a.s.sured that he was alone, he approached a crevice in the rocks, near the falls. With another hasty look about, he reached in and pulled a lever.

Instantly a most marvellous change took place, incredible almost beyond belief. The volume of water that came over the falls actually and rapidly decreased until it almost stopped, dripping slowly in a thin veil. There was the entrance of a cave--literally hidden behind the falls!

Del Mar walked in. Inside was the entrance to another, inner cave, higher up in the sheer stone of the wall that the waters had eroded.

From the floor to this entrance led a ladder. Del Mar climbed it, then stopped just inside the entrance to the inner cave. For a moment he paused. Then he pressed another lever. Almost immediately the thin trickle of water grew until at last the roaring falls completely covered the cave entrance. It was a clever concealment, contrived by damming the river above and arranging a new outlet controlled by flood-gates.

There Del Mar stood, in the inner cave. A man sat at a table, a curious gear fastened over his head and covering his ears. Before him was a huge apparatus from which flared a big bluish-green spark, snapping and crackling above the thunder of the waves. From the apparatus ran wires apparently up through cables that penetrated the rocky roof of the cavern and the river above.

It was Del Mar's secret wireless station, close to the hidden submarine harbor which had been established beneath the innocent rocks of the promontory up the coast. Far overhead, on the cliff over the falls, were the antennae of the wireless.

"How is she working?" asked Del Mar.

"Pretty well," answered the man.

"No interference?" queried Del Mar, adjusting the apparatus.

The man shook his head in the negative.

"We must get a quenched spark apparatus," went on Del Mar, pleased that nothing was wrong here. "This rotary gap affair is out of date. By the way, I want you to be ready to send a message, to be relayed across to our people. I've got to consult the board below in the harbor, first, however. I'll send a messenger to you."

"Very well, sir," returned the man, saluting as Del Mar went out.

Out at Fort Dale, Lieutenant Woodward was still entertaining his new friend, Professor Arnold, and had introduced him to Colonel Swift, the commanding officer at the Fort.

They were discussing the strange events of the early morning, when an orderly entered, saluted Colonel Swift and handed him a telegram. The Colonel tore it open and read it, his face growing grave. Then he handed it to Woodward, who read:

WASHINGTON, D. C.

Radio station using illegal wave length in your vicinity. Investigate and report.

BRANDON,

Radio Bureau.

Professor Arnold shook his head slowly, as he handed the telegram back.

"There's a wireless apparatus of my own on my yacht," he remarked slowly. "I have an instrument there which I think can help you greatly.

Let's see what we can do."

"All right," nodded Colonel Swift to Woodward. "Try."

The two went out and a few minutes later, on the sh.o.r.e, jumped into Arnold's fast little motor-boat and sped out across the water until they swung around alongside the trim yacht which Arnold was using.

It was a compact and comfortable little craft with lines that indicated both gracefulness and speed. On one of the masts, as they approached, Woodward noticed the wireless aerial. They climbed up the ladder over the side and made their way directly to the wireless room, where Arnold sat down and at once began to adjust the apparatus.

Woodward seemed keenly interested in inspecting the plant which was of a curious type and not exactly like any that he had seen before.

"Wireless apparatus," explained Arnold, still at work, "as you know, is divided into three parts, the source of power, the making and sending of wireless waves, including the key, spark, condenser and tuning coil, and the receiving apparatus--head telephones, antennae, ground and detector. This is a very compact system with facilities for a quick change from one wave length to another. It has a spark gap, quenched type, break system relay--operator can hear any interference while transmitting--transformation by a single throw of a six-point switch which tunes the oscillating and open circuits to resonance."

Woodward watched him keenly, following his explanation carefully, as Arnold concluded.

"You might call it a radio detective," he added.

Even the startling experience of the morning when she was carried off and finally jumped from the little tramp steamer that had attempted to cut the cable did not dampen Elaine's ardor. She missed the guiding hand of Kennedy, yet felt impelled to follow up and investigate the strange things that had been happening in the neighborhood of her summer home since his disappearance.

I succeeded in getting her safely home after Burnside and I rescued her in the hydroaeroplane, but no sooner had she changed her clothes for dry ones than she disappeared herself. At least I could not find her, though, later, I found that she had stolen away to town and there had purchased a complete outfit of men's clothes from a second hand dealer.

Cautiously, with the large bundle under her arm, she returned to Dodge Hall and almost sneaked into her own home and up-stairs to her room.

She locked the door and hastily unwrapped the bundle taking out a tattered suit and the other things, holding them up and laughing gleefully as she took off her own pretty clothes and donned these hideous garments.

Quickly she completed her change of costume and outward character. As she surveyed herself in the dainty mirror of her dressing-table she laughed again at the incongruity of her pretty boudoir and the rough men's clothes she was wearing. Deftly she arranged her hair so that her hat would cover it. She picked a black mustache from the table and stuck it on her soft upper lip. It tickled and she made a wry face over it. Then she hunted up a cigarette from the bundle which she had brought in, lighted it and stuck it in the corner of her mouth, letting it droop jauntily. It made her cough tremendously and she threw it away.

Finally she went to the door and down-stairs. No one was about. She opened the door and gazed around. All was quiet. It was a new role for her, but, with a bold front, she went out and pa.s.sed down to the gate of the grounds, pulling her hat down over her eyes and a.s.suming a tough swagger.

Only a few minutes before, down in the submarine harbor, the officers of the board of foreign agents had been grouped about Del Mar, who had entered and taken his place at their head, very angry over the failure to cut the cable. As they concluded their hasty conference, he wrote a message on a slip of paper.

"Take this to our wireless station," he ordered, handing it to one of the men.

The man took it, rose, and went to a wardrobe from which he extracted one of the submarine suits. With the message in his hand, he went out of the room, buckling on the suit.

A few minutes later the messenger in the submarine suit bobbed up out of the water, near the promontory, and climbed slowly over the rocks toward a crevice, where he began to take off the diving outfit.

Having finished, he hid the suit among the rocks and then went along to the little river, carefully skirting its banks into the ravine in which were the falls and the wireless cave.

In her disguise, Elaine had made her way by a sort of instinct along the sh.o.r.e to the rocky promontory where we had discovered the message in the tin tube in the water.

Something, she knew not what, was going on about there, and she reasoned that it was not all over yet. She was right. As she looked about keenly she did see something, and she hid among the rocks. It was a man, all dripping, in an outlandish helmet and suit.

She saw him slink into a crevice and take off the suit, then, as he moved toward the river ravine, she stole up after him.

Suddenly she stopped stark still, surprised, and stared.

The man had actually gone up to the very waterfall. He had pressed what looked like a lever and the water over the falls seemed to stop. Then he walked directly through into a cave.

In the greatest wonder, Elaine crept along toward the falls. Inside the cave Del Mar's emissary started to climb a ladder to an inner cave. As he reached the top, he glanced out and saw Elaine by the entrance. With an oath he jumped into the inner entrance. His hand reached eagerly for a lever in the rocks and as he found and held it, he peered out carefully.

Elaine cautiously came from behind a rock where she had hidden herself and seeing no one apparently watching, now, advanced until she stood directly under the trickle of water which had once been the falls. She gazed into the cave, curiously uncertain whether she dared to go in alone or not.

The emissary jerked fiercely at the lever as he saw Elaine.

Above the falls a dam had been built and by a system of levers the gates could be operated so that the water could be thrown over the falls or diverted away, at will. As the man pressed the lever, the flood gates worked quickly.

Elaine stood gazing eagerly into the blackness of the cave. Just then a great volume of water from above crashed down on her, with almost crushing weight.

How she lived through it she never knew. But, fortunately, she had not gone quite far enough to get the full force of the water. Still, the terrific flood easily overcame her.

She was swept, screaming, down the stream.