It soon became apparent that the task of towing the sea-plane was not so easy as Derek imagined. The unwieldy machine--for out of its natural element it was unwieldy--yawed, dipped, and strained at the towing-hawser until Derek ordered the second boat to make fast astern of the sea-plane and run at half throttle in order to steady the awkward tow. With the ebb tide the passage through the "gateway" was soon completed. Another ten miles would find the sea-plane and her tug out in the open sea. In the tide-rip off Fort Churst the behaviour of the sea-plane gave rise to some anxiety, but, upon gaining the exposed waters of the English Channel, the rate of progress was uniformly maintained. Presently Derek noticed that a bank of fog was bearing down before a stiff southerly, or on-shore breeze. Already the outlines of Thorbury Head, nine miles away, were blotted out, while, on the starboard hand, the long line of low, yellowish cliffs was cut up into sections by the rolling, fleecy vapour. Consulting the chart Derek found that his course was due west magnetic, which would pass at least a mile to the south'ard of the dangerous headland. Allowing for the reduced speed of the boats and the tow, he calculated that it would take about an hour to bring Thorbury Head broad on the beam. Down swept the fog, enveloping everything. From the steering-wheel it was almost impossible to distinguish the boat's stem-head; while astern the sea-plane was absolutely invisible. At the end of forty minutes Derek began to feel a bit doubtful of his position. Miles astern he could hear the monotonous, mournful wail of the Bodkin Lighthouse. The sea, hitherto calm, was now setting in with a long roll, breaking heavily upon the invisible shore with a continuous, sullen roar. "It seems rather shallow, sir," remarked the coxswain, as he shook the drops of moisture from the rim of his sou'wester. "Shall I take a cast, sir?" "Yes, please." In his anxiety about keeping the boat on her course Derek had forgotten the indispensable lead-line. A cast gave two and a quarter fathoms, whereas, according to the chart, there ought to be a depth of nine. "Steer south-west," ordered Derek. "There's something strange about this business," he added in an undertone. "Breakers ahead, sir!" A partial lifting of the fog enabled the range of visibility to extend to nearly a quarter of a mile. As far as the eye could see the water was one seething mass of huge waves, from which there was no escape. The boats were trapped in the dangerous Thorbury Bay.