VELOCITY AND PERIOD 



31 



Table 14. — Correlation between the age and the steepness of growing waves 



[Derived from an average curve fitted to empirical data, in a study by Sverdrup and Munk, Scripps Institu- 

 tion of Oceanography] 



A given wave may repeatedly break ; anyone can satisfy himself on 

 this point by looking out over the water when a brisk breeze is blowing, 

 for it is often possible to watch an individual crest steepen until it 

 breaks with a consequent decrease in its height and steepness, then 

 builds up to the breaking point for a second time or sometimes even for 

 a third time before it is lost to view among the neighboring waves. 

 And we have no doubt that every individual wave of a stormy sea 

 breaks in this same way time after time. Thus the history of the 

 compound wave is one of constant alterations in its steepness, altera- 

 tions of which the theoretical calculations of height and length give 

 no hint and on which no information is available from observations. 



THE VELOCITIES AND PERIODS OF WAVES 



The fact that is perhaps the most difficult for the layman to accept, 

 when first he observes waves at sea, is that it is not the direct push of 

 the wind against their backs that causes the waves to advance, but 

 that once a wave of oscillation has been set in motion, it will continue 

 to run across the surface of the sea, even in a flat calm. This fact is 

 easily demonstrated; if one drops a stone onto a calm water surface, 

 it is easy to see that the resultant wavelets run out in all directions, 

 far beyond the site of the original disturbance. Perhaps the most 

 striking illustration of the rule that wave forms may continue to 

 advance long after the disturbance immediately responsible for them 

 has ceased, is afforded by the bow waves that a steamer sets up in her 

 passage through the water. These may run so far, that, in thick 

 weather, the first notice a watcher on the land may have that a ship is 

 passing offshore comes when the waves she has set up break on the 

 beach. 



Theoretically, the velocity of a freely running wave in deep water 

 is determined chiefly by its length, the rule being that the longer the 

 wave, the higher its velocity. And while the relative steepness of a 

 wave does have some theoretical effect on its velocity, this effect is so 

 small that it can be ignored for all practical purposes. Consequently, 

 an old swell, long but now low, travels at least as fast as the much higher 



