must have been left-overs from other cruises. By this time
the little fleet was approaching the nearest of the shoals, some three
miles from shore.

"Blimp" was now well ahead, presently signalling back.

"Found a sea-hornet for you, resting in the mud."

"Good enough! We'll draw his sting," the "Grigsby's" radio reply
promised.

Darrin caused a signal to be made to two of the mine-sweepers to come in
close to him. The "Reed" still continued on her way further out.

Aircraft are of the greatest help in discovering submerged submarines.
Depending on the altitude at which they fly, air observers are able to
see, in reasonably smooth water, submarines that are moving at from
eighty to a hundred feet beneath the surface. A submarine that is
"resting" with her nose in the mud close to shore has more to fear from
aircraft than from all other possible foes.

The aircraft men, though they can drop bombs upon such lurking craft,
cannot do so with anything like the accuracy that is possible to the
crews of vessels on the surface. Hence when aircraft and destroyers hunt
together it is almost always left to the surface craft to give the "grace
blow" to the resting submarine, as also to a submarine in motion beneath
the waves.

As the "blimp" moved over the shoal in question a smoke bomb left the car
and hovered almost motionless in the air, though briefly. This indicated
that the submarine lay on the bottom directly underneath the smoke bomb.

"And the commander of that Hun craft knows that we are approaching,"
Darrin commented, as the "Grigsby" raced roaringly forward. "He can hear
the noise of our propellers. If his engines are ready, he'll likely back
off into deeper water."

Thrice more the "blimp" passed over the submarine that was invisible to
surface eyes, and each time let loose a smoke bomb.

"Now, you're directly in line," came the radio message from above. "Move
dead ahead. Will tell you when you are passing over. We'll signal the
word 'drop'."

The meaning of "drop" would be clear enough. It would mean that the
"Grigsby" was instantly to release, over the stern, a depth bomb.

As the "Grigsby" neared the spot speed was considerably reduced. Overhead
hovered the "blimp," ready for instant signalling of one word. The
command had already been passed to the men stationed by the depth bomb to
let go as soon as the messenger gave the word from the operator.

As Darrin glanced upward he saw the "blimp" nearly ov