in the height of the tide, and that the height of the tide at any port is 

 the sum of these separate oscillations. This conclusion— almost the 

 only practical result of the dynamical theoiy— may be stated thus : 

 the height of the water at any place is the sum of the heights due to 

 several simple harmonic motions, whose periods depend on the 

 motions of the sun and moon. ^ 



Each of these simple harmonic motions may be termed a tidal 

 constituent, or, for brevity, a tide. To render clearer this complex 

 dynamical action, Laplace conceived each of these partial tides as 

 produced by a corresponding imaginary star moving uniformly in the 

 plane of the equator. 



The periods of these tides being known, their other data are 

 deLermined from actual observations by the application of a profound 

 analysis. 



A Committee of the British Association, under the presidency of 

 Sir William Thompson, is at present engaged in reducing series of 

 tidal observations made in different parts of the world. One of its 

 first reports was made at the meeting held in Brighton. 



The larger a sea is, the more perceptible must be the phenomena 

 of the tides. In a fluid mass the impressions which each molecule 

 receives communicate themselves to the whole mass ; and thus it is 

 that the action of the sun, which is insensible on an isolated molecule, 

 produces on the ocean such remarkable effects. The Black Sea and 

 the Caspian have no tides, and the greatest range of the tide in the 

 Mediterranean is not more than four feet. 



But a sea which is too small for the production of perceptible 

 tides by the sun and moon, may have derivative tides ; that is to say, 

 undulations which are excited primarily by the disturbing effect of the 

 luni-solar tide-wave, and subsequently propagated by mechanical 

 action among the particles of water. The tides in the English 

 Channel are of this nature. 



It appears a universal rule throughout the English Channel, that 

 at any great distance from either shore the tide-current runs up the 

 Channel nearly three hours after high water, and down the Channel 

 nearly three hours after low water ; and that on the English side of 

 the Channel, especially opposite the entrances of bays, the direction of 

 the current revolves in 12 hrs. 20 min. through all the points of the 



