OF GEOLOGICAL DYNAMICS. 93 



to the moon's transits ; and of the amount of rise 

 and fall, than we have at present. 



1 6. But since Professor Huxley has raised the 

 definite question What interchange of water and 

 ice would keep the rotation of the earth constant 

 from the miocene period ? I must point out that 

 it can be answered only when we know how many 

 centuries have elapsed, supposing we assume (as 

 he does with me, for the sake of argument), a 

 uniform datum of tidal retardation ; and must 

 remark that he has omitted to multiply his esti- 

 mated thickness of ice by this unknown number 

 of centuries. The subject is certainly somewhat 

 perplexing, owing to the ambiguity of the words 

 commonly used in expressing such matters ; of 

 which we have a familiar instance in the state- 

 ment, " clock too fast," or " clock too slow," mean- 

 ing clock before, or clock behind. Our estimate of 

 tidal retardation is such as to make the earth, 

 regarded as a clock, come to be twenty-two 

 seconds of time behind at the end of the century, 1 



i Or 25 a times 22 s , that is, 13750% or 3 h 49'" ic s , at the end of 

 25 centuries. 



