"This," replied the Sub, pointing to a small, almost unnoticeable disc let in flush with the steel lid. "By Jove, rather!" agreed the lieutenant. "An ebonite plug with a copper core! Yes; look here. There's a corresponding gadget on the deck. The two would come in contact when the holding down bolts of the hatch are released and the cover flies back. I fancy we were wise not to meddle with those hatch covers, or our curiosity would have landed us in a hole." "She's stuffed with explosives, then?" "Precisely," agreed Wakefield. "Once the circuit is completed by opening any of these hatches, up she goes, and anyone on board with her. We've seen enough. We'll clear out." "What's the reason?" inquired Meredith. "Ask Morpeth," was the reply. "He'll probably tell you that details of his stunt have leaked out. Hello! Seaplanes coming back? Look alive there!" The boarding-party hurried to the boat. Quickly the rowers gave way. It was a race between a comparatively slow-moving boat and a pair of swift seaplanes. The former had to cover about two hundred yards: the latter a distance of from two to three miles. The aircraft would have won hands down had they not banked and circled. As it was, there was time for Wakefield and his party to regain the mystery ship. "Fritz has smelt a rat," reported the R.N.V.R. officer. "That U-boat's chock-a-block with explosives." "Good enough!" declared Morpeth, ringing for "Easy ahead, both engines." "See that the smoke-screen gear is ready, Wakefield. We may want it, badly." Q 171 increased her distance from the booby-trap to a good two cables' length, then she turned until she could bring her broadside torpedo-tubes to bear upon the anchored U-boat. Diving steeply, the first seaplane swooped down to within three hundred feet. From underneath her fuselage a black object dropped swiftly--then another. Four seconds later the first missile struck the water, exploding with a deafening report unpleasantly close to the Q-boat's starboard quarter and deluging the after quick-firer's crew with spray. The second bomb fell further away. Morpeth gave no signal to the anti-aircraft gun, although the departing seaplane offered a tempting target. His cool and ready wit saw an opening and he took it. Both Hun machines were now flying on a parallel course, the first one manoeuvring to return to the attack. Incautiously they were approaching the anchored U-boat. Like an arrow from a bow, a gleaming steel cylinder leapt from the Q-boat's side. Striking the water with a shower of spray, it dived obliquely and made straight for the Hun's booby-trap, its trail clearly defined by the milky foam on the surface. Suddenly there was a lurid flash that seemed to outshine the light of the sun. A roar so stupendous that it shook Q 171 from stem to stern gave warning that the torpedo had reached its mark.