94 POPULAR LECTURES A XL) ADDRESSES. 



after just beginning at the beginning of the 

 century to go slow, and going gradually slower 

 and slower, at a uniform rate of retardation 

 during the century. Thus to get behind by 

 twenty-two seconds at the end of the century 

 implies going slower by '22 of a second per annum 

 at the middle of the century and '44 of a second 

 per annum, at the end, than at the beginning of 

 the century. This, therefore, gives a retardation 

 of '44 of a second per annum per century, or of 

 0044 of a second per annum per annum ; an effect 

 equal in amount to what would be produced by 

 the melting of -044 of a foot of ice per annum from 

 ice caps of twenty degrees round each pole. Thus 

 to produce an amount of retardation equal to that 

 which we estimate as due to the tides, ice must 

 melt at the rate of -044 of a foot per annum, or 

 4'4 feet per century from the polar ice caps. 1 But 



1 The attraction of the polar ice upon the ocean referred to by 

 M. Adhcmar and Mr. Croll, was not taken into account in my calcu- 

 lations in the Rede Lecture of 1866, from which these figures are 

 quoted. Its effect is to render a somewhat less thickness of ice, but 

 ^renter depivx-ion of water in the equatorial regions, necessary to 

 produce the same increase of rotational velocity. 



