ON RECENT IMPROVEMENTS IN THE PORT OF DUBLIN. 169 



fold in the space of twenty years. Accordingly it was determined, after 

 mature consideration, to extend the North Wall, and construct a large 

 tidal basin with 24 feet at low water inside and 22 feet along the river 

 face, so as to float the largest commercial vessels at all states of tides. 

 The masonry was commenced in 1871, and up to the present about 2500 

 lineal feet of wall have been built on a novel principle which avoids the 

 trouble and expense of cofferdams, pumping, staging and other tem- 

 porary works, the expenditure on which frequently exceeds the cost of 

 the permanent work to which they are merely ancillary. The new mode 

 of construction consists in the use of blocks of masonry of unprecedented 

 size in the foundations below low water level, as represented in the 

 diagrams which accompany this paper. Each block is 29 feet high, 

 11^ feet long, and 21 feet 4 inches broad at the base, and weighs 350 

 tons ; they are built on land on a block wharf (Plate II.), and about three 

 months after completion they are lifted by a powerful floating shears 

 (Plate I.), and conveyed to their destination in the quay where each 

 block forms 11-| feet in length of the lower portion of the wall as far 

 as low water level, and when a number of these blocks have been thus 

 laid in position the superstructure up to the coping level is built over 

 them in the usual manner by tidal work, the total height of the wall 

 being 45 feet. Besides the large floating shears for lifting and moving 

 the blocks about, there is one other special appliance — namely, a diving 

 bell (Plate III.), also of unprecedented size and peculiar in construction. 

 This bell, which weighs 80 tons, is used for excavating and levelling the 

 river bed on which the blocks lie. The chamber is cast-iron, 20 feet 

 square and 6^- feet high, with a tube or funnel 3 feet in diameter, and 

 rising to a height of 44 feet over the bottom of the bell ; and this is 

 the greatest depth of water for which the present bell is intended, 

 though by adding to the length of the funnel it might be worked in 

 greater depths. The upper end of the funnel forms an air lock 6£ 

 feet high, with double doors and suitable cocks for admitting the com- 

 pressed air from the chamber into the lock, or for letting that in the 

 lock escape into the external atmosphere, and by this arrangement the 

 workmen can pass up and down without lifting the bell off the bottom or 

 stopping the work of excavation. Inside the chamber are two large iron 

 trays, and the men shovel the excavated earth into these trays. When 

 they are filled the bell is lifted a few feet off the ground, and the 

 barge hauled some yards to the rear of the wall where the trays are dis- 

 charged, by pulling out a detent, and the barge is then brought back to 

 its working position, and the bell lowered as before. 



The operation of lifting and setting a block is as follows : — The float- 

 ing shears is brought bow-on to the block wharf during flood tide, and 

 the lifting chains are attached to iron suspending bars which pass 

 through each block. The chains are then hauled in by the winches on 

 board, and water is pumped into a large tank at the after- end of the 

 vessel to counterbalance the weight of the block, which is then floated 

 away to its destination and lowered into place the following low water, 

 so that at one step 11^ feet forward of wall are built up to low water level. 



The cost of both floating shears and diving bell was under 25,000?., 

 and the whole of this was repaid in the first GOO feet of wall by the 

 superior economy of this system over ordinary cofferdam and pumping 

 work, and the relative saving now amounts to about 16,000Z. per annum. 



It would obviously be useless to construct deep water quays if the 



