168 NOTES ON DOCKS AND DOCK CONSTRUCTION. 



vated material, and thus raising the bottom of the basin, and 

 consequently elevating the water, and with it the vessel, which 

 is by this process raised considerably higher than the level of 

 the water in the river. The water being then run out, strong 

 beams of timber are next placed transversely under the ship 

 at two, three, or four different places, varying according to the 

 length of the ship. At the place where the beams are to be 

 placed, a trench is dug under the vessel, three beams are then 

 passed under her, and placed side by side, the same being 

 repeated at as many places as deemed necessary. The mud 

 thrown in is then removed and the banks levelled, so as to 

 expose the sides and bottom of the ship. For undocking, or 

 rather for lowering the ship into the dock after repair, hawsers 

 coiled into the shape of a cone are used. The diameter of each 

 cone at the base is about 5 or 6 feet, and at the top about 2 feet 

 less ; as the coiling is going on, it is made solid by filling the 

 interior with mud. Four of these cones are generally used, two 



600'. 3'- 



FIG. 145. 



on each side of the ship, unless it is of more than 300 tons 

 burthen, then the number is increased. After the coils are 

 placed in position, the beams are removed, and the excavation 

 completed to the required depth. The vessel, which rests entirely 

 on the cones, is then gradually lowered by withdrawing simul- 

 taneously from the base of each cone, a coil or flake, causing the 

 vessel to bodily subside at a slow rate to the required depth. 

 The cross-bund or dam closing the entrance is then removed, 

 and the water flowing in floats the vessel at high tide. Vessels 

 of 400 to 500 tons have been built and undocked in this manner, 

 at Vizagapatam. 



Avonmouth Lock 1 (Figs. 145 to 150). The total over-all length 

 1 M.P.I.C.K, vol. lv. p. 19. 



