In building the north coast line which connects Sydney with the Queensland border, a feat of a totally different character from the zigzag was completed. This is the massive bridge, 3000 feet in length, which carries the track across the Hawkesbury River, 36 miles distant from the capital. It is divided into seven spans, each of which measures 416 feet in length, supported on substantial masonry piers. The erection of this structure, which still ranks as the largest work of its type in Australia, occasioned considerable difficulty, both in regard to the piers and the setting of the steel-work into position. Indeed, it is doubtful as to which section of the work provoked the greater anxiety. The difficulty with the piers was the great depth to which the engineers had to descend to secure a foundation, because in mid-stream the 40 feet of water flows over a bed of mud ranging up to 120 feet in thickness. The only practicable means by which this essential subaqueous work could be carried out was by sinking a huge steel cylinder filled with concrete. The bottom section of this huge tub, or caisson, as it is called, was closed, and after it was completed on shore it was towed out to the site where the pier was to be erected and sunk by the introduction of the concrete. The under side of the caisson was fitted with a knife edge, by means of which it could cut its way through the soft soil, the driving force for this purpose being the weight of the superimposed concrete. The mud over the area representing the superficies of the cylinder bottom was removed from the inside to enable the mass to settle down. The steel shell was built up continually from the water-level in rings, until a solid foundation was gained. When this was reached and deemed satisfactory the spaces through which the spoil from below had been withdrawn were likewise filled with concrete, so that the contents of the cylinder really form a huge pillar of concrete homogeneous from end to end.