WA VES. 31 



The difference in the effect produced by waves upon a com- 

 posite breakwater having a long rubble slope carried up to or 

 above the level of high water of spring tides, and one in which 

 the level of the surface of the rubble mound is such as to admit 

 of " mound-made " translatory waves reaching the superstructure, 

 was very clearly exemplified at Holyhead during a storm which 

 occurred on the west coast towards the end of the year 1889. 



The mound of the Holyhead breakwater was carried up to 

 the higher level (H. W. springs) almost throughout its entire 

 length ; but for a comparatively short distance, near the seaward 

 end, it was not brought up much above the level of low water of 

 spring tides, fears having been entertained that the wave action 

 would cause the rubble to travel round the head into the 

 harbour. 



During the storm referred to, the structure was nowhere 

 affected excepting where the mound was low. At that point, 

 however, the waves struck the " bull-nose," or projecting string 

 course, at the level of 19 feet 3 inches above high water, and 

 displaced it. at the same time overturning the parapet which 

 rested upon it. 



The range of spring tides at Holyhead is the same as at 

 Alderney, namely, 17 feet. 



Crests on Waves, in the open ocean, are formed by the wind 

 urging the top of the wave which is its most exposed portion 

 forward at a greater speed than the body of the wave, and thus 

 causing that portion of the wave to break by pushing it over. 



A wave will sometimes carry its crest for a long distance, 

 and in such a condition it is dangerous to vessels, especially those 

 of small size. Such waves do not, however, disperse, and cannot 

 therefore be viewed as " breakers " in the true sense of the term. 

 This cresting or breaking of the tops of waves is most noticeable 

 during the time that they are being raised by the wind, and 

 before they attain a velocity commensurate with that of the 

 wind. 



A wave crests and breaks in a somewhat similar manner 

 when crossing a sunken reef or shoal. In this case, however, 

 instead of the crest being urged forward, the body of the wave 

 is held back and caused to mount. The free and regular circu- 

 lation of the particles being thus interrupted, the top, owing to 

 its momentum, and to the fact that it is the part least affected 

 by bottom friction, over-runs the other portion of the wave, and 



