The German submarine bases at Zeebrugge and Ostend on the Belgian coast have been repeatedly bombed from the sea and shelled by British monitors with indifferent results. With the adding of super-U-boats to the German submarine fleet and the increased transatlantic traffic of the Allies the necessity for effectually sealing these bases has long been apparent. Theoretically the nature of the entrance to the harbors of both places, resembling the neck of a bottle, about 250 feet wide, made such a task easy by the sinking of block ships. Practically it was most difficult, on account of both sea obstructions and the shore batteries. On the night of April 22-23 British naval forces, commanded by Vice Admiral Keyes, with the co-operation of French destroyers, and hidden by a newly devised smoke-screen, invented and here employed by Wing-Commander Brock, attempted to seal up the harbors. At Zeebrugge the enterprise was entirely successful. The Intrepid and Iphigenia were sunk well within and across the narrow channel, the Thetis at the entrance. All three were loaded with cement, which became solid concrete after contact with the water and can be removed only by submarine blasting. A detachment of troops was also landed on the mole from the Vindictive and engaged the crews of the German machine gun batteries stationed there. An old submarine was placed under the bridge of the mole and detonated. A German destroyer and some small craft were sunk. Before the blockships were placed a torpedo had been driven against the lock gates which lead from the channel into the inner harbors. The expedition retired with the loss of fifty officers and 538 men, of whom sixteen officers and 144 men had been killed. At Ostend, the entrance to whose harbor is protected by no mole, the block ships Sirius and Brilliant were not effectively placed. Against this port the experiment was, therefore, repeated on the night of May 9-10. The Vindictive, with a cargo of concrete, was planted and sunk at the entrance to the channel, but not entirely blocking it. ITALIAN RAID AT POLA