THE TIDES 163 



<?al cause of the tides, but that the centrifugal force of the 

 earth's axial rotation is! Nor, granting the premises 

 laid down by their leader, can one find any fault with their 

 logic, as such. Suppose, say they, that the earth did not 

 rotate on its axis at all, then there would be no centrifugal 

 force and, by the same token, there could be no equatorial 

 ring and, incidentally, no tide. Test the matter for your- 

 self: Substitute in the Corollary zero wherever the 

 quantity 85,472 feet appears and you will find that, 

 no matter how stupendous the tidal forces of the sun and 

 moon might otherwise figure out, Newton 's odd method of 

 computing would, did the planet not rotate, inevitably 

 reduce them all to nothing. Here is what no less a person- 

 age than the late lamented Sir Robert Stawell Ball, the 

 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 of 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 



