106 



BREAKERS AND SURF 



Irregularities, however, of the sorts just cited, do not vitiate the 

 underlying rule that the periods of waves that preserve their identity 

 are at least approximately the same when they break as when they 

 are farther out. Consequently, if one times the intervals between suc- 

 cessive breakers, as can easily be done on any ordinary watch that has 

 a second hand, one can at least roughly calculate the lengths that these 

 same waves had while they were still out in deep water or vice versa, 

 and many calculations of this sort have been published for various 

 parts of the world. Average periods, for example, of 8 to 12 seconds, 

 for the waves included in table 30, correspond to lengths of about 328 

 feet to about 737 feet offshore, and an average of 10 seconds between 

 breakers timed by us on the southwest coast of Ceylon, January 1902, 

 corresponds to an average wave length offshore of about 512 feet. 



Table 30. — Frequency distribution of waves of different periods at South Beach, 

 Marthas Vineyard, from observations made between January and April 1944- 

 Each case is the mean of 20 consecutive waves 



It is fortunate for anyone planning to land through the surf that 

 the period of a wave does not alter as it nears the land (unless by its 

 union with another) , for thanks to this fact, measurements of the pe- 

 riods of waves offshore, which are comparatively easy to make (p. 61), 

 give at least a rough indication of the periods between breakers on 

 a neighboring beach. And this is a matter of some practical im- 

 portance, for the longer the time interval is between successive break- 

 ers, the easier it is to bring a boat in through them, or to go out again 

 from the shore. 



Alteration in height over a shoaling bottom. — The most important 

 feature of breakers to anyone who has to land through them is their 

 height. And it has been known to seafarers, doubtless since the days 

 of the Phoenicians, that swells often grow considerably higher just 

 before they break. The most noticeable alteration in their heights 

 as they move in over a shoaling bottom is thus the reverse of what 

 occurs for their lengths and their velocities. 



It is not so generally known, because only recently discovered, that 

 this increase in height is preceded by a very small initial decrease, 

 which begins when the depth is about one-half as great as the length 



