XL] <&eoixjghal |W0rm. 249 



the accuracy of any of the calculations made by such 

 distinguished mathematicians as those who have made 

 the suggestions I have cited. On the contrary, it is 

 necessary to my argument to assume that they are all 

 correct. But I desire to point out that this seems to be 

 one of the many cases in which the admitted accuracy of 

 mathematical process is allowed to throw a wholly 

 inadmissible appearance of authority over the results 

 obtained by them. Mathematics may be compared to a 

 mill of exquisite workmanship, which grinds you stuff of 

 any degree of fineness ; but, nevertheless, what you get 

 out depends upon what you put in ; and as the grandest 

 mill in the world will not extract wheat-flour from 

 peascods, so pages of formulae will not get a definite 

 result out of loose data. 



In the present instance it appears to be admitted : 



1. That it is not absolutely certain, after all, whether 

 the moon's mean motion is undergoing acceleration, or 

 the earth's rotation retardation. 1 And yet this is the 

 key of the whole position. 



2. If the rapidity of the earth's rotation is diminishing, 

 it is not certain how much of that retardation is due to 

 tidal friction, how much to meteors, how much to 

 possible excess of melting over accumulation of polar 

 ice, during the period covered by observation, which 

 amounts, at the outside, to not more than 2,600 years. 



3. The effect of a different distribution of land and 

 water in modifying the retardation caused by tidal 

 friction, and of reducing it, under some circumstances, 

 to a minimum, does not appear to be taken into 

 account. 



4. During the Miocene epoch the polar ice was cer- 

 tainly many feet thinner than it has been during, or 



1 It will be understood that I do not wish to deny that the earth's rotation 

 may be undergoing retardation. 



