40 



water mark the slope becomes steeper, about 5 horizontal to 1 vertical 

 for a depth of 15 feet ; from this change of inclination to the bottom, 

 no matter how deep, the slope assumes a much steeper character, varying 

 from 1| and 2 horizontal to 1 vertical. The general uniformity in the 

 depth at which these rubble mounds assume their final change of inclina- 

 tion, tends to confirm the opinion that the disturbing action of waves on 

 our coast does not extend lower than 15 fc3t below low- water mark. It 

 has been exemplified, however, after the completion of some of our break- 

 waters, that the disturbing action of the waves extends about 5 feet lower 

 or to a depth of 20 feet ; but this is no doubt owing to the recoil and 

 increased agitation — produced by the upright face of the superstructure. 



Instead of the upright wall as adopted at Holyhead, Portland, and 

 Alderney, the superstructure of the Plymouth breakwater presents a 

 sloping face to the sea. It is evident that a sloping face is less exposed 

 to the violent shocks and blows of the waves than an uj^right wall ; waves 

 in rushing up a slope are allowed as on a beach to expand and exhaust 

 themselves gradually ; whereas on striking an upright wall they act like 

 a ram and tend to overturn the structure, but at the same time the effort 

 of the wave is more effectually destroyed. 



The force exerted by waves in exposed positions is very considerable, 

 and sometimes forms an important element of consideration in reference 

 to harbour work. It is generally assumed that unbroken waves 

 never exceed, under any circumstances, 43 feet in height 

 or 560 feet in length, and that their power of destruction 

 varies directly with the height. The latter depends upon 

 what is termed the "fetch," or distance from the weather shore; 

 and, according to Mr. Stevenson, it is approximately, in feet, equal to 

 once and a half the square root of the fetch in nautical miles. 

 From this, it seems that at a distance of about 800 miles from the 

 weather shore waves attain the maximum height. Now, the 

 heaviest gales recorded at the Liverpool Observatory during the 

 year 1874 and 1875, were from W. S. W., and theii' force was 

 respectively equal to 50 and 70 pounds per sc^uare foot of flat surface 

 With such gales here, the Isle of Man, distant about 30 miles, would form 



