﻿Chemistry and Physics. 395 



the retardation of the earth's rotation, which resulted from 

 Adams's value. The last sentence of the paragraph here runs as 

 follows : " It is proper to add that Adams lays but little stress 

 on the actual numerical values which have been used in this com- 

 putation, 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 Thom- 

 son'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 " Natural Philosophy " 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 -^3-0 instead of about -3^-, as it is at present. It must neces- 

 sarily 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 adjust- 

 ment would leave traces in preponderance of land in equatorial 

 regions. The existence of large continents and the great effective 

 rigidity of the earth's mass render it improbable that the adjust- 

 ment, if any, to the appropriate figure of equilibrium would be 

 complete. The fact, then, that the continents are arranged along 

 meridians, rather than in an equatorial belt, affords 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 consid- 

 erably 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 sufficiently large margin of 

 uncertainty. 



It will be observed that the argument reposes on our certainty 

 that the earth possesses rigidity of such a kind as to prevent its 

 accommodation to the figure and arrangement of density appro- 

 priate to its rotation. In an interesting discussion 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 ("Climate and Time," 1885, p. 336). Now, 

 since the equatorial protuberance of the earth when the ellipticity 

 is -g^-g- is fourteen miles greater than when it is -g-J-g-, 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 remaius that denudation would suffice 

 to obliterate external evidence of some early excess of ellipticity. 



