THE TIDES. 



a b c d e, &c. That it is actually the case may be demonstrated 

 by placing on the surface a series of floating bodies, each of 



Fig. 4. 



which will be observed to rise or sink with the water in the 

 manner here described. 



It appears, therefore, that the advance of the wave from 

 A c E to c d h is in fact produced not by any advance of the 

 water, but by its different points rising and sinking alternately 

 in the vertical direction. It will thus be understood how the 

 form of a wave may actually have a progressive motion, while 

 the water that composes it continues always to hold the same 

 position over the bottom. The real motion of the particles of 

 the liquid by which the waves are produced is an alternate 

 vertical motion upwards and downwards through a height equal 

 to the difference between the level of the crest and the hollow 

 of each wave, or what is the same, through twice the height 

 of the crest of the wave above that level at which the water 

 would settle if it were absolutely quiescent and free from all 

 undulation. 



If a cloth were laid loosely over a number of parallel rollers 

 placed at equal distances asunder, so that it would fall between 

 roller and roller, it would represent the form of a series of 

 waves. If a progressive motion were given to the rollers, the 

 cloth being kept stationary, the progressive motion of the waves 

 would be produced ; the cloth would seem to advance. 



It is the same cause which makes a revolving cork-screw, 

 held in a fixed position, seem to be advancing in that direction 

 in which it would actually advance if the screw were passing 

 142 



