Late comers still crowded in.  Men now reported that everything was ready.  Steadily the time of high water approached.

“Saw the sole pieces!” finally rang out the order.

That was a thing that must be done by two gangs, one on each side, and evenly, too.  If one gang got ahead of the other, they must stop and let the second catch up.

“Zip—­zip—­zip,” came the shrill singing tone of the saws.

Was everything all right?  Kennedy and Sprague were still circling overhead, at various altitudes.  I redoubled my attention at the glass.

Suddenly I saw Craig’s flag waving frantically.  A muffled exclamation came from my lips involuntarily.  Marlowe, who had been watching me, leaned closer.

“What is it—­for God’s sake?” he whispered, hoarsely.

“Stop them!” I shouted as I caught Kennedy’s signal.  At a hurried order from Marlowe the gangs quit.  A hush fell over the crowd.

Kennedy was circling down now until at last the air-boat rested on the water and skimmed along toward the ways.

Out on the ways, as far as they were not yet submerged, some men ran, as if to meet him, but Kennedy began signaling frantically again.  Though I had not been expecting it, I made it out.

“He wants them to keep back,” I called, and the word was passed down the length of the ship.

Instead of coming to rest before the slip, the plane turned and went away, making a complete circle, then coming to rest.  To the surprise of every one, the rapid staccato bark of the Lewis gun broke the silence.  Kennedy was evidently firing, but at what?  There was nothing in sight.

Suddenly there came a tremendous detonation, which made even the launching-slip tremble, and a huge column of water, like a geyser, rose in the air about eight hundred feet out in the river, directly in front of us.

The truth flashed over us in an instant.  There, ten feet or so in the dark water out in the river, Craig had seen a huge circular object, visible only against a sandy bottom from the hydro-aeroplane above, as the sun-rays were reflected through the water.  It was a contact submarine mine.

Marlowe looked at me, his face almost pale.  The moment the great hulk of the Usona in its wild flight to the sea would have hit that mine, tilting it, she would have sunk in a blast of flame.

The air-boat now headed for the shore, and a few moments later, as Craig climbed into our stand, Marlowe seized him in congratulation too deep for words.

“Is it all right?” sang out one of the men in the gangs, less impressionable than the rest.

“If there is still water enough,” nodded Craig.

Again the order to saw away the sole pieces was given, and the gangs resumed.  “Zip—­zip,” again went the two saws.

There were perhaps two inches more left, when the hull quivered.  There was a crashing and rending as the timbers broke away.