CAISSONS. 



259 



FIG. 2CO. 



30: o- - 



the caisson has a lifting power of 60 tons, which is only 

 exhausted when it reaches the floating draught of 20 feet. 



A weak point in this de- 

 sign appears to be that, in the 

 absence of pumps for clear- 

 ing the lower tank, the caisson 

 cannot be prepared for lifting 

 without first pumping out the 

 dock. 



Bute Docks. 1 Figs. 260 to 

 263 show the caisson closing 

 the entrance of the public 

 graving dock. It is a floating- 

 ship caisson of somewhat un- 

 usual section, constructed of 

 wrought iron, 60 feet long, 26 

 feet wide at its greatest beam, 

 and 13 feet in least width at 

 the centre, and 30 feet 6 inches 

 in depth. It is constructed 

 of angle-iron frames, columns, 



20 FEET 



FIG. 2G2. 



FIG. 263. 



beams, and skin-plates, with oak stems and keel, and fir deck- 

 planking. 



It is divided by a water-tight deck and bulkheads into 

 sixteen compartments, communicating with one another by 



1 Engineering, vol. xxxviii. p. 165. 



