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compass, in the same direction as the hands of a watch, the observer's 

 face being directed up the Channel ; and that on the French side they 

 liirn in the opposite direction. Thus, at some distance from shore, 

 we see that the time of high water does not coincide with that of slack. 

 This is well-known to sailors, who term the tide-current which flows 

 up Channel for three hours after high water tide and half-tide. 

 Nevertheless, they have often noted down the time of slack water as 

 that of high water, and thus caused no small confusion and trouble in 

 the discussion of their observations. 



The dynamical theory of Laplace gives no explanation of this 

 phenomenon ; but the present Astronomer Royal has discussed the 

 mathematical laws of the tides considered as waves propagated through 

 the sea ; and his theory, besides giving similar results to Laplace's as 

 to diurnal and semi-diurnal tides, explains the tides in rivers, which 

 the other fails to do. 



We have seen that the tides in the Channel are derivative : the rise 

 and fall of the tide at any place is due to an undulation of the surface 

 which is propagated through the water, by means of comparatively 

 small oscillations of its component particles. 



Theory shows that, in the case of the tide wave, in seas like the 

 English Channel, each particle of water describes about its position 

 of rest an elongated ellipse in a vertical plane, having its longer axis 

 horizontal and in the direction in which the wave is tiavelling. 



Let us trace the path of a particle in the surface, which we will 

 suppose to start from A. It will travel round towards B. At B its 

 motion will be entirely horizontal and up channel. But C ^ is the 

 greatest height to which the particle attains. Thus, when it is at B, 

 high -water occurs ; the particle continues to move onward, but its 

 height diminishes ; thus, as the tide falls, the current continues to 



