RAILWAY CONSTRUCTION. 121 



men, and the reverse arrangement for the ingress of men and 

 materials. The shape and dimensions of the air-lock may be 

 varied according to circumstances, but the principle will remain 

 the same. 



When the cylinder has been lowered to what is considered a 

 sufficient depth, it is usually loaded with a certain amount of 

 dead weight in the shape of old iron or other convenient material, 

 and allowed to remain loaded for some days to ascertain if it will 

 sink any further. Should this test be found satisfactory, the 

 dead weight is removed, and the interior of the cylinder pumped 

 dry and carefully filled with good cement concrete. 



Cylinders for foundations are generally made circular in 

 section, that form being the most convenient for turning and 

 facing the flange-joints. They can, however, be made oval in 

 section, or of any section that may be found most suitable for 

 the work required. Figs. 116 and 117 give the particulars of a 

 double-line railway bridge carried on cylinder piers across a 

 river. The detail sketches explain the form of cutting edge, 

 flange joint, and method of bracing. This bridge is one that was 

 reconstructed and widened from a single-line to a double-line 

 bridge. Traffic was carried over on one line while the second 

 line was being erected, hence the reason why one strong central 

 girder was not adopted. 



Cylinders of 7 feet diameter and upwards are sometimes filled 

 with concrete in the lower portion, on which is built either a 

 circular lining or a solid mass of masonry or brickwork up to 

 the level of the girder-blocks. In some cases the cylinders 

 proper, together with their concrete filling, terminate a little 

 above the water-level, and upon these foundations are erected 

 strong cast-iron columns, plain or ornamented in design, to carry 

 the girders and roadway. The cylinder itself is generally con- 

 sidered merely as a casing or medium for obtaining a foundation, 

 the weight of the superstructure being carried on the internal 

 filling or lining. 



Caissons constructed of plates of wrought-iron or steel are 

 much used for the foundations of large piers in deep water. 

 Practically they may be considered as cylinders on a large scale, 

 with the difference that whereas cylinders are generally con- 

 tinued up to the under side of the girders of the superstructure, 

 caissons are only carried up to a short distance above the water- 

 level. A caisson forms a strong water-tight iron cofferdam, from 



