In the meantime, Captain Griswold of the Ventura had been watching the struggle as his vessel sped away from the scene.  There was a strange light in his eyes and he muttered to himself.  At last he muttered an imprecation.

“He’s a brave boy,” he said.  “I can’t run away and leave him like that.”

He brought the head of the vessel around in spite of the protests of some of the passengers, and headed back for the submarine.

“Man the forward gun there!” he cried.

For the Ventura, like other allied ships plying in the seas in those days, carried small guns for defensive purposes.  The gun crew sprang to obey this order and the gun was trained on the submarine.

“Fire!” shouted Captain Griswold.

“Crash!”

The gun spoke and a geyser of water was kicked up just beyond the submarine.

At this point the officer left in command of the submarine seemed to realize his own danger.  He sprang to the conning tower, unmindful of the fact that his commander was struggling in the water.

“Down, men!” he cried.

But it appeared that the German sailors were made of sterner stuff than was the officer.  They refused to go below until their commander had been brought safely aboard.  In vain the officer pointed out their danger.

Jack struck out after the German commander as the latter swam for the submarine.  The lad was a powerful swimmer and he felt confident he could overtake the man before help could reach him.

The destroyer Essex had now drawn close.  Frank had been afraid to order a shot at the submarine for fear the shell might hit Jack in the water.

“Take the bridge, Mr. Hetherton!” he cried.  “Lower a boat, men!”

The boat was lowered in a trice and Frank and a score of sailors sprang in.  The launch darted toward Jack at full speed, Frank standing erect and with the quartermaster at the rudder.

They were close enough to see the struggle between Jack and the German commander in the water.  Frank saw the man break loose from Jack and strike out for the submarine.  He saw Jack make after him, and he saw something more.

Half a dozen German sailors leaped into the water and made for Jack, who apparently did not realize his own danger, so interested was he in the pursuit of the German commander.

“Faster!” cried Frank, and drew his revolver.

Now, for the first time, Jack realized his danger.  But it was too late to draw back, and it is doubtful if he would have done so anyway.

“I’m going to get that fellow,” he gritted between his teeth, referring to the German commander.

One of the German sailors struck at the lad with a knife.  Jack caught the man’s arm with his left hand and twisted sharply.  There was a snap, and the knife dropped into the water.  The sailor uttered a cry of pain and turning, struck out for the submarine with his good arm.