248 LAY SERMONS, ADDRESSES, AND REVIEWS, [xr. 



remains, therefore, the other half to be accounted for ; 

 and here, in the absence of all positive knowledge, three 

 sets of hypotheses have been suggested. 



(a.) M. Delaunay suggests that the earth is at fault, in 

 consequence of the tidal retardation. Messrs. Adams, 

 Thomson, and Tait work out this suggestion, and, " on 

 a certain assumption as to the proportion of retardations 

 due to the sun and moon," find the earth may lose 

 twenty-two seconds of time in a century from this cause. 1 



(b.) But M. Dufour suggests that the retardation 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, accumu- 

 lating at the rate of one foot in 4,000 years, would 

 account for the remainder of retardation. 2 



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

 thesis of his own with respect to the cause of the hypo- 

 thetical 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 <nve 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 undiscover- 

 able 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." 3 



I do not presume to throw the slightest doubt upon 



1 Sir W. Thomson, loc. cit., p. 14. * Loc. cit., p. 27. 3 Ibid. 



