There was then only one possible solution of the problem. It was to employ armed vessels of sufficient gun-power to do the work quickly, and of shallow enough draught to get to a fighting range quickly. If the thing were not done quickly, an attack from the masked banks might be fatal. If the guns of such a vessel were corrected by observers in aeroplanes, they might be enabled to do the trick. Fortunately, at the very opening of the war, the Admiralty had purchased from the builders three river monitors, then under construction in England for the Brazilian Government. They drew but a few feet. Their free board was low, their centre structure afforded but a small mark; the two 6-inch guns they carried fore and aft were protected by steel shields. They had been employed with marked success against the Germans in their first advance to the coast of Belgium. When the enemy, having established himself in the neighbourhood of Nieuport, had time to bring up and emplace long-range guns of large calibre, the further employment of these river monitors on this, their first job, was no longer possible. For the moment, then, they seemed to be out of work, and here was an undertaking exactly suited to their capacity. It was not the sort of undertaking for which they had been designed. But it was one to which, undoubtedly they could be adapted. Of the three monitors _Mersey_ and _Severn_ were therefore sent out to Mafia Island, which lies just off the Rufigi Delta and had been seized by us early in the proceedings.