August I, 1878] 



NATURE 



365 



smaller piers. It has been found throughout the operations 

 that in a river subject to such vicissitudes as the Tay, 

 the handling of a very heavy mass is a far less intricate 

 and difficult matter than that of lighter weights, and 

 during the execution of the work connected with the 

 larger spans the operations were not in the least inter- 

 rupted by weather which would have been absolutely fatal 

 to the smaller piers. 



Work on the 2/^^-feet Span Piers. 



' The under-water work of the large piers can be 

 conveniently divided into three parts : that of build- 

 ing, floating, and sinking the caisson ; that of build- 

 ing, floating, and fixing the brick pier; and that of 

 bringing it up to the required height five feet above high 



water. The first two are, of course, by far the most 

 important. The lower caisson consisted of f inch mal- 

 leable iron plates riveted together so as to form 31 

 feet cylinders 20 feet high. After erecting this structure 

 on the foundation on the foreshore prepared for it, a 

 lining of brickwork 14 inches thick was built inside. 

 This lower part was to form part of the permanent struc- 

 ture. On top of it and connected with it by a bolted 

 horizontal joint, another length of plates was built bring- 

 ing the whole up to 40 or 44 feet of height, according 

 to the depth of water. This top part merely served a 

 temporary purpose, and in order to resist the effect of the 

 waves and current it was provided with angle iron stifFeners 

 to a far greater extent than the permanent part, which 

 derived sufficient stiff"Qess from its lining of brickwork. 



Fig. 4. — Sinking of large caissons. 



At four points of the circumference T iron strips ran 

 vertical from top to bottom, and were riveted to the 

 caisson. In these strips or links there occurred three and 

 a quarter inch holes at intervals of i foot, and by putting 

 steel pins of slightly smaller diameter through these holes, 

 it became possible to obtain a hold with the hydrauHc 

 rarns by means of a girder with a claw-like end placed on 

 their tops. The hydraulic rams were placed on the four 

 comers of a square system of girders having projecting 

 ends for the barges. On the under-side of these girders 

 a claw similar to that found on the top of the rams made 

 it possible to take the weight of the pier either on the 

 four lower claws, or the four top claws, which were con- 

 nected with the hydraulic rams. 



Two pontoons, each 70 feet long, 20 feet wide, and 

 7 feet deep, lifted the weight of girders and the piers 

 standing in between them off the ground by means of the 

 lower claw. It was then floated out ; and so powerful 

 was the resistance which the heavy mass, weighing about 

 200 tons, offered to the current, that two strong tug 

 steamers had to be employed for this purpose. Arrived 

 at its destination, it was securely moored. The first one 

 floated in this manner had to withstand the fury of a three 

 days' gale, during which it had to be left out in the river, the 

 waves washing right over the barges, the hatchways having 

 been nailed and caulked to keep the water out. 



To lower the caisson the rams were pumped up 12 

 inches and the steel pin which connected the upper claw 



