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NOTE IX. 



THE THEORY OF THE TIDES. 



II. SINCE Newton s time many philosophers have turned 

 their attention to the theory of the tides. Newton left 

 it in so imperfect a state that much remained to be done. 

 ~\Ye shall therefore in the briefest possible manner men 

 tion a few of those steps that have been since made. It 

 would require a treatise by itself to discuss them com 

 pletely. A traveller about to visit a new country looks 

 at the map first : we intend to furnish this map, and must 

 refer to other works those who may desire to study so 

 interesting a subject. 



There are at present three theories of the tides. They 

 are usually called, the equilibrium theory, the theory of 

 Laplace, and the wave theory. The differences of these 

 theories will be best understood by considering them sepa 

 rately. 



(1.) In the equilibrium theory, the rotation of the earth 

 is supposed to have no effect on the form of the tide, so 

 that the times and heights of high water, and all the 

 other circumstances of the tides, at any place may be cal 

 culated on the supposition that both the earth and the 

 moon remain fixed in the position they are in at the mo 

 ment under consideration. Whether this assumption be 

 legitimate, or not, is a question we shall presently have 

 to consider. The whole equilibrium theory is built on 

 the assumption of its truth. 



The general explanation of the tides, according to this 

 theory, is very simple. The moon attracts both the earth 



