216 LAY SERMONS, ESSAYS, AND REVIEWS. [XL 



(which is hypothetically assumed to exist) may be due in part, or 

 wholly, to the increase of the moment of inertia of the earth by 

 meteors falling upon its surface. This suggestion also meets 

 with the entire approval of Sir W. Thomson, who shows that 

 meteor-dust, accumulating at the rate of one foot in 4,000 years, 

 would account for the remainder of retardation. 1 



(c.) Thirdly, Sir W. Thomson brings forward an hypothesis 

 of his own with respect to the cause of the hypothetical 

 retardation of the earth s rotation : 



&quot; Let us suppose ice to melt from the polar regions (20 round 

 each pole, we may say) to the extent of something more than a 

 foot thick, enough to give I l foot of water over those areas, or 

 0*006 of a foot of water if spread over the whole globe, which 

 would, in reality, raise the sea-level by only some such undis- 

 coverable difference as three-fourths of an inch or an inch. This, 

 or the reverse, which we believe might happen any year, and 

 could certainly not be detected without far more accurate obser 

 vations and calculations for the mean sea-level than any hitherto 

 made, would slacken or quicken the earth s rate as a timekeeper 

 by one-tenth of a second per year.&quot; 2 



I do not presume to throw the slightest doubt upon 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 workman 

 ship, 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 formulas will not get a definite result 

 out of loose data. 



1 Sir W. Thomson, loc. cit., p. 27. 2 Ibid. 



