The difficulty of connecting lightships and isolated lighthouses to the mainland by submarine cables, owing to the destructive action of the tides and waves on rocky coasts on the wll- shore ends, led many inventors to look for a way out of the difficulty by the adoption of some form of inductive Smith.
Sand bars keep filling up the mouths of these channels, necessitating frequent dredging and extension of the breakwaters, work undertaken by the Federal government, which also maintains a most comprehensive and completeystem of aids to navigation, including lighthouses and lightships, fog alarms, gas and other buoys, life-saving, storm signal and weather report stations.
Its commerce is much facilitated by the system of canals which bring it into communication with Belgium, the coal-basins of Nord and Pasde-Calais, the rich agricultural regions of Flanders and Artois, and the industrial towns of Lille, Armentieres, Roubaix, Tourcoing, Valenciennes, &c. The roadstead is indicated by lightships and the entrance channel to the port by a lighthouse which, at an altitude of 193 ft., is visible at a distance of 19 m.
The entrance to the port is free from ice nearly all the year round, is excellently buoyed, and lighted by two lightships and eight lighthouses, among the latter the remarkable Rothesand Leuchtturm, erected 1884-1885.
With flat and shelving shores, the shoal-banks off the main mouths of the delta form the chief danger to shipping, and this is guarded against by a good service of lighthouses and lightships.