xS 77.] 
Probable Origin and Age of the Sun. 
323 
that, if what has already been advanced in regard to the 
origin of the sun’s heat be correct, it will follow that the ar- 
gumentfor the recent age of the earth, based upon the assump- 
tion that the sun could have derived its store of heat only 
from the condensation of its mass, must be wholly aban- 
doned, and that, in so far as this argument is concerned, 
there is no known limit to the amount of heat which the 
sun may have possessed, or to the time during which it 
may have illuminated the earth. 
Argument from Tidal Retardation.— It is well known that, 
owing to tidal retardation, the rate of the earth’s rotation is 
slowly diminishing ; and it is therefore evident that if we 
go back for many millions of years, we reach a period when 
the earth must have been rotating much faster than now. 
Sir William’s argument is,* that had the earth solidified 
several hundred millions of years ago, the flattening at the 
Poles and the bulging at the Equator would have been much 
greater than we find them to be. Therefore, because the 
earth is so little flattened, it must have been rotating, when 
it became solid, at very nearly the same rate as at present. 
And as the rate of rotation is becoming slower and slower, 
it cannot be so many millions of years back since solidifica- 
tion took place. A few years ago I ventured to point outt 
what appeared to be a very obvious objection to this argu- 
ment, viz., thatthe influence of sub-aerial denudation in alter- 
ing the form of the earth had been entirely overlooked. It has 
been proved, as we have seen, that the rocky surface of our 
globe is being lowered, on an average, by sub-aerial denuda- 
tion, at the rate of about 1 foot in 6000 years. It follows as a 
consequence, from the loss of centrifugal force resulting from 
the retardation of the earth’s rotation occasioned by the 
fridtion of the tidal wave, that the sea-level must be slowly 
sinking at the Equator and rising at the Poles. This, of 
course, tends to protedt the polar regions and expose equa- 
torial regions to sub-aerial denudation. Now it is perfedtly 
obvious that unless the sea-level at the Equator has, in con- 
sequence of tidal retardation, been sinking during past ages 
at a greater rate than 1 foot in 6000 years, it is physically 
impossible the form of our globe could have been very much 
different from what it is at present, whatever may have been 
its form when it consolidated, because sub-aerial denudation 
would have lowered the Equator as rapidly as the sea sank. 
But in equatorial regions the rate of denudation is no doubt 
much greater than 1 foot in 6000 years, because there the 
* Trans. Geol. Soc. of Glasgow, voh iii., p. 1. 
f Nature, August 21, 1872. Climate and Time, p, 335. 
£ 3 
