HYDROGRAPHICAI> SURVEY. — WAVES. 219 



six feet above the general level of the sea. Such 

 an elevation of the water occasioning an equal de- 

 pression, produces waves of twelve feet perpendicu- 

 lar height. Accidental or extraordinary waves, 

 however, such as where cross seas meet, or where 

 parallel waves over-run one another, are sometimes 

 much higher. The first cause of waves is doubt- 

 less the action of the wind ; but the undulations 

 which continue for many hours after the producing 

 cause ceases to act, are attributed to the same causes 

 as those which occasion the continuation of the vi- 

 brations of a pendulum for some time after any 

 impulse. 



The apparent progressive motion of waves has 

 been shown, by Sir Isaac Newton, to be in the sub- 

 duplicate ratio of their breadth, and the time in 

 which a wave moves its breadth forward, (measured 

 from the top of one wave to the top of the next), 

 to be about the same as that in which a pendu- 

 lum will perform one single oscillation, " whose 

 length between the point of suspension and the cen- 

 ter of oscillation, is equal to the breadth of the 

 wave." Thus, while the particles of water have no 

 horizontal motion whatever, the ridge of each wave 

 may move with a velocity of 16 or 18 miles per 

 hour. The progressive motion of waves resembles 

 considerably the progress of a vibration on a very 

 long tight cord or wire. If a cord of 20 or 30 yards 

 in length, moderately extended in the air, be struck 

 near one end, a vibration resembling a wave will 



