For bridges of moderate span, over soft ground or over shallow fresh water, strong cast-iron screw piles can be adopted with great advantage. Fig. 112 shows a very usual form of screw pile, made with an external screw at the lower end and with a sharp cutting edge to facilitate penetration into the ground. The upper portions are made in suitable lengths, and all to one pattern and template, for convenience in carrying out the work. The screwing into the ground is generally effected by means of a capstan or cross-head fixed to the top of the first working length of pile, and which is pulled or turned round by ropes worked from stationary windlasses. In some cases long bars or levers are attached in radiating positions to the capstan-head, and a number of men are employed to walk round and round, pushing the levers, and in this way screwing the pile into the ground. As the pile goes down the capstan-head has to be removed, and additional lengths bolted on, until the pile enters a solid stratum, or is considered deep enough for the duty it has to perform. The last or top length has generally to be cast to a special length to bring the work up to the exact height to receive the girders. The core of excavated material passes up into the interior of the pile, and in some cases becomes so compressed or tight as to require the use of an internal augur to remove a portion of it to enable the screwing to proceed. The pile shown in Fig. 112 is one of a number which were successfully screwed into the ground to depths varying from 42 to 48 feet. A toothed or 116 serrated edge, as in Fig. 113, is sometimes given to the lower edge for screw piles which have to cut their way through a hard stratum. All bolting flanges should be accurately turned and fitted to ensure close, parallel surfaces when bolted together. The joint shown at A, Fig. 112, is one the writer has used to a large extent for the bolting flanges of cast-iron screw piles and cylinders. It is very simple in form, readily coated with white lead to ensure a water-tight joint, and as the upper length is practically recessed, or let into the lower length, the exact continuity of the different castings is secured. Solid screw piles of wrought-iron or steel, similar to Fig. 114, are used for some descriptions of work. These are generally made in long lengths, in sizes varying from 4 to 8 inches in diameter, and with screw blades of wrought-iron or cast-iron fixed in the most secure manner to resist the strain produced when screwing into the ground. The couplings for these solid piles must be very carefully made, all contact surfaces truly faced and fitted, bolts turned, and bolt-holes drilled.