144 THE EVOLUTION OF 



the Moon, and the other at the part of the Earth's surface 

 that is most remote from it. If the period of the Earth's 

 rotation was the same as that of the Moon's revolution 

 round it, the Moon would continually face the same 

 regions of the Earth, and in the course of time, possibly 

 in a few months or years, the water would reach a 

 position of equilibrium, forming permanent high tides at 

 the opposite ends of the diameter that would then be 

 permanently directed to the Moon. 



This simple condition is, however, profoundly modified 

 by the Earth's rotation. As the Earth turns under the 

 Moon in a period of slightly less than twenty-five hours, 

 the regions presented to the Moon those at which the 

 water tends to accumulate are continually changing, and 

 before any portion of water could move appreciably 

 towards them, the forces acting upon it would change 

 and it would be urged in some other direction. The pro- 

 blem thus becomes extremely complicated. The general 

 result, however, is, that in its continual endeavour to 

 move towards the ends of the terrestrial diameter that 

 is at each given instant pointing to the Moon, the water 

 on the Earth's surface is thrown into the continual motion 

 that we recognize as tidal ebb and flow. 



If the movement of water were unresisted by friction, 

 tidal ebb and flow would possess no cosmical significance, 

 but friction is experienced in the motion of the tidal wave 

 over the surfaces of shores and estuaries, and in internal 

 motions of water itself. The destruction of motion by 

 friction develops heat, and the Earth is consequently 

 warmed by its tides ; moreover, since heat is a form of 

 energy, some other form of energy, equivalent in amount, 

 must disappear in producing it. From considerations of 

 a not very difficult nature, it can be shown that this 

 energy is that of the Earth's rotation, so that we are 



