w TIDAL EVOLUTION. 



naught, and that there are no forces at work in nature which 

 do not always find their compensating opposites, I believe 

 does not hold good in the case of tidal friction. Silently and 

 slowly though it be, the tides are nevertheless uninterruptedly 

 acting as a brake upon the rotation of the earth, and in mil- 

 lions of years the accumulated effect will not only become 

 appreciable but even of a startling magnitude. Many attempts 

 have been made to compute this increase in the length of the 

 day, but of course the results must be taken with a good deal 

 of reserve. Dr. J. B. Mayer, after an elaborate analysis of 

 the subject, arrives at a value of 1-16 of a second for every 

 2500 years. 



Perhaps the best authority on the matter is Sir William 

 Thomson, who in a careful calculation of the age of the 

 earth's crust shows that it cannot be less than 10,000,000 

 years, because the figure of our globe differs so little from 

 equilibrium in planetoid compression that is, the polar com- 

 pression is nearly the same as that which the present diurnal 

 motion would produce in a molten world. Nearly the same, 

 but not quite. What may be the value of this trifling dif- 

 ference? The product of this discrepancy will furnish us with 

 an approximation of the relative amount of the lengthening 

 of the day. The same high authority has arrived at a valua- 

 tion of i per cent for 20,000,000 years. Out of curiosity I 

 have myself calculated what this i per cent means in fractions 

 of diurnal motion, and have found it to roughly correspond to 

 864 seconds, or 14^2 minutes. Therefore, if any assurance 

 can be placed on the figures which Sir William Thomson sub- 

 mits, it would appear that the earth today does require 14^ 

 minutes more to perform one revolution on its axis than it did 

 20,000,000 years ago. It only needs to be added that this is 

 not presented as an accurate result, but rather as an inter- 

 esting speculation. 



But in accordance with Galileo's third law of motion, 

 that to every action there must always be an equal and oppo- 

 site reaction, it may be asked in what way this reaction 

 exhibits itself in the tidal action of the earth-moon system. 

 The moon, as we have seen, acts upon the earth by retarding 



