Joe glanced back, and when he saw that the stranger's florid face had changed in hue from a deep red to a greenish white he knew that the motion of the boat was indeed taking its effect. He forebore an impulse to chuckle at their passenger's plight. "Give her a little more gas," ordered the stranger, in a curiously feeble voice. "You're not going fast enough." He brandished the revolver threateningly. Frank obligingly increased the speed of the _Sleuth_ but the rocking motion only became more pronounced. The stranger gulped, but he did not lower the weapon. "That's better," he said, without enthusiasm. "I'm going to give him something to be seasick about," whispered Frank. Without warning he suddenly bore down on the wheel and swung the motorboat about so that it was lying broadside to the waves. "Here--what's the matter?" asked the stranger. "Where are you going now?" "We're off our course. I'm heading in toward shore a little more so we can get out of the wind." This explanation satisfied the stranger, although it became speedily apparent that the new course did not. The _Sleuth_ received the full force of the long rollers. The waves were not high enough to be dangerous, but the swells gave an undulating motion to the craft that swiftly increased the stranger's illness. "He's slipping," whispered Joe. Frank glanced back again. The stranger was indeed "slipping." His teeth were tightly clenched. His face was almost green. His expression was that of a man who is deathly sick. But he still clung to the revolver and he still waved it feebly at the boys. "Head her in toward Bayport," he demanded. "Do you want to make me sick?" "This'll fix him," said Frank. "Get ready." He bore down on the wheel again. The _Sleuth_ swung around at right angles to her previous course. The abrupt, swerving motion finished the stranger. With a groan, he slumped forward in his seat, and bowed his head on his arms. Joe sprang up. With one bound he reached the man with the gun. The stranger realized what was happening, and struggled to his feet. He raised the weapon, but Joe struck out and dashed the revolver from his hand. It described a flashing arc, then fell into the water with a splash. Sick as he was, the man swung out viciously and his fist caught Joe on the side of the face, staggering him. Joe quickly recovered himself and plunged forward, grappling with the man. They swayed to and fro in the middle of the boat, then fell, still struggling. But although Joe was young and wiry he was not strong enough to cope with his antagonist and Frank soon saw that the stranger was having the better of the battle. He glanced ahead, saw that the _Sleuth_ was heading into a long, low bank of fog but that there were no other boats in sight, then abandoned the wheel.