RAILWAY CONSTRUCTION. 81 



The bridging across any 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 consider- 

 able 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. 



If the number of sailing craft passing up and down the river 

 be moderate, and, perhaps, only passing at high water, the 

 authorities may permit a low-level viaduct with an opening 

 bridge. 



There are thus the two systems: the high-level viaduct, 

 which allows trains to pass over and vessels to pass under at 

 any and all times, and the low-level viaduct with opening bridge, 

 which, if open for vessels, is closed for trains, or vice versa. 



Every crossing of a navigable river will have to be considered 

 and dealt with according to its own individual requirements. 

 An arrangement suitable for the one may not be admissible or 

 prudent for the other. A frequent and important train service 

 might be much interfered with by an opening bridge, and, in 

 a similar manner, an opening bridge might cause much inter- 

 ruption and detention to the navigation of the vessels on 

 the river. 



Where a low-level viaduct with opening bridge can be 

 adopted, there will be a very great saving of expenditure ; and 

 there are numbers of such viaducts in existence, accommodating 

 a large railway and river traffic without inconvenience. Even 

 with a low-level viaduct the height from water-level to the 



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