2IO 



PRINCIPLES OF STRATIGRAPHY 



meters (nearly i6jk2 feet), while storm waves may have a height of 

 30 feet or over (10-11.5 meters), reaching 50 feet (15-]- meters) in 

 exceptional cases. On the shore the breaking wave or the surf 

 may be as high as 100 feet, or even higher. In mediterraneans the 

 waves are as a rule smaller than in the open ocean. In the Roman 

 mediterranean the maximum height is probably not over 5 meters 

 (16.38 feet). A height of 4 meters is regarded as the usual maxi- 

 mum for storm waves in the North Sea, though a maximum height 

 of 6 meters, a length of 45 meters, and a period of 9 seconds have 

 been reported. 



The orbits in which the water particles of a given wave move 

 have a diameter corresponding to the height of the wave, while 

 the time required for the completion of the circuit by the water 



Fig. 32. Diagrams to illustrate wave form, and its change with change in 

 size of orbit, strength of wind, etc. The heavy arrows indicate 

 the wind direction and the direction of wave-form advance. The 

 smaller (curved) arrows indicate the movements of the water par- 

 ticles. (Original.) 



particle corresponds to the period of the wave. Thus when a par- 

 ticle has moved from the top of the orbit, where it forms a part of 

 the crest of the wave, to the bottom, a trough has replaced the crest, 

 the crest has moved half a wave length forward, or half the wave 

 period has been completed, corresponding to half the revolution of 

 the particle in its orbit. By the time the entire orbit has been 

 completed the wave crest has traversed the entire wave length ; the 

 period is completed. The length of a wave period varies ordinarily 

 for ocean waves from 5.8 to 9.5 seconds, corresponding to an 

 orbital velocity of i.i to 2.3 m. per second, and may sometimes 

 exceed 4 m. per second. The orbital velocity (v) is obtained from 



JlTT 



the formula, v = — , where h is the height of the wave (or 



T 



the diameter of the orbit) and t the wave period. (tt = 



3.14159265359-I-; 3.1416 approximately.) 



