IMPORTANCE OF SURF 



101 



Table 27. — Pressures of breakers on west coast of Scotland, in 1845, as recorded by 



spring dynamometers 



[From Gaillard, after Stevenson] 



Table 28. — Pressures recorded on spring dynamometers by waves of different 

 dimensions. The first three wave heights (2 to 6 feet) were for the breaker zone 

 at St. Augustine, Fla.; the last three (12 to 18 feet) were for Duluth Canal, Lake 

 Superior, where readings were taken at two different levels relative to the mean 

 level of the lake at the lime 



[Adapted from Gaillard] 



The force with which a breaker will strike any given object does 

 not necessarily correspond to its total energy; it may, in fact, be 

 very much less, for while the energy of a wave depends solely on its 

 shape and size, the force that it may exert on any obstacle depends on 

 the shape of the object struck, i. e., on how nearly streamline the latter 

 may be, as well as on its size. Thus, a breaker 12 feet high and 200 

 feet long, which would exert a pressure of 1,600 pounds per square foot 

 on a vertical object lying squarely stransverse to its path, such as a 

 barge lying stranded, side on, in the surf zone, would affect the same 

 barge much less, proprtionately, if she were lying bow on. 



The obstacles on which waves may beat vary so infinitely in their 

 contours that it is not practical to make exact mathematical calcula- 

 tions of wave force for given cases unless their shapes are very simple. 

 But pressures, such as those tabulated above for St. Augustine, are 

 ample to account for the displacement of concrete blocks weighing 3,600 

 to 21,600 pounds that actually occurred there during the period of ob- 

 servation, even after due allowance has been made for the shapes of the 

 blocks. And in view of the much higher pressures that have been 

 recorded elsewhere, it is not astonishing that many cases are on record 

 where blocks of stone or cement, or masses of concrete used in break- 

 waters, have been shifted from their beds for longer or shorter dis- 



