ON TEE PROBABLE ORIGIN AND AGE OF TEE SUN. 



359 



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, 1 

 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 flat- 

 tened, it must have been rotating, when it be- 

 came solid, at very nearly the same rate as at 

 present. And as the rate of rotation is becom- 

 ing slower and slower, it cannot be so many mill- 

 ions of years back since solidification took place. 

 A few years ago I ventured to point out ' 2 what 

 appeared to be a very obvious objection to this 

 argument, viz., that the influence of subaerial 

 denudation in altering the form of the earth had 

 been entirely overlooked. It has been proved, 

 as we have seen, that the rocky surface of oui 

 globe is being lowered, on an average, by sub- 

 aerial denudation, at. the rate of about one foot in 

 6,000 years. It follows, as a consequence, from 

 the loss of centrifugal force resulting from the 

 retardation of the earth's rotation occasioned by 

 the friction 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 protect 

 the polar regions and expose equatorial regions 

 to subaerial denudation. Now, it is perfectly 

 obvious that, unless the sea-level at the equator 

 has, in consequence of tidal retardation, been 

 sinking during past ages at a greater rate than one 

 foot in 6,000 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 

 subaerial denudation would have lowered the 

 equator as rapidly as the sea sank. But in equa- 

 torial regions the rate of denudation is, no doubt, 

 much greater than one foot in 6,000 yeai-s, because 

 there the rainfall is greater than in the temper- 

 ate regions. It has been shown that the rate at 

 which a country is being lowered by subaerial 

 denudation is mainly determined not so much by 

 the character of its rocks as by the sediment- 



1 "Transactions of the Geological Society of Glasgow," 

 vol. iii., p. 1. 



8 Nature, August 21, 1S72 ; " Climate and Time," p. 

 335. 



carrying power of its river-systems. Consequent- 

 ly, other things being equal, the greater the rain- 

 fall the greater will be the rate of denudation. 

 We know that the basin of the Ganges, for ex- 

 ample, is being lowered by denudation at the rate 

 of about one loot in 2,300 years ; and this is prob- 

 ably not very far from the average rate at which 

 the equatorial regions are being denuded. It is 

 therefore evident that subaerial denudation is 

 lowering the equator as rapidly as the sea-level 

 is sinking from loss of rotation, and that conse- 

 quently we canuot infer from the present form of 

 our globe what was its form when it solidified. 

 In as far as tidal retardation can show to the con- 

 trary, its form, when solidification took place, 

 may have been as oblate as that of the planet 

 Jupiter. There is another circumstance which 

 must be taken into account. The lowering of the 

 equator, by the transference of materials from the 

 equator to the higher latitudes, must tend to in- 

 crease the rate of rotation, or, more properly, it 

 must tend to lessen the rate of tidal retardation. 



The argument may be shown to be incon- 

 clusive from another consideration. The ques- 

 tion as to whether the earth's axis of rotation 

 could ever have changed to such an extent as to 

 have affected the climate of the poles, is at pres- 

 ent exciting a good deal of attention. The sub- 

 ject has recently been investigated with great 

 care by Prof. Houghton, 1 Mr. George Darwin, 9 

 the Rev. J. F. Twisden, 3 and others, and the gen. 

 eral result arrived at may be expressed in the 

 words of Mr. G. Darwin : " If the earth be quite 

 rigid no redistribution of matter in new continents 

 could ever have caused the deviation of the pole 

 from its present position to exceed the limit of 

 about 3°." 



Mr. Darwin has shown that, in order to pro- 

 duce a displacement of the pole to the extent of 

 only 1° 46', an area equal to one-twentieth of the 

 entire- surface of the globe would have to be 

 elevated to the height of two miles. The entire 

 continent of Europe elevated two miles would not 

 deflect the pole much over half a degree. As- 

 suming the mean elevation of the continents of 

 Europe and Asia to be 1,000 feet, Prof. Hough- 

 ton calculates that their removal would displace 

 the pole only 199.4 miles. 



It may now be admitted as settled that if the 

 earth be perfectly rigid the climate of our globe 

 could never possibly have been affected by any 



1 "Proceedings of the Eoyal Society." vol. xxvi., p. 51. 



2 " Proceedings of the Eoyal Society," vol. xxv., p. 32S. 



3 Paper read before the Geological Society, February 

 21, 1S77. 



