250 NOTES ON DOCKS AND DOCK CONSTRUCTION. 



they can only be removed from the entrance at a given height 

 of tide, to give them a sufficient height above the bottom to 

 clear the batter of the sides. They require a greater number of 

 men to work them, and with a high wind are difficult to manage 

 on account of the surface exposed. 



Box floating caissons have been designed by Mr. Kinipple 

 that can be removed from the entrance when raised sufficiently 

 only to clear the sill. To accomplish this, the sides of the en- 

 trance are made vertical, and the ends of the caisson, instead of 

 being square, are splayed. 



To guard against the risk of the caisson rising through the 

 water-tank being inadvertently emptied, or the ballast being 



FIG. 246. 



FIG. 245. 





Caisson 



Bolt 2" dimn. 



& 



Bolt 2" dia 



Water Line 



id 



FIG. 247. 



displaced, strong iron girders are generally fixed to the 

 masonry at either side of the entrance (Figs. 245, 246), or, in 

 the case of floating caissons, by holding-down straps, as shown 

 by Fig. 247. 



The question as to when caissons should be adopted for 

 closing entrances in preference to gates will depend upon local 

 conditions, and, to some extent, upon personal predilection. 

 It may, however, be taken as a general rule that caissons are 

 more especially adapted for closing dry docks and locks and 



