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most famous English-speaking astronomer of our day, 

 says on this subject (Story of the Heavens, p. 539) : 



The tides are, however, doing work of one kind or another. 

 A tide in a river estuary will sometimes scour away a bank and 

 carry its materials elsewhere. We have here work done and en- 

 ergy consumed, just as much as if the same task had been accom- 

 plished by engineers directing the powerful arms and navies. We 

 know that work cannot be done without the consumption of en- 

 ergy in some of its forms ; whence, then, comes the energy which 

 supplies the power of the tides? At a first glance, the answer to 

 this question seems a very obvious one. Have we not said that 

 the tides are caused by the moon? And must not the energy, 

 therefore, be derived from the moon? This seems plain enough, 

 but, unfortunately it is not true. It is one of those cases, by no 

 means infrequent in Dynamics, where the truth is widely different 

 from that which seems to be the case. An illustration, perhaps, 

 will make the matter clearer. When a rifle is fired, it is the finger 

 of the rifleman that pulls the trigger ; but are we, then, to say that 

 the energy by which the bullet has been driven off has been sup- 

 plied by the rifleman ? Certainly not ; the energy is, of course, 

 due to the gunpowder, and all the rifleman did was to provide the 

 means by which the energy stored up in the powder could be lib- 

 erated. To a certain extent we may compare this with the tidal 

 problem; the tides raised by the moon are the originating cause 

 whereby a certain store of energy is drawn upon and applied to 

 do such work as the tides are competent to perform. This store 

 of energy, strange to say, does not lie in the moon; it is in the 

 earth itself. Indeed it is extremely remarkable that the moon 

 actually gains energy from the tides by itself absorbing some of 

 the store which exists in the earth. This is not put forward as an 

 obvious result, it depends upon a refined dynamical theorem. 



We must clearly understand the nature of this mighty store 

 of energy from which the tides draw their power and on which 

 the moon is permitted to make large and incessant drafts. Let us 

 see in what sense the earth is said to possess a store of energy. 

 We know that the earth rotates on its axis once every day. It 

 is this rotation which is the source of the energy. Let us com- 

 pare the rotation of the earth with the rotation of the fly-wheel 

 belonging to a steam engine. The rotation of the fly-wheel is 

 really a reservoir, into which the engine pours energy at each 

 stroke of the piston. The various machines in the mill, worked 

 by the engine, merely draw upon the store of the energy accumu- 

 lated in the fly-wheel. The earth may be likened to a gigantic fly- 

 wheel detached from the engine though still connected with the 

 machines in the mill. From its stupendous dimensions and from 

 its rapid velocity, that great fly-wheel possesses an enormous 

 store of energy which must be expended before the fly-wheel 



