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 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.