TEANSACTIONS OF SECTION A. 515 



* It is proper to add that Adams lays but little stress on the actual numerical 

 values which have been used in this computation, and is of opinion that the amount 

 of tidal retardation of the earth's rotation is quite uncertain.' Thus, in the opinion 

 of our great physical astronomer, a datum is still wanting for the determination 

 of a limit to geological time, according to Thomson's argument. 



However, subject to this uncertainty, with the values used by Adams in his 

 computation, and with the assumption that the rate of tidal friction has remained 

 constant, then a thousand million years ago the earth was rotating twice as fast as 

 at present. In the last edition of the ' Natural Philosopby ' the argument from 

 these data runs thus : — 



' If the consolidation of the earth took place then or earlier, the ellipticity of 

 the upper layers (of the earth's mass) must have been ~ instead of about ~, as 

 it is at present. It must necessarily remain uncertain whether the earth would 

 from time to time adjust itself completely to a figure of equilibrium adapted to the 

 rotation. But it is clear that a want of complete adjustment would leave traces 

 in a preponderance of land in equatorial regions. The existence of large continents 

 and the great eSective rigidity of the earth's mass render it improbable that the 

 adjustment, if any, to the appropriate figure of equilibriiun would be complete. 

 The fact, then, that the continents are arranged along meridians, rather than in an 

 equatorial belt, afibrds some degree of proof that the consolidation of the earth 

 took place at a time when the diurnal rotation differed but little from its present 

 value. It is probable, therefore, that the date of consolidation is considerably 

 more recent than a thousand million years ago.' 



I trust it may not be presumptuous in me to criticise the views of my great 

 master, at whose intuitive perception of truth in physical questions I have often 

 marvelled, but this passage does not even yet seem to me to allow a sufiiciently 

 large margin of uncertainty. 



It will be observed that the argument reposes on our certainty that the earth 

 possesses rigidity of such a kuid as to prevent its accommodation to the figure 

 and arrangement of density appropriate to its rotation. In an interesting dis- 

 cussion on subaerial denudation, Croll has concluded that nearly one mile may 

 have been worn off the equator during the past 12,000,000 years, if the rate of 

 denudation all along the equator be equal to that of the basin of the Ganges.^ 

 Now, since the equatorial protuberance of the earth when the ellipticity is ^ is 

 fourteen miles greater than when it is gi^, it follows that 170,000,000 years 

 would suffice to wear down the surface to the equilibrium figure. Now let 

 these numbers be halved or largely reduced, and the conclusion remains that 

 denudation would suffice to obliterate external evidence of some early excess of 

 ellipticity. 



If such external evidence be gone,'' we must rely on the incompatibility of the 

 known value of the precessional constant with an ellipticity of internal strata of 

 equal density greater than that appropriate to the actual ellipticity of the surface. 

 Might there not be a considerable excess of internal ellipticity without our being 

 cognisant of the fact astronomically ? 



And, further, have we any right to feel so confident of the internal structure of 

 the earth as to be able to allege that the earth would not through its whole mass 

 adjust itself almost completely to the equilibrium figure ? 



Tresca has shown in his admirable memoirs on the flow of solids that when the 

 stresses rise above a certain value the solid becomes plastic, and is brought into 

 what he calls the state of fluidity. I do not know, however, that he determined at 

 what stage the flow ceases when the stresses are gradually diminished. It seems 

 probable, at least, that flow will continue with smaller stresses than were initially 



' Croll, Climate and Time, 1885, p. 336. 



* I find by a rough calculation that ^ths of the land in the N. hemisphere is in 

 the equatorial half of that hemisphere, viz. between 0° and 30° N. lat. ; and that it^ths 

 of the land in the S. hemisphere is in the equatorial half of that hemisphere"viz. 

 between 0° and 30° S. lat. Thus for the whole earth, igiths of all the land lies in 

 the equatorial half of its surface, between 30° N. and S. lat. In this computation the 

 Mediterranean, Caspian, and Black Seas are treated as land. 



I. I. 2 



