CARTAGENA DOCK. 297 



of piping, by which the tanks can be drained or the dock 

 washed, or ships supplied with water to test tanks, fill 

 boilers, etc. 



Some such reasoning as the following led to the sides being 

 made in form as shown by Fig. 290. A ship to be moved into 

 a dry dock or on to a slip must have sufficient stability in 

 herself, or ballast must be added to give the necessary stability 

 before she can be moved ; therefore, in this condition she 

 requires no external aid. A pontoon when sunk, ready to 

 receive a ship, must in virtue of its construction have the 

 combined centre of gravity of the structure and contained 

 water some distance below its centre of buoyancy, and con- 

 sequently it must remain upright, even if totally submerged. 

 Now, as it has been shown that the ship must be stable, and 

 the pontoon has enormous stability, it is evident the pontoon 

 requires no sides above the water to give it stability at the 

 time when it and the ship are combined ; but while the water 

 is being pumped out of the pontoon, the combined centre of 

 gravity of the ship, pontoon, and its contained water, will rise, 

 and the combined centre of buoyancy will fall, and therefore 

 the dock must have sides to give stability as it rises, until 

 ultimately the sides are made the full length, and the maximum 

 stability is secured with a minimum of top weight and 

 displacement. The sides being low, allow light and air free 

 access to the ship, and the ship's bottom is thus easily seen and 

 rapidly dried. 



At Cartagena, 1 the arrangement adopted by Mr. Rennie con- 

 sists of an iron dock 324 feet long by 105 feet wide and a total 

 height of 48 feet outside (Fig. 293). The basement, or lifting 

 chamber, is 11 feet 6 inches deep, and is capable of lifting ships 

 of five to six thousand tons. 



The base, or pontoon, is divided longitudinally by a central 

 water-tight bulkhead. Each side is further divided by trans- 

 verse water-tight bulkheads into ten equal compartments, which 

 are again divided by a bulkhead parallel to the centre one, 

 which, however, is not water-tight, but is perforated so as to 

 admit of a partial flow of water from one side to the other. 



The side walls run the whole length of the dock, and are 

 divided by transverse bulkheads similar to those in the basement, 

 and also by horizontal bulkheads, thus separating the side walls 

 1 M.P.LC.R, vol. xxxi. p. 302. 



