The sum mentioned, no less a sum than five thousand dollars, would mean luxury to the poor man. And all for what? Simply to call up a stranger, a Mr. Locke, to tell him that the boatman demanded more money since he had telephoned before, that the cash was to be placed by him in an old packing-case from which a stationary engine had been removed that morning. It was just an exchange. That was all.

"Sure I'll do that," the boatman told Paul, and Paul, smiling craftily, gave him his hand to seal the bargain.

The boatman went back to his quarters and again called Brent Rock, making his new demands. Locke was tremendously indignant, but he wanted the suit quickly to prevent its falling into unscrupulous hands. He agreed and immediately started for the dock.

The boatman turned from his telephone and, picking up the suit, regarded it curiously. "Five thousand dollars," he muttered. "Five thousand dollars." And he shook his head wonderingly.

He was standing near an open window and was commencing to fold the suit preparatory to taking it to the end of the dock where lay the engine-case, when, without the slightest warning, three emissaries of the Automaton, who had appeared just a moment before on the dock, leaped through the window and felled him to the floor. He struggled feebly, but it was no use, and a final blow left him unconscious.

The emissaries next grabbed the diving-suit and left hurriedly by the way they had come. But they had not completed what it was they sought to do.

The old boatman was not as badly hurt as it seemed and was able to drag himself across the floor with just strength enough to pull the telephone from the table and call Brent Rock. Then as weakness again overcame him he managed to blurt out a message to Eva, who answered.

"Don't let Mr. Locke come to the dock," he managed to gasp. "He'll be killed." Then he collapsed and fainted.

Eva tried frantically to get the boatman again on the wire, but it was useless. Quickly a plan formed in her mind.

If she could only intercept Locke before he reached the dock!

She dashed out to the garage, realizing that it was almost hopeless, since Locke had been gone some time. Hoping against hope, she jumped into her speedster and swung out and down the road.

The fact was that even as she sped along toward the cove Locke was passing the arched gate of the dock.

He called at the boatman's little shack. Of course there was no reply.

To all appearances it was deserted. Thinking to find him at the very end of the dock where he had been told to place the money, he proceeded to the engine-case.

He was slightly surprised at not finding the boatman there, but as that was no part of the agreement it engaged his attention for only a moment.

He started to withdraw the money from his pocket, groping at the same time to see if the diving-suit was actually in the case.

He was bending over when suddenly there was a rush of men behind him and a blackjack in the hands of one of the ruffians just missed his head.

He fought, but their numbers were overwhelming. Like a pack of wolves they pulled him down.

Locke was quickly bound with ropes and forced into the engine-case. The cover was put on and they nailed it down solidly. To make it doubly sure this time the case was then lashed with ropes and they were knotted.

Next the emissaries carried the case to a sloping landing stage, preparatory to casting it into the river.

It was at this moment that Eva came running down the dock in wild search to intercept Locke. Wide-eyed, in the moonlight, she paused at what she saw.

The emissaries had given the packing-case its final shove. Scraping, it slid down the incline and toppled overboard. There was a great splash as it struck the water and immediately began to sink in the depths.

The engine exhaust had evidently protruded from the case, as there was a hole in its side slightly larger than a man's hand. To Eva's horror, though she had half expected it, she saw actually a hand thrust forth from this hole as if waving frantically.

The box sank lower as it rapidly filled with water.

Eva knew not what to do. Instinctively she knew that it was Locke. It was as though he had waved a last farewell.

Only the hand now showed above the surface. Finally that, too, disappeared beneath the waves.

Despairingly she turned to see if there was anything on the dock with which she might help Locke--and she saw the Automaton himself advancing from the shore toward her. She turned. The emissaries on the other end of the dock cut off any chance in that direction.

Without a moment's hesitation Eva poised herself a moment on the edge of the dock and leaped far out into the blackness of the river.

CHAPTER XIII

The box that held Locke a prisoner was now undoubtedly resting on the slimy bottom. Eva had totally disappeared. The Automaton, convinced that at last he had rid himself of his victims, waved away the emissaries and departed.

Except for the tiny lights of ships on the river and the staccato exhaust of a tugboat, the river flowed with nothing to remind one of the two tragedies of only a few seconds ago.

As far as the eye could see, the surface of the water was unbroken.

Then, suddenly, the scene changed. For from out the water, as though hurled up by a catapult, shot a man's body.

It was Locke.

By what miracle had he escaped from the watery grave?

From the time he was a small boy the study of locks and bolts, of knots and strait-jackets, of anything that could restrain or bind a man, had held a marvelous fascination for him, until now he was recognized as one of the world's greatest experts on these subjects. The great lock concerns often sent for him to test new inventions, and invariably he could point to any flaw in the constructions of them that existed. As he came to manhood his knowledge had grown apace until to many he seemed a veritable sorcerer.

It was by a trick known only to himself that he had been able to extricate himself from his desperate plight at the river's bottom. True, his flesh was lacerated. True, he was on the verge of total collapse.

But he lived.

He made his way slowly toward the dock and was resting against one of the piles when he heard a faint cry. He strained his ears to locate the direction whence it came. Once again that feeble call floated across the water, and in it there sounded something vaguely familiar.

He was more rested now and he swam farther under the dock. Again came the cry. With a thrill now he recognized the voice.

"Eva!" he called, again and again.

"Here I am," came back the echo.

With a powerful stroke he breasted the current and in a moment he was supporting her half-fainting body. Precarious though their position was, Locke felt the thrill of her words. The effect was to spur him on to fresh efforts.

Eva had become stronger now. For a few moments he swam, in order, if possible, to find some means by which they might escape from the water and reach the dock.

They had no way of knowing but that the Automaton and his emissaries might still be lurking above, ready to thrust them back into the water or to reserve for them some even more terrible fate. But it was a risk that they realized must be taken and at once. An attempt to swim to another dock could end only disastrously.

Locke soon returned with the cheering news that he had discovered a ladder that came even to the surface of the water, a landing for small boats. More than that, he had mounted the ladder, and from a short survey he had seen no sign of their enemies.

Carefully aiding Eva, Locke swam to this ladder and soon they stood upon the dock, safe.

With great caution they moved toward the street and, without harm, finally passed beneath the arched gates again and were in the city street.

Eva went at once to her father's room. His condition was one of great weakness. The laughing madness had abated in so far that the poor victim was so weak that the spasms could not maintain a very violent form.

Eva practised all those little kindnesses which are known only to women, and tears were in her eyes as she stroked his poor gray head.

How terrible was it that, after all they had attempted, all that they had suffered, they should still stand defeated in their aim to get the antidote that would cure her father's malady. However, the brave girl was not one to admit herself beaten, and even as she sat there she was planning new ways to discover who were her terrible adversaries and to bring defeat to them.