20 

now. I think we can say fairly definitely that the sun 
was formerly much hotter. There must have been a 
time when the sun’s heat was from 20 to 50 times 
more intense than it is now. That would no doubt 
have made a ereat difference to many geological 
processes. Unfortunately, I cannot say whether it 
occurred in known geological epochs. It must have 
occurred after the earth had begun to exist as a separate 
planet ; but whether it was before or after the sequence 
of geological strata began to be laid down I have no 
idea. It would not be unreasonable, however, to 
expect that in the early geological times the sun was 
several times hotter than it is at present. 
After the evolution of the solar system, we naturally 
turn to consider the evolution of the earth-moon 
system. My impression is that nothing in recent 
progress suggests any doubt that the beautiful theory 
of Sir George Darwin is substantially correct. The 
main features are that the moon at one time formed 
part of the earth, and broke away. At that time the 
rotation period of the earth was between 3 and 4 
hours, and the cause of the fracture was that the 
solar tidal force synchronised with a free period of 
natural vibration of the earth; owing to resonance 
the tidal deformation of the earth continually in- 
creased until rupture occurred. The earth’s period of 
rotation has since lengthened to 24 hours, owing to 
frictional dissipation of energy by lunar and solar 
tides ; and the back-reaction of the lunar tides on the 
moon has caused the moon to recede to its present 
considerable distance. All this has well stood the test 
of searching criticism, and must be considered as ex- 
tremely probable. Modern research has added two 
contributions ; it enables us to calculate the magnitude 
of this tidal friction at the present time, and it enables 
us to locate more exactly the region where the frictional 
dissipation is occurring. 
I believe it was Darwin’s view that the tides most 
potent in wasting energy were not water-tides but 
tides in the solid earth ; that is to say, we have to do 
with deformations of the whole earth under the tide- 
raising force of the moon’s attraction. Undoubtedly 
these deformations of the earth occur, but everything 
turns on whether the process of deformation is attended 
with serious friction. H. Jeffreys has pointed out 
that the phenomenon of latitude variation is accom- 
panied by similar deformations of the earth; and in 
this case it is clear that the friction is inconsiderable, 
for otherwise the deviations of the pole from the sym- 
metrical position would be damped out almost at 
once. It seems, therefore, very unlikely that the 
solid tides can have had much effect in the process of 
tidal evolution of the earth-moon system. Ocean 
tides are likewise of small effect as Darwin himself had 
seen. The modern conclusion is a very curious one ; 
it is in the land-locked shallow seas that nearly all the 
mischief occurs. This was discovered by G, I. Taylor, 
who found that the Irish Sea alone is responsible for 
= of the whole amount required by observation. 
The remaining land-locked basins on the earth are 
probably capable of making up the necessary total. 
The actual rate at which the earth’s rotation is being 
slowed down at the present era can probably be de- 
duced with fair accuracy from the records of ancient 
eclipses. The day is lengthening about one-thousandth 
NO. 2775, VOL. I11] 
NATURE 


[January 6, 1923. 
of a second per century or 1 minute in 6,000,000 
years. At this rate we should have to go back more 
than 10,000 million years to the time when the day 
was between 3 and 4 hours and the moon was 
born. Since the rate depends on the accidental cir- 
cumstance of occurrence of shallow seas no definite 
prediction can be made; but allowing for the much 
greater effect of the tides when the moon was nearer 
to us, it is difficult to date the birth later than 1000 
million years ago. 
Had the earth a solid crust at the time the cata- 
clysm happened? I cannot tell at all. But if it 
suits geological theories I can see no objection what- 
ever to the hypothesis that the earth had a solid crust 
at the time. No cohesion of the crust would seriously 
resist the enormous forces involved when the resonant 
vibration got started. It would not be appreciably 
more difficult than the disruption of a molten earth. 
The view that the Pacific Ocean is the hollow left at 
the place where the moon broke off seems tenable 
unless geologists find objection to it ; and in that case 
we may suppose that the water now collected in the 
hollow formerly covered the earth—or most of it. 
This change of condition of the earth may (or may 
not) have happened within geological times. When 
the earth was covered with water there would be no 
land-locked seas and no appreciable tidal friction from 
the sun (the moon being not yet born), so that we can 
allow a long previous history during which the length 
of day was nearly constant at 3 or 4 hours. That 
rather helps to make the whole theory self-consistent. 
These speculations stand very much as they did 
when Darwin put forward his theory. But I am 
tempted to add further speculations arising out of the 
location of the frictional dissipation. (I am taking 
advantage of the great opportunity for speculation 
which this address affords. Ordinarily Iam restrained, 
because people would ask, What facts can you pro- 
duce in support of your speculations? But here I 
am asking the question, Have you any facts which 
seem to support them? If not, by all means let them 
drop.) The frictional dissipation acts as a brake on 
the earth’s rotation, and we now feel confident that 
the brake is a surface-brake applied at certain points 
on the earth’s surface where the favourable conditions 
exist. The retarding force is transmitted to the 
earth’s interior, and so delays the rotation as a whole ; 
but unless the material is entirely non-plastic there 
will be a tendency for the outer layers to slip on the 
inner layers. I do not know how much the material 
a few hundred miles below the surface would be ex- 
pected to give under the strain ; it may be inappreci- 
able, but I will assume that though small it has some 
effect. 
We have then the whole crust slipping from east to 
west over the main part of the interior. Probably it 
would go very stickily, sometimes arrested by a 
jamming which would hinder it for a time and then 
going on more easily. That is helpful in explaining 
certain astronomical observations. There are irreg- 
ularities in the motions of heavenly bodies, noticed 
particularly in the swift-moving moon but shown also 
on a smaller scale in the sun and planets, which appear 
to indicate that our standard timekeeper, the earth, 
is a little irregular. Now, of course, it is the rotation 
