TIDES AND WAVES 63 



known laws and facts must be taken into con- 

 sideration. In the first place, although we 

 speak of the earth "revolving on its axis," in 

 reality it does not revolve about its absolute 

 centre — like a wheel about its axle — but in- 

 stead it rotates about the centre of gravity of 

 both the earth and moon combined, for the 

 two planets really form a single unit in the 

 solar system. This is perhaps more readily 

 understood by studying the diagram on page 

 62 in which A represents the true centre of 

 gravity of the earth, while B represents the 

 centre of gravity of both the planets together. 

 It is easy to see that if the earth is revolving 

 around the axis at B the side C must travel 

 faster than D and therefore the mobile water 

 on that side will have a tendency to be thrown 

 from the surface, just as mud is thrown from a 

 swiftly-travelling wheel, and it is this tend- 

 ency that is supposed to cause the tidal wave 

 opposite the moon. 



Tides do not always rise and fall the same 

 amount, but vary at different seasons and at 

 first this may seem peculiar if we assume that 

 they are caused by the moon. If every large 



