VARIOUS TYPES OF BREAKWATERS. 



207 



lighthouse, 1 where, with a similar profile, at the close of a gale, 

 the sea would often break over the top of the lantern." 2 



Sir John Hawkshaw, who, in company with Sir Andrew 

 Clarke, C.B., R.E., was requested by the Board of trade to report 

 upon this work, stated that 



" From the shape of the mound and of the wall, the seas were 

 thrown up to a considerable altitude ; in falling back again their 

 momentum drew away the deposit from the base of the wall, and the 

 bottom courses dropped down, having nothing to stand upon. The 

 sea then got to the rubble hearting between the two legs of the wall, 

 fetched out that, and in a short time worked through the other por- 

 tion of the wall on the harbour side, making breaches clean through." 3 



(6) The liability of a masonry superstructure to be dislocated 

 by settlement, when placed upon a mound of such huge dimen- 

 sions, and in such an exposed position as that at Alderney. 



The settlement of the mound at Alderney equalled one-twentieth 

 of its height, and amounted to fully 6 feet at the seaward end, 

 wherg the depth at low water springs was about 133 feet. 



This large settlement caused cracks and fissures to appear in 

 the superstructure, the air and water within which, being compressed 

 by the wave-stroke, 4 forced out the face stones, and resulted in serious 

 damage. 



(c) The danger of loose or weak hearting, as pointed out in 

 Chap. V., pp. 80-82 



(see Fig. 14, p. 80). 



The waves made 

 breaches right through 

 the breakwater (Fig. 46.) 



(d) The inexpe- 

 diency of giving much 

 batter to the sea face 

 of a breakwater, in- 

 asmuch as it relieves 

 the face stones of 



weight and facilitates their withdrawal. 



1 The superiority of a vertical face over a curved sloping face, as regards the 

 throwing up of water, was very clearly demonstrated during the building of the 

 new Eddystone lighthouse which has a vertical cylindrical base while the old 

 one, with its curved battered face, was yet standing. 



2 Min. Proc. Inst. C.E., vol. xxxvii. p. 93. 



8 Ibid., p. 95 4 See pp. 107 et seq. 



FIG. 46. The unshaded portions show the damage in progress, 

 the dotted lines representing the ultimate breach. 



