"My diagnosis was quite correct as far as it went, but it did not go quite far enough; still--by the special intervention of the sweet little cherubim who sits up aloft to keep watch o'er the life of poor Jack--my plan of operation was quite as sound as if I had all the facts of the case spread out before me. Had the U-boat really been lurking round waiting for a pot at some of the ships trying to save his supposed remains--something that we never gathered any definite evidence on--our screening tactics would probably have prevented his success; while the trawlers, with their sweep, furnished the best antidote for the little surprise party that he already _had_ prepared for us. "Scarcely had the trawlers entered the oily area than the jar of a heavy under-sea explosion jolted against the bottom of the _Flash_, which, a thousand yards distant, was just beginning to work up to full speed. Almost immediately three or four other explosions followed, coming so close together as to make one rippling detonation of tremendous violence. An instant later I saw several columns of grimy foam shoot skyward, two or three of them so close together that they seemed to 'boil' into each other as they spilled and spread in falling. Although neither of the trawlers appeared to be immediately over any of the explosions, both of them received terrific shocks. One of them I distinctly saw rear up till it seemed almost to be balanced on its rudder-post as a round hump of green water drove under it, while the scuppers of the other spurted white as they cleared the flood that a spreading foam geyser had thrown upon the deck. It seemed impossible that either of them could survive such shocks as I knew they must have received, and I fully expected to see nothing better than two foundering wrecks emerge from the smother which hovered above the scene of the explosions. Imagine my surprise, then, when two junk-like profiles (they were both of the marvellously sea-worthy 'Iceland trawler' type) came bobbing serenely into sight again, and I noted with my glass that neither appeared to have suffered serious damage. On the score of lives, a tom-cat has nothing the best of a trawler. If it had been otherwise our whole fleet of them--and they, with the drifters, form the main strands of the finer meshes of our anti-U-boat net--would have been wiped out many times over. "At the instant the jar of the first explosion made itself felt, the thought flashed through my mind that there actually was a U-boat lying on the bottom, and that the explosive charge on the sweep had been detonated against its hull. The 'bunched' explosions immediately following also lent themselves to this theory, and it was not till the distinct columns of blown water began rising in the air that I surmised the real cause of them--mines, probably laid so close together that the explosion of the first had set off the others. This fact we were shortly able to establish beyond a doubt.