August 11, 1887] 



NA TURE 



355 



tide was 33 pounds per square inch above the atmosphere. The 

 la'^t of these caissons was got down to its final depth in 

 October 1885. 



In sinking the southern grouj) of caissons, the air-pressure 

 hardly ever exceeded 22 pounds per square inch, the silt and clay 

 acting as a lute ; and the working shifts were of six hours' 

 duration, about twenty-seven men being down at a time. 



Recffi'ery of Canted Caisson. — With the exception of the north- 

 west pier in the southern group, the whole of the piers were 

 completed with ve.^ularity. But the caisson for that particular 

 pier, weighing with concrete s )me 3000 tons, while ready at the 

 site for placing in its final position, by some means becane 

 waterlogged on New Year's Day, 1885, and on the tide falling 

 slid forwards on the mud about 15 feet, and canted over through 

 25°. After an ineffectual attempt to right it by pumping, 

 regular siege was laid to it ; but not until the autumn following, 

 after nine months of incessant work, was a timber jacket or 

 cofferdam completed, which enabled the pumps at last to obtain 

 command over the leaks. The caisson then floated again, and 

 after repair was sunk in position in the ordinary manner, arriving 

 at its final depth in 1886. After the excavation had been com- 

 pleted, the chambers were rammed with concrete and grouted 

 up, the concrete and anchorage and masonry were completed, 

 and the temporary cofferdam was ready for removal. 



Mt>i Employed. — No difficulty arose in obtaining a sufficient 

 number of men inured to work under air-pressure, as M. 

 Coisseau, of the firm of MM. Couvreux and liersent, of Paris, 

 brought his staff of trained excavators from the Antwerp harbour 

 improvement works, and contracted for the work to be executed 

 under air-pressure. 



Raising Viaduct Gi<ders. — After the masonry of the approach 



viaduct piers had been carried up to a convenient height, a 



temporary stage was built, upon which the girders were erected 



and riveted up. Steel cross-beams with pairs of hydraulic jacks 



-ere placed under the ends of the girders over the piers ; and a 



age surrounding the piers was suspended from the main girders. 



10m this platform the men in charge of the rams conducted the 



ioerations of lifting and blocking up the girders ; and the 



masons afterwards completed the stonework in the vacant 



spaces. By this plan the giiders were raised to their final 



height in July of the present year. The whole of the ten spans 



u the south side were lifted simultaneously as soon as they were 



,ul riveted up. The materials for the piers were first raised in 



trucks, by a steam hoist on the jetty, to a tramway laid on beams 



between the bottom members of the girders, and afterwards 



'lowered into position by w inches over each pier, these winches 



being driven by running ropes from engines on the girders at 



alternate piers. These ap})roach spans now require only the 



l>arapet and a few other details for completing them in all 



res]>ects, ready for the permanent way. 



Erecting Steel Work over Main Piers. — On the completion of 

 the masonry, the operation of erecting the steel work was com- 

 menced on the northern piers early in 1885 by riveting up the 

 bed plates, and lowering them into position over the heads of 

 the foundation bolts. Their surfaces were afterwards smoothed 

 by emery wheels, and coated with crude petroleum, to prepare 

 them for receiving the bea'ing plates of the cantilever bases or 

 skewbacks. These, as already mentioned, have freedom for a 

 limited amount of sliding, and the gauges at present attached 

 how that the sliding movements follow the changes of temperature 

 w anticipated. 

 The skewbacks, forming the junction of five tubul.ar and five 

 ■ctangular members over the piers, were then erected, and were 

 . jnnected with the horizontal members at the same level, which 

 had been built together on a stage. After the connexions had 

 l)een riveted up, a commencement was made upon the upper 

 arts over the piers ; these parts have since been erected without 

 iiy form of fixed scaffolding, and the operation is still in progress 

 ver the Inchgarvie piers. 



The lifting gear for raising the erecting platforms con- 

 sists of a ])air of plate frames, one below the other, fixed 

 aside each i2-feet pier-column, by pins passing through the 

 «ings of the frames and the ribs of the column. The lower 

 frame supports a»hydraulic lifting press; and upon the ram 

 rests a through box-girder cross-beam, at right angles to the 

 length of the bridge, passing through voids in ihe columns, where 

 kites are temporarily left out for this purpose. These cross- 

 beams support lattice-girders in pairs, one on each side of the 

 olumn, which extend a little more than the full length of the 

 ide of the quadrangle formed by the piers. Upon the top of 



all comes the main deck, furnished with gantries, cranes, oil- 

 heated rivet-furnaces, &c. , complete in all respects for carrying 

 on the chief operations of erection. On the bottom level of the 

 girders is a lower deck, with the ends housed in to form tem- 

 porary shelters for the men. The box and other girders are built 

 up of parts which will eventually be used in the permanent struc- 

 ture. Communication between the level of the jetty and the 

 platf jrms is made by hoists, drawn up between wire-rope guides 

 by the winding engine on the level of the jetty, which lifts the 

 material by wire ropes to the platform ; s.ifety clutches are 

 attached to each cage, for seizing the guide ropes in case the haul- 

 ing gear were to give way. During lifting operations, access to 

 the platforms is gained by ladders laid up the cross-bracing be- 

 tween the main columns over the piers. 



The process of raising the platforms is as follows. Water- 

 pressure at about 30 cwts. per square inch is conveyed from 

 pumps on the jety to the lifting presses by wrought-iron 

 piping taken up the inside of the columns, and is turned 

 into a cyinder, lift'.ng the load off the series of pins in the 

 top frame. The pins are then withdrawn, and the ram lifts the 

 box-girder, carrying with it the loose frame, until opposite the 

 next series of holes in the ribs of the columns, into which the pins 

 are then inserted ; the pressure is released, and the box-girder 

 again rests upon the upper frame. In the return stroke the ram, 

 hanging by its shoulders from the upper frame, by means of its 

 piston form now hauls up the lower frame, from which the pins 

 have been withdrawn ; and when this has been repinned, it is 

 ready to support the press for another upward stroke. By this 

 means the platforms have been gradually raised, generally through 

 lifts of 16 feet at a time, until arrived at the summit. On their 

 way up they have been utilized for building the tubular cross- 

 braces and other work ; and at the present time those at the 

 southern and northern piers form the stage for erecting the top 

 members between the heads of the main columns. The platforms 

 at Inchgarvie are now only 40 feet below the height to which they 

 will have ultimately to be raised. 



In building the pieces together, they are connected by service 

 bolts, until the hydraulic riveters are brought into action. For 

 the open work the riveters are of the gap type ; but for the closed 

 tubular work, a special adaptation was devised by Mr. Arrol, by 

 which the rivets are closed in any part of the built tubes. When 

 these machines arrive at the top of the columns, after having com- 

 pleted the riveting on the way up, they are taken apart ready for 

 application elsewhere. 



Erecting Cantilevers. — The building out of the first projecting 

 bays of the cantilevers is being conducted on the system just 

 described, with such modification as to suit the altered circum- 

 stances. The bottom members are first erected, and have been 

 built by means of overhanging frames in panels, resting upon the 

 completed portions of the tube, and so constructed that, as fast 

 as the work is riveted up by the annular riveting machines, and 

 the forward portions of the cage-like framing are brought into 

 bearing, the back frame-; can be unshipped and taken forwards 

 to the working fice. Upon the top of this framework a mov- 

 able hydraulic crane is placed for lifting the pieces into position, 

 which are brought alongside from the pier by carriers suspended 

 from a single rail of angle bar. As soon as the limit is reached 

 at which these members can support the projecting work, in- 

 clined supporting stays are introduced, which connect the 

 bottom member at this part with a temporary horizontal tie 

 stretching between the main columns at about the level of the 

 cross-bracing ; thence the inclined stays slope down again, and 

 are attached to the bottom member on the other side of the pier. 

 After this has been done, platform girders with decks are built 

 at a convenient level to rest on cross-beams carried by rising 

 frames, which are introduced between the corners of the first 

 vertical member of the bridge : this member having been pierced 

 beforehand with a series of pin-holes, in readiness for a lifting 

 action similar to that used in the main colunns. The ends of 

 the platforms nearest the piers are raised by suspension bars, 

 by thi action of hydraulic rams attached to the main columns 

 at a higher level. From these platforms, as in the previous 

 cases, the erection of all parts commanded by them is carried 

 on as they rise. 



The erection of the secondary parts proceeds simultaneously 

 with that of the main members, the railway girders being built 

 by corbelling out from the supports, and the other parts by light 

 stages when the parts themselves cannot serve as a means of 

 support to extend the work. As will readily be understood, the 

 erection of these sections calls for greater nerve and judgment on 



