WAVE-SCREENS AND FLOATING BREAKWATERS. 227 



plates backward and forward at the same moment, the effect of 

 such action being to almost neutralize the force of the waves 

 upon these plates, freeing the structure in a great measure from 

 the violent action of the sea tending to destroy it. 



A small structure of this kind was, I believe, erected at the 

 head of a jetty at Glenelg, near Adelaide (Australia), in the year 

 1858. Its form on plan was that of a crescent, facing in the 

 direction of the prevailing winds, and its length was about 

 240 feet. The depth of water at low water springs was 11 \ feet, 

 the tidal range being 6 feet. 



In a position sheltered by nature to the extent that Adelaide 

 is, a structure of this kind might give some protection by 

 combing down small waves ; but in a short time, especially in a 

 warm climate, the immersed louver-plates would, unless frequently 

 scraped, become thickly coated, or even caked into a solid mass, 

 by marine growth. 



The removal of this from time to time would entail con- 

 siderable cost, and would, further, be injurious to the plates; but 

 if not done, the water would be unable to pass between them, 

 and the screen would practically become a close breakwater, any 

 real or assumed advantages of the louver-plate system being 

 thus altogether nullified. 



The idea of only carrying the louver-plates down to about 

 12 feet below low water probably originated in the old-fashioned 

 and long-ago-exploded notion that wave-action did not extend 

 below such level at all events, to any appreciable extent. 

 Later research and experience have, however, demonstrated the 

 fallacy of such an idea, as has been pointed out in previous 

 chapters. 



A structure such as the one just described could only inter- 

 cept small surface waves, and would, without doubt, be quickly 

 destroyed in a heavy sea. Indeed, this view seems to be borne 

 out by Mr. Hays's own statement. In the pamphlet before 

 referred to I find him saying (p. 5) 



" It should, however, be borne in mind that this breakwater is 

 adapted only to situations where there is some depth of water; it is 

 not calculated to resist the shock of breaking waves near a beach 

 or in very shallow water." 



It would thus appear that the ideas of the inventor respecting 

 waves were not in accordance with more recent experience. 



