206 



HYDRODYNAMICS. 



If a cloth be laid over a number of parallel rollers 

 so far apart as to allow the cloth to fall between them, 

 and a progressive motion be then given to them, the 

 cloth remaining stationary, a good representation of 

 waves will be afforded, and the cloth will appear to ad- 

 vance ; or if a strip of cloth be laid on a floor, repeated 

 jerks at one end will produce a similar illusion. 



It is only the form of the wave, and not the water 

 which composes it, which has an onward motion. Let 

 the dark line in Fig. 175 represent the surface of the 



Fig. 175. 





water. A is the crest of one of the waves, and being 

 higher than the surface at B, it has a tendency to fall, 

 and B to rise. But the momentum thus acquired car- 

 ries these points so far that they interchange levels. 

 The same change takes place with the other waves, 

 and the' dotted line shows the newly-formed surface as 

 the water thus sinks in one place and rises in another. 

 The same process is again repeated, and each wave 

 thus advances further on, and the progressive motion 

 is continually kept up. 



BREADTH AND VELOCITY OF WAVES. 



Each wave contains at any one moment particles 

 in all possible stages of their oscillation ; some rising 

 and some falling ; some at the top and some at the bot- 

 tom ; and the distance from any row of particles to the 

 next row that is in precisely the same stage of oscilla- 



