With the syphon arrangement the iron pipes must be laid down the slopes of the cutting and under the road-bed of the permanent way. The pipes must be continuous, strong, and firmly connected at the joints to prevent leakage. The inlet and outlet ends of the pipes should be securely built into receiving-tanks of masonry, brickwork, or concrete, to ensure an uninterrupted flow of the stream, and also to prevent any of the water from percolating through under the pipes and on to the railway. As a precautionary measure, it is well to place iron gratings some little distance in advance of the syphon pipes to intercept and collect any brushwood, straw, or other things which might be brought down with the stream. Fig. 73 gives an example of the syphon arrangement as constructed with two cast-iron pipes placed side by side. Railway works carried out in cities and large towns, whether they take the form of cuttings, embankments, arching, or tunnels, are certain to cause a very considerable disturbance of existing drains, corporation sewers, gas-pipes, water-mains and underground telegraph wires. Some of these underground works may be so peculiar and complicated as to necessitate a slight deviation from the course originally intended for the line. Suitable provision will have to be made for each of the items interfered with by the railway, and the substituted work must be carried out to the satisfaction of the constituted authorities within the municipal boundaries. [Illustration: Fig. 72, 73] Bridges.--Amongst the many bridges and viaducts which have to be built during the making of a railway those constructed over rivers and waterways are generally the most important The bridging across any 81 navigable river or tidal water can only be effected in compliance with conditions imposed by the authorities controlling the navigation rights. These conditions will place restrictions as to the number and distance apart of the piers, as well as the height from high water level to the under side of the arches or girders. For rivers having a constant traffic of sea-going vessels of large tonnage and lofty masts the authorities will demand great height or headway as well as large spans; and if to this be added a deep water-way and bad foundations, the work to be constructed becomes one of considerable magnitude. The banks of the river must be carefully studied to find the most favourable point for crossing, and in some cases it may be prudent to make a detour of two or three miles. The crossing at a great height involves the construction of the approach lines at a great height also. If the river is in a deep valley with high sloping sides the natural contour of the ground facilitates the formation of the approach lines; but with a river on a low, wide, open plain, inclined approach lines add enormously to the cost of construction, as well as to the cost of permanent working.