TRANSACTIONS OF SECTION J. 489 



a space so large that the tide rushing in and out would have 

 sufficient power to keep a channel open of the required depth. 

 On a low sandy coast this will only have the desired effect if 

 the tidal current is confined between walls projecting into the 

 sea, so as to create a sort of artificial rocky headland, so that 

 the violence of the waves against the stones may disturb the 

 bottom, which enables the shore-currents to carry away the 

 material. Of course there must be a free get-away for the 

 material after it has contended with the entrance and travelled 

 past, and in this generally lies the secret of success in such 

 structures. 



A harbour in the form of an island off the coast is another 

 method of dealing with the difficulty of travelling beaches, 

 and, if well designed and placed, it is no doubt the best solu- 

 tion of it ; but such harbours are necessarily costly, and on 

 that account cannot always be adopted. One of this de- 

 scription was built at Callao, in Peru, about fifteen years ago, 

 and has served its purpose, and stood w^ell in spite of the 

 earthquake-wave wdiich passed over it during its construction. 

 Sir John Coode is building one of this description on the 

 coast of Natal, but I have not yet heard of its success or 

 failure. 



In many parts of the world, as well as in New Zealand, 

 rivers have been furnished with breakwaters and training-walls 

 with the object of enabling them to maintain deeper water than 

 they carried in their natural condition. In most cases such 

 works have been successful, sometimes very much so, as in Grey- 

 mouth and Westport, the Sulina mouth of the Danube, and 

 the South Pass of the Mississippi. Where such artificial mouths 

 to rivers have been constructed, opposing the drift-material of 

 the beaches, they are seen to act very like natural headlands of 

 rock. The commotion of the v/aves disturbs the bottom round 

 about the projecting ends of the breakwaters, and the shore- 

 currents carry away the disturbed material, whether it comes 

 drifting along the beach or is brought down the river ; con- 

 sequently tho success of such structures will be subject to 

 the same laws which determine whether a rocky point shall 

 continue to stand in the sea with deep water round it, or shall 

 be surrounded with a shingle or sand beach. If the supply of 

 drift-material be very large and composed of heavy shingle, 

 while the currents out of the river are feeble, such a work will 

 probably be a failure ; but if the supply be moderate, the 

 material light, and the river-currents strong, the w^ork will be 

 successful. In any case success will depend on the un- 

 obstructed get-away of all material which has accomplished 

 the ordeal of passing the entrance, and any structure on the 

 lee side which interferes with the travel of the material will be 

 injurious to the principal structure. Of course river harbours 



