over, it was also the first great structure built entirely of mild steel. The two main spans are each 1700 feet clear, and have a headway in the centre 500 feet in length, with not less than 150 feet clear height above high-water mark. These immense spans support exceptional loads to the main piers, and as some of these piers are founded at a depth of almost 90 feet below high water, the making of the foundations alone was a work of exceptional magnitude.
PHOTO BY SIR WILLIAM ARROL & CO., LTD.
THE MATERIAL LOCK USED IN SINKING CYLINDERS WITH COMPRESSED AIR.
In past engineering practice it has been the habit of engineers to avoid struts of large dimensions, but in this structure a large proportion of the main members are struts of exceptional size and length. They are, in most cases, of tubular form, rigidly stiffened at close intervals, and so satisfactorily was the design worked out that the Board of Trade sanctioned a stress of 7½ tons to the square inch on these members. The main spans of the bridge are approached by viaducts of granite piers and steel girders of an ordinary type, the principal point of interest in connection with them being the great height of the roadway above the water, or ground, level.
Throughout the building of the Forth Bridge Mr. Arrol was the active spirit, and everything was on a gigantic scale. The workshops that had been built for Sir Thomas Bouch's bridge were utilised, and, in addition to these, other large shops were built near the site of the works, for the manipulation of the 60,000 tons of steel required in the structure. The bridge itself was of such a novel type that special plant had to be designed for carrying out the work, and it is in this connection that the impress of Mr. Arrol's personality is seen in practically every stage of the great undertaking.
In the shops hydraulic power was used to a very great extent, not only for the handling of the material, but
