LEGHORN DOCK. 199 



line of the dock (Fig. 185). When these courses were set, they 

 formed the base for carrying a staging 48 feet high, with 

 corresponding road timbers on the dock sides. Steam-travellers 

 placed upon these gantries continued the masonry nearly to the 

 water level, above which ordinary shears were used. The centre 

 staging was 150 feet in length, divided into five bays; the 

 rear bay being taken down and re-erected in front as the work 

 advanced. 



The floor and lining of the dock generally was formed of 

 the best quality hard crystalline limestone, found in the islands 

 of Malta and Gozo, in blocks weighing from 3 to 8 tons. The 

 sub-floor and backing was partly of an inferior quality of the 

 same stone and partly of the stone found suitable from the 

 excavation of the dock. 



The whole of the masonry was set in mortar composed of 

 1 part Pozzuolana to '60 parts of lime, ground in a mill with 

 edge-runners for 15 minutes. 



Extension of a Dry Dock at Leghorn. 1 The dry dock at the port 

 of Leghorn, constructed in 1863, having become too short to 

 accommodate the improved class of ships frequenting the port, 

 an extension to the extent of 68 feet 8 inches was decided upon 

 (Fig. 186). 



The dimensions of the original dock were : 



Metres. 



Length on blocks 356'32 



at mean water level ... 373-21 



Width at block level 37'50 



water level 73-16 



Depth of water over sill 22-96 



The foundation was muddy sand, so slightly permeable as 

 to admit of the work being carried out in the open with ordinary 

 pumping. 



Since the completion of the dock, the surrounding conditions 

 have changed. The construction of a wet dock and a canal 

 brought the water close up to the old dock, and rendered the 

 ordinary method of executing the extension by open cutting 

 and pumping inapplicable. An attempt made to carry out 

 the construction of the wells proved a failure, in consequence 

 of the influx of water being so great as to overcome all 

 attempts at drainage, except at a very great risk of affecting 

 the stability of the old work. A method of execution by the 



1 Le Gfn'e Civil, vol. xvi. p. 337. 



