832 GEOLOGICAL REFORM X 



of the earth (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.^ 



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

 h3^othesis of his own with respect to the cause 

 of the hypothetical retardation of the earth's 

 rotation : — 



" 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 1"1 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 undiscoverable 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 observations 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." ^ 



I do not presume to throw the slightest doubt 

 upon the accuracy of any of the calculations made 

 by such distinguished mathematicians as those 

 1 Sir W. Thomson, loc. cit. p. 27. ^ /jj^?. 



