take their whole main fleet to sea for the purpose of seeking advantageous conditions for bringing a portion of our Grand Fleet to action.

Movements of modern fleets under war conditions necessitate the use of various types of small craft to precede them—e.g., mine-sweepers for clearing channels for the fleet to pass through, destroyers for supporting the mine-sweepers and for driving back the enemy's small craft, light cruisers for scouting purposes, etc. Mine-sweepers and torpedo craft, by virtue of their small size, are unable to keep the sea for long periods. It will therefore be realised that, in the event of the High Seas Fleet putting to sea for operations in southern waters, the Flanders coast provided Germany with an advanced base from which their light craft could operate.

The German torpedo craft based in Flanders, therefore, would be able to serve a double purpose, viz., that already mentioned and that of attacking our patrol craft, our coast and our merchant vessels when opportunity offered. The mine-sweepers could also serve a double purpose in that they were required to sweep channels for the ingress and egress of submarines based in Flanders whilst being suitably placed for sweeping duties in advance of the main fleet. That Flanders was also suitable for aircraft bases is as well known as it is obvious; but it may not be generally understood that such aircraft would also be of special value to the main fleet under the conditions stated above. Thus, to sum up, the occupation of the Flanders coast by the German sea forces would be of treble value—to provide, firstly, a base for the submarines employed on commerce destruction; secondly, a base for the advanced flotillas and aircraft operating in conjunction with the main fleet in the event of the latter coming south; and, thirdly, a base from which to attack our southern coasts or sea-patrols and from which to indulge in air raids against British and French territory.

The foregoing consideration of the possible uses of Flanders to the German Navy shows the inherent value of an advanced base in that locality; the intrinsic value obviously depended upon the existence of suitable harbours for use as bases. Let us now examine the geographical situation.

CHART SHOWING THE RELATIVE POSITIONS OF DOVER, ZEEBRUGGE, HELIGOLAND, AND THE EXITS FROM THE NORTH SEA.

In the latter part of 1917 the Flanders coast, as far westward as Nieuport, was in the possession of the Germans. The northern extremity of the line separating the German and Allied armies was situated approximately on the Yser Canal, which emanates from Nieuport harbour. The latter was dominated by the gun-fire of both armies; its use was, therefore, denied to both. The only other harbours on the coast of Flanders were Ostende, Blankenberghe, and Zeebrugge. These will be described in some detail presently.

The Flanders Coast

The Flanders coast consists mainly of flat country barely elevated above the level of the sea. Sand-hills along the shore act as a barrier between the sea and the land. Parallel to the shore the tidal current runs to and fro with considerable velocity. The tendency for the tidal current to wash away the sand from the shore is partially countered by the use of groynes, such as are similarly used to maintain our own coastline in many parts of England. Although the groynes on the Flanders coast are carried well out into the sea—they are often 100 yards in length—the movement of sand along the coast is very considerable, and, as will be explained later, has a strong influence on the harbour situation in that locality.

The approaches to the coast are beset with shoals reaching to a distance of eight miles from the land. These shoals have always provided serious obstacles to navigation. During times of peace the charts of this locality had been kept corrected by virtue of continual surveying. The shoals were frequently moving and new shoals appeared from time to time. The channels required almost constant dredging. For obvious reasons during the occupation of Flanders by the Germans it was not possible for the Allies to continue either the surveying or the dredging.

Before the war navigation off this coast required the use of many facilities such as buoys and lighthouses. At the best of times buoys are not very dependable as navigational aids owing to their tendency to break away in heavy weather or to drag their anchors along the bottom. Their positions need to be "fixed" from time to time by means of angles to shore objects, or by methods of astronomical observation, and then compared with the positions shown on the charts. Lighthouses, however, unless they are of the small type without lighthouse-keepers, are more efficient aids to the navigator. During the war the lighthouses east of Nieuport were only used by the Germans during short periods when specially required for their own craft; the majority of the buoys were withdrawn and the remainder were moved to new positions which were frequently altered to prevent the Allies from making use of them. Thus, during the war, the charts available to the Allies were very soon obsolete; no others were obtainable. Navigation off the Flanders coast, for Allied vessels of any size, therefore, became decidedly hazardous.

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.

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 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.

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.

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,