286 HARBOUR CONSTRUCTION. 



amount of traffic which passes through the Amsterdam ship 

 canal, at the west end of which Ymuiden harbour is situated, 

 and of the success of the whole undertaking, it is evident that 

 those who planned the work were fully justified in the view 

 they took. 



And here it may be remarked how largely the. success or 

 failure of a work is affected by the factor of revenue. A 

 harbour similar in all respects to that at Ymuiden, but lacking 

 its large revenue, would prove an absolute failure. The same 

 may be said of many other harbours around our own coasts and 

 elsewhere, where the amount of dredging required to keep them 

 open is very great. 



The value of tidal and upland waters for scouring away and 

 preventing accumulations of silt, also the evils resulting from a 

 reduction of the volume of tidal water, by reclamations or other- 

 wise, have already been dwelt upon (Chap. IX.). 



In order to increase the scouring power of the ebb tide, large 

 quantities of water are sometimes impounded at high water, and 

 released by means of sluices during the latter part of the ebb. 

 The effect of such sluicing is not, however, generally felt beyond 

 the pier-heads ; and in long channels, where the entrance is far 

 removed from the sluices, the power of the water is often lost 

 long before the harbour entrance is reached. 



The construction of harbours on sandy coasts is admitted to 

 be a difficult problem, and in many instances it would doubtless 

 be better to avoid, than attempt to overcome, the difficulties 

 arising from sand-travel. This can probably best be done by 

 placing the sheltering breakwater in deep water beyond the 

 field of sand-travel. 



If, in addition to affording shelter, it should be intended to 

 use such a breakwater as a quay, it might be connected with the 

 shore by means of an open viaduct. In such a work quay-room 

 could, if necessary, be increased by running out jetties shore- 

 wards under lee of the breakwater, or the ends of the break- 

 water might be returned so as to form a basin. 



Works designed on this principle were proposed for the port 

 of Calais by M. Dupuy de Lome and Mr. Scott Russell ; also for 

 Madras by Sir Andrew Clarke, E.E. ; and subsequently for Port 

 Elizabeth, Cape Colony, by Sir John Coode ; but in no case was 

 effect given to these proposals. 



It is not necessary that a harbour should be what is termed 



