TIDES AND WAVES 67 



blowing, the floating timber will merely rise 

 and fall without moving along. 



A still better idea of the motion of waves 

 may be obtained by moving some object along 

 the under side of a piece of paper or cloth. 

 The "wave" of paper or cloth thus produced 

 will appear to move forward just as the water 

 of real waves looks as if it was moving, but in 

 reality the cloth or paper remains stationary 

 just as the water remains practically station- 

 ary, while the wrinkles of the material, which 

 represent the waves, actually move along. 

 When waves are caused by the force of the 

 wind upon the surface of the water they form 

 in fairly regular lines at right angles to the 

 course of the wind, but as they reach shallow 

 water they alter their course and break upon 

 the shore in a line nearly parallel with the 

 coastline. This is due to the fact that the 

 portions of the waves nearest to the shore are 

 retarded by the shallow water while the por- 

 tions still over deep water proceed more rap- 

 idly and thus the waves gradually swing 

 around in line with the coast. This same re- 

 tarding effect of shoal water upon waves 



