AND ITS INHABITANTS 29 



fall of the earth's fluid envelope. In so far as the body of 

 the earth yields, it is an elastic yielding which involves no 

 measurable expenditure of energy. The oceanic tidal waves 

 tend to continually face the moon and the earth revolves be- 

 neath them, like a wheel revolving between two opposite 

 brake-shoes. This generates tidal friction which tends to slow 

 down the axial rotation of the earth. There is no question as 

 to the correctness of this theory, but there is a very serious 

 question whether the forces are not so weak as to be without 

 any geologic consequences, at least under the present rigid 

 condition of the earth's interior. 



Some of the latest work has been given to measuring directly 

 the retardative influence of the tides, if such exists. Mac- 

 Millan has made an estimate of the loss of energy by friction 

 of the oceanic tides. He used the formulae employed by en- 

 gineers for the loss of head due to friction and viscosity, and 

 applied them to the ocean. His conclusion is that the day 

 would be lengthened by one second in about 500,000 years. 

 Even if this figure be in error tenfold or a hundred-fold it is 

 still in great contrast with the conclusion of Adams in the 

 middle of the last century, that the earth was losing time at the 

 rate of 22 seconds per century, a figure raised to 23.4 seconds 

 by Darwin and lowered to 8.3 seconds by Newcomb. Mac- 

 Millan's method brought to bear as a retardative agency prac- 

 tically all the friction of the tides, irrespective of their positions 

 or directions of motions, and seems to show that the water 

 tides do not have and have never had an appreciable effect on 

 the earth's rotation. 5 



Tidal retardation if it has ever been an important factor in 

 planetary history must then be chiefly due to a body tide. In 

 so far as there is a mere elastic yielding of the body no energy 



5 MacMillan, W. D., On the loss of energy by friction of the tides. In The 

 tidal and other problems. Carnegie Institution of Washington, Pub. No. 107, 

 1909, pp. 71-75. 



