Over a vast observation minefield the boats glided serenely. Fifty feet beneath their keels were cylinders of powerful explosive that at the touch of an electrically-connected key ashore would blow a hostile ship to atoms. Farther on there were mechanical contact-mines, moored fathoms down so that a vessel of the deepest draught could pass unscathed but should a U-boat attempt to nose her way in by creeping just above the bottom of the sea, her fate would be swift and terrible. "Keep a sharp look-out for the gateway," ordered Derek, as he placed a fresh man at the helm. "It's getting a bit misty, and we don't want to run full tilt against the boom." The boats were now nearing the innermost line of anti-submarine defences of the western approach to the greatest naval harbour in the world. Right across the water-way was a triple line of massive wire hawsers, supported by barrels at frequent intervals. So much was visible; what was not visible was a wondrous complication of nets, explosive charges, and other effective anti-submarine defences. Britain's sure and safe shield was taking no undue risks with Fritz and all his evil works. To enable authorized vessels to pass, a gateway had been constructed. Between two large craft moored a cable's length apart there was a movable section of the barrier, and towards this the two motor-boats steered. "Motor-boats ahoy!" hailed an officer from one of the guard-ships. "You are to proceed to Bull Roads and await further orders from the S.N.O." Against this mandate there was no appeal. The word of the Senior Naval Officer was more than law. Doubtless it meant irritating and apparently needless delay, but, whatever the object of the order, it had to be put into effect without delay. "Aye, aye, sir!" shouted Derek in reply. He knew perfectly well that non-compliance would result in a six-pounder shell fired across his bows, and almost immediately a salvo from the guardship's quick-firers. "Port helm!" continued Daventry, addressing the coxswain. Round swung his boat; the one astern instantly followed suit, and a course was shaped for Bull Roads, an open anchorage barely two miles distant. Arriving here the boats had orders to anchor, and for four long hours they rolled heavily in the tide-way. Naval patrol-boats of all sorts and sizes passed continually, but none appeared to pay the slightest attention to the two strangers within their gates. It was not until well into the afternoon that a patrol-boat eased down within a few feet of Derek's craft. "You can proceed," announced the officer. "Why have we been detained?" asked Derek, wondering at the bald announcement and the lack of explanation. The sub-lieutenant R.N.R. shrugged his shoulders. "Ask me another, old sport," he replied. "If you want to carry on do so at once, before the Old Man puts another stopper on you. _Bon voyage!_"