FOR GREAT BRITAIN AND IRELAND. 77 



The observations of the writer were to this effect, that on 

 a slope of eleven and a quarter degrees, there is no recoil 

 or backsend from the waves, which spend their violence 

 thereon up to a slope of nearly forty-five degrees, but with 

 the approach to the last-named gradient a visible recoil 

 commences, and increases fast between that and seventy 

 degrees, to which extreme steepness the shingle will often 

 run for ten or fifteen feet below high water-mark, when the 

 shingle beach is perpendicular to or directly faces the pre- 

 valent wind through the curvature of the shore. When 

 this is the case, a heavy swell breaking against the top of 

 the slope, is reflected back with so much violence that 

 the backsend may be plainly seen to travel as far as 

 four hundred feet, more or less, according to the power of 

 the undulation or ground-swell at the time, in the seaward 

 direction. 



These observations on the angles of recoil and expen- 

 diture of wave-force manifestly have an important bearing 

 on the subject of the gradient of any artificial beach or 

 talus, and point to the wisdom of preserving a portion of 

 natural sea beach, unless a pressing necessity exists for 

 occupying the same by a quay, in which case it may 

 be necessary to resort to one of the before-mentioned 

 methods. 



BOOMS FOR EXCLUDING WAVES. 



Harbours of small reductive power are sometimes 

 closed by logs of timber named booms, the ends secured 

 by being lowered into grooves of masonry on each side of 

 the entrance of the basin inside. The lowest log rests 

 on a sill-piece at the bottom of the channel, and the logs 

 being continued to the level of the coping, a barrier or wall 



