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CHAPTER XX. 



PRELIMINAEY. 



In Book I. a single phenomenon was accepted as 

 siifficient to indicate that a force whose action is a 

 matter of commonplace knowledge, is in play in the 

 ocean in the same manner as that in which its action 

 can be observed in ordmary phenomena. 



In Book II. it is shown that the action of that 

 force in the ocean tends to cause the elaborate circu- 

 lation through the system of ocean currents there 

 described. 



In Book III. it has been ascertained that the 

 force which causes the foregoing system of cu'culation 

 is intrinsically the same as that which causes the 

 well-known phenomena of the tides. 



And it has been shown that that force must tend 

 to give both the moon and its tide a relative motion 

 over the surface of the earth by the same action as 

 that which circulates any particle of water in the 

 ocean. And a cursory consideration of the motions 

 of the moon and the planets has shown that they are 

 not at variance with the action of that force, 



That analysis has shown that the vis -inertia of 



