August, 1913. 



KNOWLEDGE. 



295 



pockets on the surface, and thus the configuration 

 of continents and oceans arose. Sooner or later, 

 yet another era would be ushered in by the 

 commencement of those processes of denudation and 

 deposition which, throughout the long ages following, 

 have reared new continents upon the ruins of the 

 old, and which still manifest their slow-moving but 

 irresistible activities before our eyes. At what period 

 signs of life appeared and the earth first became 

 inhabited we cannot stay to discuss, but must revert 

 to our original theme. 



I. — Kelvin's Theories. 



During mine-sinking operations it is noticeable 

 that as the shaft penetrates deeper and deeper the 

 temperature slowly and regularly increases. In 

 different places the temperature-gradient is not the 

 same ; for external conditions necessarily disturb it 

 in the topmost strata, and we have not yet delved 

 deep enough to reach the lower regions where 

 uniformity prevails. But from observations made 

 an average can be taken, and from this it is possible 

 to deduce mathematically an estimate of the earth's 

 age. The problem resolves itself into the following. 

 Knowing the conductivity of the earth's crust, and 

 assuming a certain uniform initial temperature, what 

 time must elapse before it cools to such an extent 

 that the temperature-gradient over a certain thick- 

 ness is that which we find to obtain now ? The 

 mathematical expression of this has been solved by 

 Kelvin, who arrives at the result that the whole 

 period since the earth was molten amounts to about 

 twenty million years, and certainly could not exceed 

 four hundred million years. 



Another method of his depends upon the braking 

 action which the tides exert upon the rotation of the 

 earth. Since the earth rotates more quickly than 

 the tidal bulge, it follows that the frictional effect 

 thus brought into play must have slowly diminished 

 the period of the earth's revolution. As a time- 

 keeper, in fact, the earth loses some twenty-two 

 seconds in each century, and though this appears 

 a negligible quantity it nevertheless mounts to 

 appreciable proportions when extended over millions 

 of years. Consider, then, for a moment, what this 

 retardation implies. We all know that the earth is 

 not a perfect sphere, but oblately spheroidal, having, 

 in fact, almost exactly the shape it would assume if 

 it were all molten and revolving at its present rate. 

 If the earth, however, had been rotating very much 

 faster when in the liquid condition, and solidi- 

 fication had then taken place, the shape would 

 differ considerably from what we find it to be now. 

 Even if it had been rotating three per cent, faster, 

 the difference would be quite noticeable ; for a greater 

 polar flattening would result and the disposition of 

 the oceans could not be as at present ; and since 

 mathematical reasoning demonstrates that one 

 hundred million years ago a three per cent, increase 

 in the rotation would obtain, the deduction is drawn 

 that any greater antiquity than one hundred million 

 years can only be regarded as improbable. 



The remaining argument is drawn from a con- 

 sideration of the origin of solar heat and its 

 dissipation by radiation, and led to the conclusion 

 that the sun has illuminated the earth only during a 

 period which is probably less than even one hundred 

 million years and certainly not much more. 



This computation, however, and to some extent 

 the second, are based upon data which are neces- 

 sarily vague and indeterminate, and in consequence 

 much reliance cannot be placed upon the results. 

 As will be shown presently, even the first of the 

 Kelvin estimates has to be rejected on the score of 

 recent discovery ; so now there is not one of his 

 methods of estimation to which exception cannot be 

 taken. 



In their day, nevertheless, they served a useful 

 purpose ; for by awakening geologists from their 

 dream of an unlimited credit on the bank of eternity, 

 and compelling their reluctant limitation to a less 

 exorbitant draft, the above intrusion on the part of 

 physics at least prepared the way for fresh lines of 

 argument of less doubtful validity. Naturally, 

 Kelvin's estimates were at first indignantly dis- 

 credited, though without any apparent effect upon 

 the firmness of Kelvin's attitude. In vain his 

 opponents appealed to their ancient traditions — 

 geologists to their succession of strata, and palaeonto- 

 logists to their evolution of types, both of which 

 were believed to require an aeon of time— he simply 

 ignored all with disarming unconcern. 



But though the most eminent of the physicists 

 thus arbitrarily defined his isolated position, there 

 were others in his own sphere of activity who 

 refused to recognise the finality of his results, and 

 were disposed rather to compromise with geologists 

 on the question. Thus, Sir G. Darwin, in a review 

 of the three theories, pointed out some of the 

 uncertainties which surrounded them ; whilst 

 Professor Perry, in his inimitable style, advanced 

 cogent reasons for believing that the Kelvin com- 

 putation erred considerably on the side of under- 

 estimation. Indeed, Perry was quite willing to 

 concede an estimate four times greater than the 

 greatest of Kelvin's. 



These more reasonable views now stand fully 

 justified in the light of more recent knowledge. 

 The first and, as we have mentioned, the most 

 trustworthy of Kelvin's arguments was vitiated 

 during the last few years of the nineteenth century 

 by the discovery of radioactivity and its allied 

 phenomena. This is a point, however, which has to 

 be deferred to a later stage. It is evident that some 

 other methods of attacking the problem were urgent, 

 and as such have happily been forthcoming we shall 

 proceed to discuss them. 



II. — Solvent Denudation. 



FYom a consideration of the weathering forces 

 which on every hand mould the features of con- 

 tinents, Professor Joly derives a method for 

 determining the age of the earth since the time 

 when the latter attained its consistentior status. 



