46 



Lock g<ates of large span are generally so constructed that when 

 closed they form an arc of a circle, thus producing tightness or compres- 

 sion at the meeting posts and side walls, tending to prevent leakage. 

 With gates of medium size this is obtained by making them meet at an 

 obtuse angle, so that M'hen closed against the water pressure they mutually 

 strut each other, and moreover the opening and closing of the gates is facil- 

 itated. The entrance to Whitehaven Dock is 50 feet wide ; during a spring 

 tide, the stress in direction of each gate is 233 tons and the resulting 

 thrust on each side wall is 250 tons. With forces such as these, together 

 with the hydrostatic pressure on the gate floor, it is evident that the work 

 in the vicinity of lock gates requires to be not only massive but as 

 impervious as possible. 



Sluices or outlets are generally formed in the gates or side walls for 

 the purpose of keeping the lock floor clean, and to counteract any shoaling 

 or silting tendency in the vicinity of the entrance ; their efficiency will, of 

 course, dejjend in a great measure upon the volume and velocity of the 

 water emitted. The latter is proportional to the square root of the depth 

 of the outlet below the surface of the water ; that is to say, with four 

 times the depth the velocity will be only doubled. If the stream is 

 opjjosed by a stratum of standing water the efficiency of the sluicing will 

 be nullified, and it -will only tend to create whirlpools and eddies. 



In connection with harbours and wet docks it is necessary to have some 

 place for the reception of vessels so that they can be examined and repaired 

 in the dry. For this purpose the "Graving or Dry Dock" is the most popular. 

 When a vessel is once in and the gates closed, the water is allowed to run 

 out, or is pumped out by steam power, according to circumstances, thus 

 leaving the dock perfectly dry until the vessel's repairs are complete, when 

 the water is again admitted and the vessel floated out. Other means are 

 adopted for repairing vessels in the dry, or free from the flow of the tide, 

 such as the " Patent Slip," the " Hydraulic Lift Dock," and the 

 *' Floating Dock. " The Patent Slip is only used for vessels of medium 

 size, and consists of a cradle with rollers, upon which the vessel is laid 

 and then drawn up an incline beyond high water mark. One of these 

 may be seen in use at the south side of the Whitehaven Harbour. 



