THE OCEAN AND ITS WORK 



199 



are set in motion and any floating object is seen to rise and fall as the 

 crests and troughs of the waves pass under it, but it is not borne along. 

 As each wave glides under the object, it is moved forward a short dis- 

 tance, but as soon as the crest has passed beneath it, it comes back 

 to its former position. In storm waves, however, the friction of the 

 wind drives some of the surface water along and thus produces sur- 

 face currents (p. 217). The height of a wave is the vertical distance 

 between the trough and the crest, and the wave length is the distance 

 from crest to crest. The wave length in heavy storms varies but 

 little from 600 feet, although waves more than twice that length have 

 been observed in the southern ocean. Each particle of water in a 

 wave moves in a vertical orbit (Fig. 185), i.e., if a wave is ten feet in 



a b c d e f y h a' b' c r 



Fig. 185. — Diagram illustrating the orbital movement of water in waves. The 

 particles of water move forward in the crests and backward in the troughs, each particle 

 moving in a closed orbit. 



height the diameter of the orbit is ten feet. In open seas storm waves 

 may be 20 to 30 feet high, and waves of 50 feet have been reported ; 

 it is, perhaps, doubtful if waves exceeding 50 feet in height are. ever 

 developed in the open ocean. Waves 10 to 15 feet high are propa- 

 gated at a rate of about 60 miles an hour. 



Wave motion is propagated indefinitely downward, but rapidly de- 

 creases from the surface to the bottom (Fig. 185), so that at compara- 

 tively shallow depths even sand is not disturbed ; the force of wave 

 motion is one fifth at 65 feet (20 m.), one fiftieth at about 190 feet 

 (50 m.), and perhaps not effective below 230 feet (70 m.). The 

 depth to which agitation extends is in the ratio which the length bears 

 to the height. Thus, a wave 30 feet long and 10 feet high would move 

 the water 6 inches vertically at a depth of 10 feet, whereas a wave of 

 the same height and three times the length would agitate the water 

 18 inches below the bottom of the wave. (Wheeler.) In violent 

 storms it is possible that there is some motion at 3000 feet, but, in 



