i$8 HARBOUR CONSTRUCTION. 



shaped entrance has the reverse effect and they are liable to 

 lead heavy seas up the channel, thus frequently causing an 

 inconvenient range at wharves or shipping-berths, even when 

 these are situated far from the harbour- mouth. Moreover, 

 vessels entering the port on the ebb tide, or leaving it on the 

 flood, have the force of the current concentrated upon them, and 

 they cannot get out of each other's way with the same facility as 

 when more width is given. 



Instances are not wanting where, owing to the roughness of 

 the sea within a narrow entrance-channel, the leading vessels of 

 those seeking shelter during a gale have collided and completely 

 blocked the channel, the following ships running in on the top 

 of them. 



The fact that piers projected in this manner do not enclose 

 an area such as to appreciably enlarge the tidal compartment, 

 and so increase the scour at the entrance, is a conspicuous defect, 

 the more so since the construction of such jetties almost in- 

 variably causes an advance of the foreshore, and a consequent 

 reduction of depth. 



The employment of dredgers in such channels is often 

 attended with much inconvenience to the shipping. 1 



Isolated breakwaters, as at Plymouth, Cherbourg, and other 

 places, naturally provide an entrance at either end, between the 

 breakwater and the shore (Fig. 25, p. 157). This gives mariners 

 a choice either in entering or leaving the harbour, and sometimes 

 proves very convenient. 



Additional stillness within a harbour, coupled with the 

 advantages of a double entrance, may be obtained by protecting 

 the main entrance by means of an outlying breakwater placed 

 at a suitable distance to seaward of it. This system has been 

 adopted at several of the Mediterranean ports, and at other 

 places. 



Single breakwaters projected from the shore, as at Holyhead, 

 Portland, Dover (Admiralty pier), etc. (Fig. 26, p. 15V), can 

 scarcely be said to form harbours, the areas so sheltered partak- 

 ing more of tne nature of roadsteads. The amount of shelter 



O 



afforded in such cases depends much upon the direction of preva- 

 lent gales and the trend of the coast ; and it may, as at Holyhead 

 and Portland, very nearly approximate to that of close harbours. 



1 The foregoing remarks must not be taken as applying to the gradual enlarge- 

 ment of a river-channel towards its mouth. 



