I could not help thinking that, although he was glad to see us, there was a certain air of restraint about him.

Quickly Elaine related the story of finding the bomb in the rocks and the peculiar events and our escape which followed.  Once, at the mention of the searchlight gun, Professor Arnold raised his hand and coughed back of it.  I felt sure that it was to hide an involuntary expression of satisfaction and that it must be he who had sent the gun to Elaine.

He was listening attentively to her, while I stood by the rail, now and then looking out over the water.  Far away I noted something moving over the surface, like a rod, followed by a thin wake of foam.

“Look!” I exclaimed, “What’s that?”

Elaine turned to me, as Arnold seized his glasses.

“Why, it seems to be moving directly at us,” exclaimed Elaine.

“By George, it’s the periscope of a submarine,” cried Arnold a moment later, lowering his glasses.

He did not hesitate an instant.

“Get the yacht under way,” he ordered the captain, who immediately shouted his orders to the rest.

Quickly the engine started and we plowed ahead, that ominous looking periscope following.

In the submarine harbor to which he had been taken, Del Mar found that he had been pretty badly shaken up by the accident to his car.  His clothes were torn and his face and body scratched.  No bones were broken, however, though the shock had been great.  Several of his men were endeavoring to fix him up in the little submarine office, but he was angry, very angry.

At such a juncture, a man in a dripping diving-suit entered and pulled off his helmet, after what had evidently been a hasty trip from the land through the entrance and up again into the harbor.  As he approached, Del Mar saw that the man’s hand was bound up.

“What’s the matter?” demanded Del Mar.  “How did you get that?”

“That fellow Jameson and the girl did it,” he replied, telling what had happened in the cave.  “Some one must have given them one of those new searchlight guns.”

Del Mar, already ugly, was beside himself with rage now.

“Where are they?” he asked.

“I saw them go out to the yacht of that Professor Arnold.”

“He’s the fellow that gave her the gun,” almost hissed Del Mar.  “On the yacht, are they?”

An evil smile seemed to spread over his face.  “Then we’ll get them all, this time.  Man the submarine—­the Z99.”

All left the office on the run, hurrying around the ledge and down into the open hatch of the submarine.  Del Mar came along a moment later, giving orders sharply and quickly.

The hatch was closed and the vessel sealed.  On all sides were electrical devices and machines to operate the craft and the torpedoes—­an intricate system of things which it seemed as if no human mind could possibly understand.

Del Mar threw on a switch.  The submarine hummed and trembled.  Slowly she sank in the harbor until she was at the level of the underwater entrance through the rocks.  Carefully she was guided out through this entrance into the waters of the larger, real harbor.