490 REPORT— 1891. 



have the additional disadvantage of bringing down material 

 from the interior, and this would be a difficulty which would 

 make success in such structures very uncertain, were it not 

 that in the New Zealand rivers which have been furnished 

 with these improvements prevailing currents and waves are 

 always in one direction, and the frequent floods are so power- 

 ful as to sweep the shingle brought down the river quite out- 

 side the breakwater, where the waves carry it on to the lee- 

 ward beach, and it travels on as it has always done. 



It appears to be evident that the farther seaward such 

 training or guiding breakwaters project the more certain will 

 be their effect in maintaining deep water, partly because on 

 the weather side heavy material will find considerable difficulty 

 in advancing against the waves to the end of the projecting 

 wall, and consequently it will undergo a large amount of rough 

 usage and grinding-down before it gets so fine as to be able to 

 travel with the shore- or wave-currents, which must flow on in 

 whichever direction they are forced to go ; but the finer the 

 material the deeper the water will be in the neighbourhood of 

 projecting rocks, and among currents passing round them. 

 Also, the farther the breakwater projects the farther will the 

 beach be from the end of the leeward breakwater ; and as the 

 supply of material is not increased, and under the supposition 

 that the get-away is unobstructed, the leeward beach should 

 suffer no alteration, and as a consequence the leeward break- 

 water will maintain its original depth of water, into which as 

 into a pit all accumulations drifted round the windward break- 

 water or out of the river will fall and will be washed on to 

 the beach by the waves and carried on their accustomed 

 journey. 



Such prognostigations and reasonings as the above will 

 often be found verified in actual experience ; they will also 

 sometimes be found to fail, for there is always much uncertainty 

 as to what will or will not happen in the construction of 

 artificial harbours, and after a failure has disappointed the most 

 careful observation and long experience it will be discovered 

 that some back current or unlooked-for eddy was not noticed^ 

 the disturbing effect of some opposing waves was not thought 

 of, or the force of such and such a current as compared with 

 the quantity or weight of the material it had to deal with was 

 neglected, and then the poor engineer finds the sweet fruit of 

 all his thought and care turned to bitter ashes just as he hoped 

 to enjoy his long-expected repast. 



