“Into the tube with you, now,” whispered Jack.  “Crawl well forward—­right up to the forward end of the tube—­so.  Get hold of the crossbar of the cap.  Hold on hard.  Now, when we close the rear port, and open the forward cap, with a little rush of compressed air, the cap will open forward and up, dragging you out into the water.  Now, then, got a good hold?”

“A grip like death itself,” laughed Hal.

“Be ready, then.”

Captain Jack closed the rear port of the tube, and turned on some compressed air, which also drove the forward port open and up.  A moment later the submarine boy tapped at the door of the state-room.

“Has anything happened?” smiled Mr. Farnum.

“Hal Hastings is missing, sir,” reported Jack.

“Missing?” demanded the boatbuilder, leading his guests out into the cabin.

Young Benson pointed to the pile of clothing, just as Hal had left it on the floor.

“Get to the surface,” commanded Mr. Farnum.  “We shall have to look into this.”

Soon the conning tower of the “Pollard” reappeared above the waves.

“Hal is safe, gentlemen,” reported Captain Benson, from the tower.

An instant later he opened the manhole of the tower, allowing all hands to step out on deck.

Grinning delightedly, Hal stood in the bow of the small shore boat.

“How did he get there, from a submarine on the bottom?” asked Commander Ennerling, in astonishment.

“That is one of the secret features of this boat,” laughed Mr. Farnum.  “Now, gentlemen, if you will kindly come below again, we’re going to sink.”

Hardly had the submarine touched bottom before Mr. Farnum again conducted his guests back to the state-room.  When Captain Jack summoned them forth, they returned to find Hal Hastings, laughing in a way that showed his white teeth, standing there in his dripping garments.

“From what you have seen, gentlemen,” said the builder, seriously, “I am sure you will understand that we have mastered a new feature, of great value in submarine boating.”  The three Navy officers struggled to conceal their wonder.

“Make for the surface, Captain Benson,” directed the owner.

When the passengers aboard the submarine stood once more on the platform deck, the yard’s owner signaled for the shore boat to lay alongside.  Into this small boat he took his guests.  The boat was rowed away two or three lengths, immediately after which the “Pollard” again sank.

Two or three minutes passed.  Then Captain Jack’s head shot above the surface.  He made for the boat, hanging onto the gunwale.

“It would be bad judgment to call you young fellows mermaids,” said Commander Ennerling, dryly, “but you are surely merboys.”

A moment later Hal Hastings’s head came above the surface.

“Mr. Pollard and young Somers could as easily leave the boat and join us,” explained Mr. Farnum.  “However, if the last man aboard leaves the boat then there is no way provided for a return to the ‘Pollard,’ and we would be placed at great expense in raising her.  I think we have, however, shown you enough to make you believe that we have mastered some new wrinkles in submarine work.”