THE TIDES 



113 



(X), and twelve hours later under the further protubérance (Y) 

 — that is to say, bas two bigb-waters daily and twice daily 

 passes tbrougb a low-water position (along tbe Hne PP'). 



A similar gravitational attraction is exercised by the moon, 

 but in this case it is tlie moon which moves around the earth ; 

 or, strictly speaking, both bodies move around their common 

 centre of gravity. Owing to the différence in size between 

 the moon and earth, this common centre of gravity actually 

 lies within the earth, at about three-quarters of the radius from 

 the earth's centre. 



This mutual rotation around a common centre of gravity 

 produces two forces — • 



19c. — Equilibrium Theory of the Tides. 



(i) A centrifugal force which is similar at ail points of the 

 earth's surface. 



(2) An attractive force by the moon on the earth (and, of 

 course, by the earth on the moon). 



The above figure shows the relative strength of the tide- 

 producing force of the moon at différent points at the earth's 

 surface compared with the centrifugal force due to the earth's 

 rotation. 



At ni, the middle point of the earth, the attraction of the 

 moon is equal to the centrifugal force, and AB = AC. At X, 

 which is nearer to the moon than A, the attraction of the moon 

 is greater than the centrifugal force, and XY is greater than 

 XZ. At L the attraction of the moon is less than at A, and 



8 



K 



