The tides on the coasts, in addition to running alternately eastward and westward with considerable velocity, also caused large differences in sea-level amounting to fifteen feet between the highest and lowest states of the tide. [Sidenote: The Canal System] Mention has already been made that the only harbours on the coast eastward of Nieuport were situated at Ostende, Blankenberghe, and Zeebrugge. But these were not natural harbours. They had been artificially cut out of the coast-line by means of {9} dredging. The entrance channels were preserved by piers built out into the sea and by dredging operations designed to retain the desired depths of water. [Illustration: BIRD'S-EYE VIEW OF CANALS] Ostende, before the war, had been much used as a commercial harbour, and was therefore provided with numerous wharves, basins, and docks. It was a suitable harbour for all classes of submarines and torpedo craft. Blankenberghe was a little harbour about nine miles east of Ostende and three miles west of Zeebrugge. Its depth was exceedingly small; it could, therefore, only be used for shallow draught vessels such as fishing boats, motor boats, and the like. It was true that the rise of tide, amounting to approximately fifteen feet, would enable larger vessels to enter or leave near the time of high water, but any naval vessel stationed in a harbour from which it can only proceed to sea during a limited portion of the twenty-four hours at once loses much of its value. Zeebrugge could accommodate vessels up to a considerable size: the harbour works and depths will be described in detail presently. Although these three places provided the only harbours on the coast, there was a harbour of great importance at Bruges, about eight miles inland from Zeebrugge. {10} Bruges harbour was also entirely artificial, consisting of locks, basins, and waterways built on the canal system. Bruges was connected to the sea by means of canals running to Zeebrugge and Ostende, these canals converging on the waterways of Bruges in such a manner that vessels of a certain limited size could pass from Ostende to Zeebrugge, and vice versa, without actually proceeding into the open sea. A series of small canals also connected Bruges to Antwerp, via Ghent, but this canal system, being only constructed to accommodate barges, did not materially add to the value of Bruges as a harbour for sea-going vessels. Of the three canal systems connecting at Bruges, the canal to Zeebrugge easily held first place in importance. This canal was built by the Belgians. It was commenced in 1896 and completed in 1907. Six and a quarter miles in length, it was almost entirely straight throughout. It could accommodate torpedo-boat destroyers or submarines, both of the largest size, and could, if required, have been used by light cruisers. [Sidenote: The Harbours in Flanders]