" came back the reply. "They lie about
four hundred feet apart and heading in the same direction. I can find
them again, sir, but I didn't go back over them for fear they'd take the
alarm and run for it."

"Perhaps they have," Darrin suggested.

"I've dropped small buoys, sir, and can lead you over them."

"Then do so, and travel at full speed. Be prepared to get out of our way
if we come fast after dropping."

Even the two cool-headed sailors who stood by the depth bomb apparatus
stiffened themselves as they found the "Grigsby" following in the wake of
the mine-sweeper. The after lookouts lashed themselves fast against
injury by any such surprise as that of the afternoon.

As the signal flashed from the mine-sweeper ahead Dave passed the order
for the bomb instantly after ordering full speed.

There was an explosion, but an ordinary one, such as this crew of the
destroyer was accustomed to.

At full speed, too, Dave tried for the second hidden enemy boat. There
was barely time to have the second bomb in place when signal and order
came.

Another terrific explosion, like that of the afternoon! It seemed as
though the waters must divide! Yet the "Grigsby," moving fast all the
time, felt the shock severely, but not like the one of the afternoon.

About the destroyer came, playing her searchlight on the waters. The
tell-tale oil patches were there, showing only too plainly that two
submarine craft had been destroyed.

"Apparently one craft carried no mines, while the other was loaded with
them," said Dave to his executive officer. "Fernald, I think I'm
beginning to get an idea of the way the enemy are working their
mine-laying game. If I'm right we'll make a record along this patch of
shoals while the hunting lasts."

Patiently Fernald listened and waited, but did not speak. He hoped to
hear what his chief's idea was, but it was not the executive officer's
place to ask for it.

"I may even be able to figure out when the best time would be for hunting
these lazy rascals resting on the bottom," Darrin continued.

Mr. Fernald began to show signs of a more active curiosity.

"But I won't say much about it," Darrin smiled, "until I've more data to
work on and have proved some part of my theory."

Lieutenant Fernald looked so much like a man who wished to speak that
Dave laughed.

"Out with it, Fernald," he urged good-humoredly. "You've an idea, too.
You may tell me if you wish."

"Why, sir," replied the ex