Del Mar took his place at the periscope, the eye of the submarine.  Anxiously he turned it about and bent over the image which it projected.

“There it is,” he muttered, picking out Arnold’s yacht and changing the course of the submarine so that it was headed directly at it, the planes turned so that they kept the boat just under the surface with only the periscope showing above.

Forward, about the torpedo discharge tubes men were busy, testing the doors, and getting ready the big automobile torpedoes.

“They must have seen us,” muttered Del Mar.  “They’ve started the yacht.  But we can beat them, easily.  Are you ready?”

“Yes,” called back the men forward, pushing a torpedo into the lock-like compartment from which it was launched.

“Let it go, then,” bellowed Del Mar.

The torpedo shot out into the water, travelling under her own power, straight at the yacht.

. . . . . . .

Elaine and I looked back.  The periscope was much nearer than before.  “Can we outdistance the submarine?” I asked of Arnold.

Arnold shook his head, his face grave.  On came the thin line of foam.  “I’m afraid we’ll have to leave the yacht,” he said warningly.  “My little motor-boat is much faster.”

Arnold shouted his orders as he led us down the ladder to the motor-boat into which we jumped, followed by as many of the crew as could get in, while the others leaped into the water from the rail of the yacht and struck out for the shore which was not very distant.

“What’s that?” cried Elaine, horrified, pointing back.

The water seemed to be all churned up.  A long cigar-shaped affair was slipping along near enough to the surface so that we could just make it out—­murderous, deadly, aimed right at the heart of the yacht.

“A torpedo!” exclaimed Arnold.  “Cast off!”

We moved off from the yacht as swiftly as the speedy little open motor-boat would carry us, not a minute too soon.

The torpedo struck the yacht almost exactly amidships.  A huge column of water spurted up into the air as though a gigantic whale were blowing off.  The yacht itself seemed lifted from the water and literally broken in half like a brittle rod of glass and dropped back into the water.

Below in the submarine, Del Mar was still at the periscope directing things.

“A hit!” he cried exultingly.  “We got the whole bunch this time!”

He turned to the men to congratulate them, a smile on his evil face.  But as he looked again, he caught sight of our little motor-boat skimming safely away on the other side of the wreck.

“The deuce!” he muttered.  “Try another.  Here’s the direction.”

Furiously he swore as the men guided the submarine and loaded another torpedo into a tube.  As the tube came into position, they let the torpedo go.  An instant later it was hissing its way at us.