He stopped abruptly as a dull boom, like a distant clap of thunder, came over the fog-ridden water. The Lieutenant's experienced ears told him that it was not thunder but the sound of heavy guns. Either there was an engagement taking place or else the Germans had crept up through the mist and were bombarding some coast town. Owing to the direction of the wind in relation to the sound, he inclined to the latter supposition. The dull reverberations of the guns continued and the crew of the _Knat_, having been piped to quarters, she steamed in the direction from which the firing seemed to come. Still, the wind was light and variable, which made it difficult to determine even approximately the locality of the sounds. The fog, too, was a serious handicap, for, should a warship be sighted, the difficulty of deciding whether she was a friend or an enemy would be enormously increased. War vessels, seen at a distance even on a clear day, look much alike, and in a thick atmosphere it is practically impossible even for an expert to distinguish friend from foe. A wireless message of inquiry, though sent in code, would, of course, defeat its own object should the vessel receiving it be an enemy. Under such circumstances as these, therefore, commanders of small craft like destroyers and submarines are under a grave disadvantage and have to proceed with the utmost caution or run the risk of making a ghastly error. After proceeding slowly for some little time, Lawless came to the conclusion that the firing was taking place somewhere to the north-west, and he accordingly steered in that direction. As the _Knat_ slowly felt her way through the fog the sound of the guns grew louder and more distinct, thereby confirming the Lieutenant in his decision. He was leaning over the bridge-rail, trying to penetrate the opaque white curtain in front of him, when there came a shout from the deck below. "Zep, sir, right over'ead, sir!" cried a man. Lawless looked up and saw, just above the low-lying mist, the dim outline of an enormous object moving slowly in the opposite direction. "A scouting Zeppelin," he muttered. "If only I had an Archie aboard I'd put a few plugs in her belly." But, unfortunately, the _Knat_ carried no anti-aircraft guns, and by no possible exercise of ingenuity could the machine-guns, mounted as they were, be made to fire in a vertical direction. So Lawless had to gnash his teeth and watch the airship floating overhead within easy range, yet as safe, so far as he was concerned, as though she were five miles up. "There goes my thousand quid, so close that I could almost touch her," he said bitterly to Trent. "Isn't it just my luck?" "Pity we didn't think to rig up one of our Q.F's vertically," remarked the Sub-Lieutenant. "We could easily have done it."