TIDAL EVOLUTION. 9 



case, and illustrates it with the example of a rifle which is fired 

 by the pulling of a trigger by the rifleman's finger. No one 

 would assert that the energy by which the rifle ball was fired 

 off was supplied by the rifleman. Certainly not. The energy 

 was of course due to the gunpowder, and all the rifleman did 

 was to provide the means by which the energy stored up in 

 the gunpowder became liberated. 



In almost the same way the tides raised by the moon are 

 simply the originating cause whereby a certain amount of 

 energy stored up in the earth itself is liberated and applied 

 to perform such work as the tides are competent to perform. 



The store house of energy upon which the tides through 

 the agency of the moon are making such large and continuous 

 drafts is to be found in the momentum stored up in the axial 

 revolution of the planet. No doubt this is an enormous store 

 house of energy. No doubt it seems to us all but inexhausti- 

 ble. But as the power drawn from it by the tides is not 

 replaced, the irresistible consequence is that the momentum 

 of rotation is being dissipated, and that this rotation is slowly 

 but surely decreasing. As a result the day which is now 24 

 hours is becoming longer. 



If I am not mistaken, it was Immanuel Kant, who, more 

 than a century ago first called attention to the fact that the 

 tides in rolling from east to west were reducing the earth's 

 diurnal motion. He showed that countless aeons of time may 

 elapse before the momentum can be completely exhausted, 

 but it is nevertheless true that the energy is decreasing and 

 the relation of the earth-moon system tending toward equilli- 

 brium. At present the axial revolution of the earth is not 

 measurably longer than it was 100 years ago. Even in 1000 

 years the change in the length of the day may perhaps be 

 only a fraction of a second. But the importance attached to 

 this dissipation of momentum lies in the fact that this phe- 

 nomenon, though acting majestically slow, is not of a rythmic 

 nature but continues right on ceaselessly and eternally in the 

 same direction. 



L,a Place's celebrated theory that opposite currents are as 

 to their effect upon the rotation of the earth as plus and minus 





