ESTUARY HARBOURS, ETC. 153 



experience in dealing with such matters can ensure success in 

 any individual case. 



The position of an entrance is to a great extent governed by 

 the depth of water which it is sought to obtain ; but the exact 

 point at which to terminate protecting piers or training banks is 

 frequently, in the early stages of a work, a matter of doubt even 

 with the most experienced engineers. It is therefore important 

 that works of this description should be laid out upon such lines 

 as to admit of their easy extension, should such appear to be 

 desirable, as the works proceed. 



Excepting under the most favourable conditions, and where 

 the volume of upland water is great, a certain proportion of the 

 material brought in by the flood tide will at slack water be 

 deposited in the river-channel in excess of what is removed by 

 the ebb. In such cases dredging must be resorted to in order 

 to keep the channel clear ; and it may here be stated that 

 there are comparatively few instances where natural forces 

 can be made to maintain a channel without such assistance. 

 This represents expenditure for maintenance, and is therefore 

 distasteful to those who have to provide the necessary funds. 

 It is, however, better to admit the fact at the outset than to 

 spoil an entrance by unduly contracting it and attempting 

 impossibilities. 



The relative value of upland and tidal waters necessarily 

 depends in a great measure upon the proportion which they 

 bear to each other. This is, however, not always the case to so 

 great an extent as might at first appear. Owing to the greater 

 density of salt water, the outgoing fresh water floats upon it, 

 and the scouring power of the latter is thus rendered inoperative 

 during the whole period of flood tide, inasmuch as the salt water 

 runs in as an undercurrent, whilst the fresh water flows out 

 over it. 



This is often very noticeable while bathing near a river- 

 mouth, not only the difference in the direction, but also in the 

 temperature of the upper and under currents, being clearly 

 discernible. 



Admiral Sir Edward Belcher states 



" The ships of war fitting at Woolwich were supplied with fresh 

 water from the Thames, pumped up from alongside at the turn of the 

 tide, and up to the first quarter of flood ; but the end of the pump or 



