366 



NATURE 



\August I, 1878 



■with the caisson inserted. By pumping up again the 

 weight was taken off the lower claw, the pin which 

 had held it there withdrawn and inserted in the hole 

 a foot higher up. The water was then allowed to 

 flow out of the rams and the weight to come down 

 on the lower pin again, during which operation the 

 caisson had necessarily been 12 inches lowered. The 

 work of adjusting it in its right place was accom- 

 plished in the same manner as described for the smaller 

 piers. The ffrst of these caissons was lowered down on 

 August 28, 1875, 3.nd the same afternoon the floating 

 girders and barges were withdrawn, and the pier left 

 standing on the sandy bottom. The tide was particularly 

 strong, and from the resistance which this mass offered 

 lo its flow washed away the sand round about it, thereby 

 causing the pier to heel over to such an extent that next 

 morning it was more than 6 feet out of level. The centre 

 of gravity of these piers, compared to the diameter of the 

 base was, however, so low that no serious apprehensions 

 were felt, and when the sand was excavated on the high 

 aide it soon came to the level again. It was always found 

 possible with a little care to keep the pier nearly level in 

 sinking by excavating on one side or another, and no 

 such artificial means of keeping them upright, as had 

 been adopted in the case of the smaller piers, became 

 oecessary. 



Sinking the Large Caissons. 

 The best means of excavating the sand out of 

 these piers had been a subject of very careful considera- 

 tion, and numerous experiments had been tried with 

 varying results, in order to find the best machinery for 

 effecting that purpose. Finally, however, an apparatus 

 constructed upon very simple principles by Mr. Reeves, 

 one of the contractor's assistant engineers, was found the 

 enost efficient. The conditions which it had to fulfil were 

 of this kind. It had to elevate the sand from the inside 

 •of the cylinder and drop it outside at a small cost, and it 

 had to be constructed in such a manner as to be easily 

 detached from the pier in case of a sudden gale. These 

 conditions were fulfilled by placing the whole apparatus 

 on a barge which could be moored alongside the pier and 

 withdrawn with little trouble and at a moment's notice. 

 On this barge were placed four air-tight tanks each having 

 a circular hole in the bottom, closed by a door on 

 which an india-rubber ring was fastened so as to obtain 

 an air-tight closing. A steam-engine worked two exhaust 

 air-pumps, and each of these pumps was so connected 

 with two of the tanks that by means of a three-way cock 

 •either the one or the other could be put in communication 

 with it. From the top of the tanks flexible pipes of 4 inches 

 diameter ran down to the bottom of the pier, and their 

 ends were in charge of the diver, who could direct them. 

 Either of these tanks being put in communication with the 

 pump would be exhausted, and a current of water mixed 

 with sand would rush up the flexible hose, filling up the 

 vacancy. A float indicated the height to which a tank 

 was filled, and before any water or sand could enter the 

 air-pipe and reach the valves, the attendant had to turn 

 the cock which brought the other tank into communica- 

 tion with the exhaust-pump and admitted air into the full 

 •one. It then discharged its contents through a hole in the 

 t)Ottom of the barge into the river. The joints of the 

 -cylinder not being water-tight, the level of the water 

 inside was always as high as that outside, and the only 

 purpose of the temporary caisson was to protect the divers 

 from the strong current, and give those in charge of the 

 operation an opportunity of observing the position of the 

 pier during its descent. Fig. 4 shows this apparatus at 

 -work by one of the large piers. 



Building the Brick part of the Pier. 

 It will be understood that by the sinking of this caisson 

 -a hole was formed in the sand, the sides of which were 

 •kept up vertically by the caisson while the bottom was 



formed by the hard gravel layer. This pit was now 

 filled with concrete to a couple of feet above the river 

 bottom, and all the iron above that point removed. The 

 concrete hardening became like a rock with a level surface 

 of 3 1 feet diameter. The remaining part of the pier up to 

 high water consisted of brickwork only. The lower portion 

 was built on a level part of the foreshore in bricks and 

 Portland cement, which became so hard that the block 

 could be lifted from four points by means of barges and 

 apparatus similar in all but the dimensions to that used for 

 the floating of the 9' 6" piers. Previous to the building a 

 layer of paper was spread on the temporary foundation in 

 order to prevent the brickwork from adhering to it. The 

 block was hexagonal in shape, measuring 27 feet in one 

 direction and 16 feet in the other. Its height would 

 depend on the depth of water at the place where it was to 

 be used, and was so regulated that after depositing the 

 block on the artificial rock foundation the top would be a 

 little above low water. The weight of the blocks was 

 about 200 tons. In the centre a hole was left answering 

 a double purpose. First, it allowed the weight of the 

 block to be kept below the point at which it would have 

 become unmanageable and than the concrete which was 

 put in this hole after the block was in situ would increase 

 the adhesion between the brickwork and the foundation. 



From low to high water line the pier was finished in 

 solid brickwork set in Portland cement, the work of 

 course being interrupted at every rising tide and taken up 

 again when the water was low enough. Four courses of 

 ashlar of an aggregate thickness of 5 feet brought the 

 pier to the height at which the cast iron columns com- 

 menced and the bolts to hold down their bases were 

 fixed in the stone. 



Superstructure. 



In the erection of the iron superstructure the same 

 principle of not employing staging in the river was 

 adhered to. The iron-work as it arrived, in pieces of 

 from 20 to 35 feet long, from the contractor's works at 

 Middlesbo rough, was landed on a jetty near the shore 

 about 260 feet long, over which a strong travelling crane 

 could be moved from end to end. Here the parts were 

 put together and riveted up. All the girders with 

 the exception of the 170-feet span and the 27-feet spans, 

 are parallel lattice girders having diagonal struts and 

 ties. The section of the booms is trough-shaped, that of 

 the compression diagonals H -shaped, while the tension 

 bars are flat, varying in thickness from three-eighths to 

 five-eighths of an inch. Those girders which have the 

 roadway on top have half struts running up from the 

 intersections of the diagonals to the top boom, while 

 those where the load is at the bottom have tie-bars 

 for the same purpose. The depth of the 245 feet spans 

 is 27 feet, their width 15 feet, and their weight about 190 

 tons. In them the rails are carried on longitudinal 

 sleepers resting on iron cross-sleepers 5 feet apart, while 

 in all the other spans wooden sleepers 3 feet apart, and 

 having a section of 12 by 9 inches, carry the roadway. 



Floating the Girders. 

 When a girder was completely erected and riveted up, 

 parts of the jetty on which it rested were removed at each 

 end, and barges were introduced at a low state of the rising 

 tide in the gaps. As the height of the water increased 

 these barges would touch the under-side of the girders, 

 and a further rise would lift them off the bearings on which 

 they had been erected, and keep them floating on the two 

 pontoons. It will be seen that in order to get the ends 

 of the girder free for depositing them on the piers, it was 

 necessary to keep the barges at some distance from these 

 ends and to introduce temporary vertical struts between 

 those parts of top and bottom boom which rested on the 

 barges. In addition to these struts some others were 

 used to steady the comparatively high and narrow girders 

 on their floating supports in case rough weather should 



