Foundations of the Universe 293 



evidences of this energy all round the word's coastlines. Estua- 

 ries are scooped out, great rocks are gradually reduced to rubble, 

 innumerable tons of matter are continually being set in movement. 

 Whence is this energy derived? Energy, like matter, cannot 1 it- 

 created from nothing; what, then, is the source which makes this 

 colossal expenditure pv^ssible. 



The Earth Slowing down 



The answer is simple, but startling. The source of tidal 

 energy is the rotation of the earth. The massive bulk of the earth, 

 turning every twenty-four hours on its axis, is like a gigantic 

 flywheel. In virtue of its rotation it possesses an enormous store 

 of energy. But even the heaviest and swiftest flywheel, if it is 

 doing work, or even if it is only working against the friction of its 

 bearings, cannot dispense energy for ever. It must, gradually, 

 slow down. There is no escape from this reasoning. It is the 

 rotation of the earth which supplies the energy of the tides, and, 

 as a consequence, the tides must be slowing down the earth. The 

 tides act as a kind of brake on the earth's rotation. These masses 

 of water, held back by the moon, exert a kind of dragging effect 

 on the rotating earth. Doubtless this effect, measured by our 

 ordinary standards, is very small ; it is, however, continuous, and 

 in the course of the millions of years dealt with in astronomy, this 

 small but constant effect may produce very considerable results. 



But there is another effect which can be shown to be a neces- 

 sary mathematical consequence of tidal action. It is the moon's 

 action on the earth which produces the tides, but they also react 

 on the moon. The tides are slowing down the earth, and they are 

 also driving the moon farther and farther away. This result, 

 strange as it may seem, does not permit of doubt, for it is the 

 result of an indubitable dynamical principle, which cannot be 

 made clear without a mathematical discussion. Some interesting 

 consequences follow. 



Since the earth is slowing down, it follows that it was once 



