CAISSONS. 



251 



basin entrances in sheltered positions, and gates for closing 

 entrances in exposed positions. 



Floating Ship Caisson. The ordinary form of a floating ship 

 caisson is shown by Fig. 248, which represents one in use 

 at Portsmouth Dock- 

 yard. 1 It consists of a 

 wrought-iron body con- 

 structed of angle- and 

 T - irons, and plated 

 sides, covered with a 

 deck or roadway of 

 Dantzic oak 4 inches 

 thick in 6 -inch widths. 

 On this deck, railway 

 bars are fixed to a gauge 

 of 4 feet 8i inches, 

 sunk into the planking 

 and fixed to the deck- 

 beams. The sides are 

 protected by a hand- 

 railing 3 feet 6 inches 

 high, constructed of 1- 

 inch and 1 J-inch tubing, 

 hinged so as to turn 

 down on to the deck 

 when the caisson is 

 removed from the en- 

 trance. Immediately 

 under the roadway deck 

 is a second deck of 

 diagonal bracing, sup- 

 porting a water-tank in the centre ; below this is the upper 

 water-chamber, strengthened by a third deck constructed of 

 diagonal bracing/the floor of the water-chamber being formed 

 by a water-tight deck. 



Through this chamber, and terminating in the roadway deck, 

 are two water-tight trunks communicating with the air-chamber 

 below, one used as a means of access, the other containing the 

 pump for removing the water from the well. 



Below the water-chamber is the air-chamber, which is of 

 1 X.P.LC.K, vol. Ixiv. p. 146. 



25 FEET 



FIG. 248. 



