Sz the popular science monthly. 



dress to Section A of the British Association at York, went into this 

 question in its commercial aspect. At present, however, we may say 

 that the power of the tides is as much wasted as is the power of Ni- 

 agara. Perhaps, when coal becomes more scarce and when the means 

 of distributing power by electricity are more developed, the tides and 

 the great water-falls will be utilized ; but that day will not be reached 

 while coal is only a few shillings a ton. 



Though we have not yet put the tides into harness, yet tides are 

 not idle. Work they will do, whether useful or not. In some places 

 the tidal currents are scouring out river-channels ; in others they are 

 moving sand-banks. From a scientific point of view the work done by 

 the tides is of unspeakable importance. To realize the importance, let 

 us ask the question, "Whence is this energy derived with which the 

 tides do their work ? The answer seems a very obvious one. If the 

 tides are caused by the moon, the energy they possess must also be de- 

 rived from the moon. This looks plain enough, but unfortunately it 

 is not true. Would it be true to assert that the finger of the rifleman 

 which pulls the trigger supplies the energy with which the rifle-bullet 

 is animated ? Of course it would not. The energy is derived from 

 the explosion of the gunpowder, and the pulling of the trigger is 

 merely the means by which that energy is liberated. In a somewhat 

 similar manner the tidal wave produced by the moon is the means 

 whereby a part of the energy stored in the earth is compelled to ex- 

 pend itself in work. I do not say this is an obvious result. Indeed, it 

 depends upon a refined dynamical theorem, which it would be impos- 

 sible to enter into here. 



But what do we mean by taking energy from the earth ? Let me 

 illustrate this by a comparison between the earth rotating on its axis 

 and the fly-wheel of an engine. The fly-wheel is a sort of reservoir, 

 into which the engine pours its power at each stroke of the piston. 

 The various machines in the mill merely draw off the power from the 

 store accumulated in the fly-wheel. The earth is like a gigantic fly- 

 wheel detached from the engine, though still connected with the ma- 

 chines in the mill. In that mighty fly-wheel a stupendous quantity of 

 energy is stored up, and a stupendous quantity of energy would be 

 given out before that fly-wheel would come to rest. The earth's rota- 

 tion is the reservoir whence the tides draw the energy they require for 

 doing work. Hence it is that, though the tides are caused by the moon, 

 yet whenever they require energy they draw on the supply ready to 

 hand in the rotation of the earth. 



The earth differs from the fly-wheel of the engine in a very impor- 

 tant point. As the energy is withdrawn from the fly-wheel by the 

 machines in the mill, so it is restored thereto by the power of the 

 steam-engine, and the fly runs uniformly. But the earth is merely 

 the fly-wheel without the engine. When the work done by the tides 

 withdraws energy from the earth, that energy is never restored. It 



