EXAMPLES OF WALL SECTIONS. 151 



the contents discharged. The barge was then hauled back to 

 its original position, and the bell again lowered. An area of 

 400 square feet was thus levelled with the greatest facility and 

 accuracy. 



For lifting, transporting, and placing the blocks in position 

 a specially constructed shear pontoon was used. This vessel 

 was rectangular in section, 130 feet long over all, 48 feet wide, 

 by 14 feet deep, and provided with two pairs of lofty shear legs ; 

 one pair forward and the other aft. The cross-head of the 

 former, which lifted the blocks, was 54 feet above the deck, and 

 carried two sets of heavy four-shear pulleys, through which, 

 and corresponding lower blocks, the pitch-chains for lifting the 

 concrete blocks were rove. The lower pulley blocks were pro- 

 vided with forged projections which gripped the upper heads of 

 the suspension rods passing through the concrete block. 



The after shear legs supported a counterbalance tank of 

 the same width as the pontoon, 30 feet long by 17 feet 

 deep, and loaded partly with concrete and partly with water. 



The centre of the block, when suspended, was 15 feet from 

 the bow of the vessel, the centre of the tank was also 15 feet 

 from the stern ; therefore, with a block suspended at one end 

 and balanced at the other, the pontoon, when on an even keel, 

 supported a weight double that of the block. 



In lifting a block, the shear pontoon was placed end on 

 to the wharf during flood tide, and the suspension bars, already 

 in position in the block, attached to the cross-heads of the 

 lower set of pulleys. The lifting chains were then hauled in, 

 and the pump started to fill the counterpoise tank ; but, as 

 its action was slower than the winches hauling in the chains, 

 the vessel soon dipped at the head a few feet more than 

 at the stern. As soon as this dip was considered sufficient, 

 the hauling in of the chains ceased and the final lift of 

 the block was done by the quiet action of the water filling 

 the counterbalance tank, and bringing the vessel on an even 

 keel. 



The block being suspended from a height of 53 feet 6 inches 

 above the deck, any serious deviation from the centre would 

 have established a powerful leverage tending to overturn the 

 vessel. To prevent this, a pair of strong timber booms provided 

 with diagonal stay chains and tightening screws were hinged 

 near the feet of the shear legs by which the block, when 



