As he skirted the shore of the river, he came to a falls.  Here he moved even more cautiously than before, looking about to make certain that no one had followed him.

It was a beautiful sheet of water that tumbled with a roar over the ledge of rock, then raced away swiftly to the sea in a cloud of spray.

Assured 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  “This rotary gap affair is out of date.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: