THE TIDAL WAV3S. 299 



tides may consequently be estimated at g^th of the effect re- 

 ceived by the earth from the sun. 



The rotatory effect which the earth at present possesses, 

 may be calculated from its mass, volume, and velocity of rota- 

 tion. The volume of the earth is 2,650,686,000 cubic miles, 

 and its specific gravity, according to Reich, = 5-44. If, for 

 the sake of simplicity, we assume the density of the earth to 

 be uniform throughout its mass, we obtain from the above 

 premises, and the known velocity of rotation, 25,840 quadril- 

 lions of kilogrammetres as the rotatory effect of the . earth. 

 If, during every second in 2500 years, 464,000 millions of 

 Km of this effect were consumed by the ebb and flow of the 

 tidal wave, it would suffer a diminution of 36,600 trillions of 

 Km, or about To^oooth of its quantity. 



The velocities of rotation of a sphere stand to each other 

 in the same ratio as the square roots of the rotatory effects, 

 when the volume of the sphere remains constant. From this 

 it follows that, in the assumed time of 2500 years, the length 

 of a day has increased i^osA ; or if a day be taken equal 

 to 86,400 seconds, it has lengthened ^th of a second, if the 

 volume of the earth has not changed. Whether this supposi- 

 tion be correct or not, depends on the temperature of our 

 planet, and will be discussed in the next chapter. 



The tides also react on the motion of the moon. The 

 stronger attraction of the elevation nearest to, and to the east 

 of the moon, increases with the tangential velocity of our sat- 

 ellite ; the mean distance of the earth and the moon, and the 

 time of revolution of the latter, are consequently augmented. 

 The effect of this action, however, is insignificant, and, ac- 

 cording to calculation, does not amount to more than a frac- 

 tion of a second in the course of centuries. 



