WHY A FILM OF OIL CAN CALM THE SEA. 497 



the form of the heap. Its length is the distance from crest to 

 crest, and its height is the distance from the crest to the surface 

 of the water before the disturbance. 



The motions of the individual particles of water are different 

 from the motion of translation which the wave has. Consider a 

 particle in a mass of s^^n. 

 water about to be 

 traversed by a wave 

 form. The action of 

 gravity on the heap 

 behind it tends to 

 press it forward, 

 where it is confront- 

 ed by a solid wall of 

 water. Under the ac- 

 tion of these two op- 



posite forces the par- r> 



tide is driven up- Fig. 1. 



ward and forward 



until the particles which have displaced it have made room for 

 themselves ; then it sinks, and finally comes to rest a little in ad- 

 vance of the place from which it started. The motion of migra- 

 tion of each individual particle is thus in a closed orbit. The 

 propagation of the wave is the advancement of a mere form. The 

 actual translation of water in the propagation of unbroken waves 

 is small. The motion of each particle takes place in a vertical 

 plane parallel to the direction of propagation of the wave. The 

 path of orbit described by each particle is approximately elliptic, 

 and in water of nearly uniform depth the longer axis of the ellip- 

 tic orbit is horizontal and the shorter vertical. When at the top 

 of its path, the particle moves forward as regards the direction of 

 propagation; when at the bottom, backward, as shown by the 



curved arrows in Fig. 1. The 

 straight arrow denotes the di- 

 rection of propagation. 



The particles at the sur- 

 face describe the largest or- 

 bits. The extent of the mo- 

 tion horizontally and vertically diminishes with the depth below 

 the surface. A particle in contact with the bottom of water of 

 moderate depth moves backward and forward in a horizontal 

 straight line, as at D. On the ocean, where the water is deep as 

 compared with the length of a wave, the paths of the particles are 

 nearly circular, and the motion is insensible at great depths. 



When waves are first raised at sea their crests are smooth and 

 rounded, as represented in Fig. 2. As the wind freshens the crests 

 vol. am. — 36 



