by no means obvious. It depends upon an elaborate 
mathematical theorem. I cannot attempt to prove this 
for you, but I think I can state the result so that it can 
be understood without the proof. If the moon acts on 
the earth and retards the rotation of the earth, so, con- 
versely, does the earth react upon the moon. The earth 
is tormented by the moon, so it strives to drive away its 
persecutor. At present the moon revolves round the 
earth at a distance of about 240,000 miles. The reaction 
of the earth tends to increase that distance, and to force 
the moon to revolve in an orbit which is continual’'y 
getting larger and larger. 
Here then we have two remarkable consequences of 
the tides which are inseparably connected. Remember 
also that we are not enunciating any mere speculative 
doctrine. These results are the inevitable consequences 
ofthe tides. If the earth had no seas or oceans, no lakes 
or rivers ; if it were an absolutely rigid solid throughout 
its entire mass; then these changes could not take place. 
The length of the day would never alter, and the distance 
of the moon would only fluctuate between narrow limits. 
As thousands of years roll on, the length of the day 
increases second by second, and the distance of the moon 
increases mile by mile. These changes are never reversed. 
It is the old story of the perpetual dropping. As the per- 
petual dropping wears away the stone, so the perpetual 
action of the tides has sculptured out the earth and moon. 
Still the action of the tides continues. To-day is longer 
than yesterday; yesterday is longer than the day before. 
A million years ago the day probably contained some 
minutes less than our present day of twenty-four hours. 
Our retrospect does not halt here; we at once project our 
view back to an incredibly remote epoch which was a 
crisis in the history of our system. 
Let me say at once that there is great uncertainty about 
the date of that crisis. It must have been at least 
50,000,000 years ago. It may have been very much 
earlier. This crisis was the interesting occasion when 
the moon was born. I wish I could chronicle the event 
with perfect accuracy, but I cannot be sure of anything 
except that it was more than 50,000,000 years ago, 
I do not admit that there is anything discreditable 
about this uncertainty. Do you not know that our his- 
torians, who have records and monuments to help 
them, are often in great confusion about dates? I am 
not going to find any fault with historians. They do 
their best to learn the truth; but I cannot help re- 
minding you that they are often as much in the dark 
about centuries as the astronomers are about millions. 
Take, for example, the siege of Troy, which Homer has 
immortalised, and ask the historians to state the date 
of that event. Some say that the siege of Troy was 
1184 B.C., others that it was 900 B.c.; both are equally 
uncertain. Schliemann says that he found the remains 
of the town burned down, but that no one knows who 
did it or when it was done, Others, again, say that there 
was never any siege of Troy at all. 
A recent instance which has attracted great and de- 
served attention is Schliemann’s discovery at Mycenz of 
what he considers to have been the tomb of Agamemnon. 
The tomb certainly did contain the remains of some 
| mighty man, if we may judge by the roo lb. weight of 
