WATER OF WAVES NOT PKOGUESSIVE. 233 



their summits, and no^r dropping it into the hollows 

 between. The same effect may be observed with the wa- 

 ter-fowl, which sits upon the surface. It often happens, 

 indeed, that tlie waves on a river roll in an opposite di- 

 rection to the current itself. 



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 remain- 

 Lug stationary, a good representation of waves will bo 

 afforded, and the cloth will appear to advance ; 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 the onward motion. Let 

 the dark line in fig. 260 represent the surface of the water. 



Fig. 2G0. 

 A 



B 



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 carries 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 repeat- 

 ed, and each wave thus advances further on, and its pro- 

 gressive motion is continually kept up. 



BREADTH AXD 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 bottom ; and 

 the distance from any row of particles to the next row 

 that is in precisely the same stage of oscillation is called 



