ENTRANCE LOCKS. 165 



struction, and frequently valuable wharf space may be gained. 

 There will also be greater economy in working, inasmuch as 

 shorter shores and timbers for stages are required, and the 

 quantity of water to be pumped is reduced. 



The depth of water over the sill will be governed by the 

 draught of the ships to be accommodated, the position of the 

 entrance that is, whether it opens upon a basin where the water 

 is always kept practically at the same level, or into a tideway 

 the range of tide, and whether it is intended that ships should 

 be able to enter at low water of spring tides. 



The length of a dock will, like the other dimensions, be 

 governed by the class of ship to be accommodated. Whilst, 

 hoAvever, it may be desirable to provide for the longest ship 

 likely to visit the port, such a dock may be too long for the 

 ordinary requirements ; in such a case, the dock may be divided 

 into two chambers. The advantages of such an arrangement 

 are that the expense of pumping out the whole of the water 

 when a moderate-sized vessel is docked is avoided, and that two 

 ships may be dealt with at the same time without inconvenience. 

 There is also this further advantage, that it may be possible to 

 complete the outer section so that it may be brought into use 

 whilst the inner section is in progress, the intermediate gates or 

 caisson forming the dam between the two. 



Entrance locks differ from dry docks in having gates or 

 caissons at both ends, and in detail as regards the width, and 

 that the altars are as a rule omitted either wholly or in part. 

 In some cases, however, the arrangement of the altars, slides and 

 slips, and means of pumping, are such as to admit of a lock being 

 used as a dock when required. 



In designing the lock, local and shipping conditions will 

 have considerable weight. For a large dock, it may be advan- 

 tageous to make the width sufficient to admit of two ships lying 

 side by side, with the object of saving time in passing ships in 

 or out of the basin, reducing the quantity of water required for 

 lockage, and avoiding the necessity of constructing additional 

 locks. 



A wide lock is advantageous where there is a large traffic by 

 lighters, or small barges, which have to be passed into the basin 

 for transhipment of cargo. Such traffic can hardly be dealt 

 with by a small lock in a satisfactory manner. 



By dividing a long lock into two parts, it can be used for 



