"Radio message from the 'blimp,' sir," called a messenger, darting from the doorway of the wireless room. "Do you wish a written copy, sir?"

Lieutenant Fernald glanced at Dave, who shook his head.

"Let's have the message orally," Fernald called down to the deck.

"'Blimp' wants to know, sir, if these two craft are the 'Grigsby' and 'Reed.'"

"Tell the operator to admit the fact," Fernald ordered.

"Officer in charge of the 'blimp,' sir, says that he was to report and help you yesterday, but that the weather was too foggy."

"Tell the operator to send back: 'Good morning. Glad to have you with us.

Signature, Darrin,'" Dave directed.

The seamen and petty officer at the anti-aircraft gun left their station.

Straight onward came the "blimp," dropping much lower just as it pa.s.sed over. From the car beneath the big gas-bag several men leaned over to wave friendly hands, a greeting that was instantly responded to by Dave's and Dan's jackies, for the dirigible, after sailing over the "Grigsby,"

turned and floated over the "Reed."

"Message from the 'blimp,' sir," again iterated the messenger on the deck. "Message says: 'We're to keep near you and try to spot submarines for you.'"

"More power to your vision," was the message sent back by Dave.

"You're working northward, toward the shoals?" asked "Blimp."

"Yes," Darrin acknowledged.

"That's a likely place to find one or two of the Hun pirates resting,"

"Blimp" continued.

"Always a good hunting ground," Dave a.s.sented, in a radio message.

This took place while the dirigible was flying back and forth, ahead and astern, between the destroyers and to either side of their course.

"It's a fine thing to be able to move at aircraft speed," said Lieutenant Fernald, rather enviously. "If we could only make such speed, sir!"

"If we could build ships that would steam sixty to a hundred miles an hour, then the enemy could build them also," Dave returned. "There would be little, if any, net gain for us. But if we could find the secret of doubling the speed of aircraft, and keep said secret from the boches, that would be an achievement that would soon end the war."

For ten miles the sweepers proceeded, with a total "catch" of only three mines, which 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 sh.o.r.e.

"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 alt.i.tude 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 sh.o.r.e 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" pa.s.sed 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 pa.s.sing 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 pa.s.sed 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 overhead.

Suddenly the messenger's startled voice roared out the message pa.s.sed by the radio operator:

"_Full speed astern!_"

In the same instant Lieutenant Fernald repeated the order over the engine-room telegraph. There was a jolting jar as the "Grigsby"

shivered, then glided back in her own wake.

"Jove! That was a narrow squeak!" came down from the sky. "That hornet laid an egg in your path. It came within an ace of b.u.mping your keel."

"Never did speed pay a prompter profit, then," uttered Darrin, his cheeks paling slightly.

For the Englishman's laconic message meant that the submarine had just proved herself to be of the mine-laying variety. Further, the Hun craft, hearing the destroyer's propellers almost overhead, had judged the moment at which to let loose a mine, which, rising to its proper level under water, would have struck the hull of the advancing destroyer.

Had that happened, the career of the "Grigsby" would have been over, and several officers' and seamen's names would have been added to the war's list of dead.

"Going to try again, sir?" asked Lieutenant Fernald, quietly, as Dave himself changed the full-speed-astern order.

"It's out of our line, I guess," Darrin confessed, with a smile. "Signal yonder mine-sweeper to close in on the job."

As a result of the message, and aided by the "blimp" overhead, the snub-nosed mine-sweeper steamed into position. First, her wire sweeper picked up the mine that had been sprung for the "Grigsby's" undoing, and backed away.

Then, under Dave's further order, after the mine had been hoisted on board, the snub-nosed craft moved in with a different type of sweeper. To different wires of this implement were attached small but powerful contact bombs. Jauntily the snub-nosed craft moved over the lurking place of the submarine, and pa.s.sed on ahead.

From the depths came m.u.f.fled sounds, followed by a big and growing spread of oil on the water.

"Enemy done for!" signalled the "blimp."

"Thank you, sir. We know it," the "Grigsby" wirelessed back.

The mine-sweeper, having pa.s.sed on ahead, now circled back, her crew grinning at sight of the ma.s.s of floating oil.

The contact bombs dangling from the sweep wires had struck against the submarine's hull and exploded, letting in the water at several points.