NEWTON S PRINCIPIA. 401 



or nearly equal, to the semi-diurnal tide. Yet in the 

 Thames it is absolutely insensible, and in other ports, as 

 well of England as of other ports of Europe and America, 

 though discoverable, it is not notorious, and has only been 

 found from the observations made by men of science. It 

 has been found to be very conspicuous at some places 

 near the equator, and some places near the pole, where it 

 ought not to be discoverable, or scarcely discoverable. The 

 tides of longer period have scarcely been observed.&quot; 



&quot; Altogether this is one of the most contemptible theo 

 ries that was ever applied to explain a collection of impor 

 tant physical facts. It is entirely false in its principles, 

 and entirely inapplicable in its results. Yet, strange as it 

 may appear, this theory has been of great use. It has 

 served to show that there are forces in nature following 

 laws which bear a not very distant relation to some of the 

 most conspicuous phenomena of the tides; and what is 

 more important, it has given an algebraic form to its own 

 results, divided into separate parts analogous to the parts 

 into which the tidal phenomena may be divided, admitting 

 easily of calculation and of alteration, and thus at once 

 suggesting the mode of separating the tidal movements, 

 and affording numerical results of theory with which they 

 are to be compared. The greatest mathematicians and the 

 most laborious observers of the present age have agreed 

 equally in rejecting the foundation of this theory, and 

 comparing all their observations with its results. And till 

 theories are perfect (a thing scarcely to be hoped for in 

 any subject, and less in the tides than any other), this is 

 one of the most important uses of theory.&quot; 



(2.) The next theory to be considered is that of Laplace. 

 In this the phenomena of the tides are treated as a question 

 of hydrodynamics. It is manifest that no true theory can 

 proceed on any other supposition. We may, as usual, 

 suppose the centre of the earth reduced to rest by the 

 application to every particle of the earth and sea a suffi 

 cient force. This we can do, for we are only concerned 

 with the motion of the sea relatively to the earth. The 



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