August 31, 1905] 



NA TURE 



443 



quantitative corrc-lation tlic lengths of the present day 

 and month, the obliquity of the ecliptic, and the inclin- 

 ation and eccentricity of the lunar orbit, must, I think, 

 have strong claims to acceptance."' 



We have pursued the changes into the past, and I will 

 refer but shortly to the future. The day and month are 

 both now lengthening, but the day changes more quickly 

 than the month. Thus the two periods tend again to 

 become equal to one another, and it appears that v/hen 

 that goal is reached both day and month will be as long 

 as fifty-five of our present days. The earth will then 

 always show the same face to the moon, just as it did 

 in the remotest past. But there is a great contrast 

 between the ultimate and initial conditions, for the ulti- 

 mate stage, with day and month both equal to fifty-five 

 of our present days, is one of great stability in contra- 

 distinction to the vanishing stability which we found in 

 the initial stage. 



Since the relationship between the moon and earth is 

 a mutual one, the earth may be regarded as a satellite 

 of the moon, and if the moon rotated rapidly on her axis, 

 as was probably once the case, the earth must at that 

 time have produced tides in the moon. The mass of the 

 moon is relatively small, and the tides produced by the 

 earth would be large ; accordingly the moon would pass 

 through the several stages of her history much more 

 rapidly than the earth. Hence it is that the moon has 

 already advanced to that condition which we foresee as the 

 future fate of the earth, and now always shows to us the 

 same face. 



If the earth and moon were the only bodies in exist- 

 ence, this ultimate stage when the day and month were 

 again identical in length would be one of absolute 

 stability, and therefore eternal ; but the presence of the 

 sun introduces a cause for yet further changes. I do not, 

 however, propose to pursue the history to this yet remoter 

 futurity, because our system must contain other seeds 

 of decay which will probably bear fruit before these further 

 transformations could take effect. 



If, as has been argued, tidal friction has played so 

 important a part in the history of the earth and moon, 

 it might be expected that the like should be true of the 

 other planets and satellites, and of the planets themselves 

 in their relationship to the sun. But numerical examin- 

 ation of the several cases proves conclusively that this 

 cannot have been the case. The relationship of the moon 

 to the earth is in fact quite exceptional in the solar 

 system, and we have still to rely on such theories as that 

 of Laplace for the explanation of the main outlines of 

 the solar system. 



I have as yet only barely mentioned the time occupied 

 by the sequence of events sketched out in the various 

 schemes of cosmogony, and the question of cosmical time 

 is a thorny and controversial one. 



Our ideas are absolutely blank as to the time requisite 

 for the evolution according to Laplace's nebular hypothesis. 

 And again, if we adopt the meteoritic theory, no estimate 

 can be formed of the time required even for an ideal sun, 

 with its attendant planet Jove, to sweep up the wanderers 

 in space. We do know, indeed, that there is a continuous 

 gradation from stable to unstable orbits, so that some 

 meteoric stones may make thousands or millions of re- 

 volutions before meeting their fate by collision. Accord- 

 ingly, not only would a complete absorption of all the 

 wanderers occupy an infinite time, but also the amount 

 of the refuse of the solar system still remaining scattered 

 in planetary space is unknown. And, indeed, it is certain 

 that the process of clearance is still going on. for the 

 earth is constantly meeting meteoric stones, which, pene- 

 trating the atmosphere, become luminous through the 

 effects of the frictional resistance with which they meet. 



All we can assert of such theories is that they demand 

 enormous intervals of time as estimated in years. 



The theory of tidal friction stands alone amongst these 

 evolutionary speculations in that we can establish an 

 exact but merely relative time-scale for every stage of the 

 process. It is true that the value in years of the unit 

 of time remains unknown, and it may be conjectured that 

 the unit has varied to some extent as the physical con- 

 dition of the earth has gradually changed. 



1 Phil Trans , pt. ii., 



NO. 1870, VOL. 72] 



, p. 833. 



It is, however, possible to determine a period in years 

 which must be shorter than that in which the whole 

 history is comprised. If at every moment since the birth 

 of the moon tidal friction had always been at work in 

 such a way as to produce the greatest possible effect, then 

 we should find that sixty million years would be consumed 

 in this portion of evolutionary history. The true period 

 must be much greater, and it does not seem extravagant 

 to suppose that 500 to 1000 million years may have 

 elapsed since the birth of the moon. 



Such an estimate would not seem e.xtravagant to 

 geologists who have, in various ways, made exceedingly 

 rough determinations of geological periods. One such 

 determination is derived from measures of the thickness 

 of deposited strata, and the rate of the denudation of con- 

 tinents by rain and rivers. I will not attempt to make 

 any precise statement on this head, but I imagine that 

 the sort of unit with which the geologist deals is 100 

 million years, and that he would not consider any estimate 

 involving from one to twenty of such units as unreasonable. 



Mellard Reade has attempted to determine geological 

 time by certain arguments as to the rate of denudation 

 of limestone rocks, and arrives at the conclusion that geo- 

 logical history is comprised in something less than 600 

 mTllion years.' The uncertainty of this estimate is wide, 

 and I imagine that geologists in general would not lay 

 much stress on it. 



Joly has employed a somewhat similar, but probably less 

 risky, method of determination.^ When the earth was still 

 hot, all the water of the globe must have existed in the 

 form of steam, and when the surface cooled that steam 

 must have condensed as fresh water. Rain then washed 

 the continents and carried down detritus and soluble matter 

 to the seas. Common salt is the most widely diffused of 

 all such soluble matter, and its transit to the sea is an 

 irreversible process, because the evaporation of the sea 

 only carries back to the land fresh v;ater in the form of 

 rain. It seems certain, then, that the saltness of the sea 

 is due to the washing of the land throughout geological 

 time. . 



Rough estimates may be formed of the amount of river 

 water which reaches the sea in a year, and the measured 

 saltness of rivers furnishes a knowledge of the amount of 

 salt which is thus carried to the sea. A closer estimate 

 may be formed of the total amount of salt in the sea. On 

 dividing the total amount of salt by the annual transport 

 Joly arrives at the quotient of about 100 millions, and 

 thence concludes that geological history has occupied 100 

 million years. I will not pause to consider the several 

 doubts and difilculties which arise in the working out of 

 this theory. The uncertainties involved must clearly be 

 considerable, yet it seems the best of all the purely geo- 

 logical arguments whence we derive numerical estimates 

 of geological time. On the whole I should say that pure 

 geology points to some period intermediate between 50 

 and 1000 millions of vears, but the upper limit is more 

 doubtful than the lower. Thus far we do not find any- 

 thing which renders the tidal theory of evolution untenable. 



But the physicists have formed estimates in other ways 

 which until recently, seemed to demand in the most 

 imperative manner a 'far lower scale of time. According to 

 all theories of cosmogony, the sun is a star which became 

 heated in the process of its condensation from a condition 

 of wide dispersion. When a meteoric stone falls into the 

 sun the arrest of its previous motion gives rise to heat, 

 just as the blow of a horse's shoe on a stone makes a 

 spark. The fall of countless meteoric stones, or the con- 

 densation of a rarefied gas, was supposed to be the sole 

 cause of the sun's high temperature. 



Since the mass of the sun is known, the total amount 

 of the heat generated in it, in whatever mode it was 

 formed can be estimated with a considerable amount of 

 precision. The heat received at the earth from the sun 

 can also be measured with some accuracy, and hence it is 

 a mere matter of calculation to determine how much heat 

 the sun sends out in a year. The total heat which can 

 have been generated in the sun divided by the annual 



1 "Chemical Denudation in Relacion to Geological Time," Bogue, 

 London, 1S79 ; or Roy. Soc, January =3 1879. 



: "An Esiimate of the Geological Age of the Eanh, Trans. Roy. Out. 

 Soc, vol. vii. series iii., 1902, pp. 23-66. 



