Let us suppose that the officer in command at the white, map-covered table finds that the battleship is gradually approaching the position indicated in the harbor as mine number nineteen; as the officer watches the moving image of the battleship, he sees it going closer and closer to the exact spot numbered nineteen or the white map.
"Be ready, Sergeant," calls the officer, warningly, to a non-commissioned officer who stands before a board on the wall on which are several electric push-buttons, each numbered.
"Yes, sir," replies the sergeant.
At this moment the officer sees the image of the battleship passing fairly over the dot on the white map that is numbered nineteen.
"Fire nineteen, Sergeant," calls the Army officer in charge.
The non-commissioned officer quickly presses electric button numbered nineteen. As he does so the electric current is sent flashing, perhaps along four or five miles of insulated wire on the bottom of the harbor. At the other end of that wire is submarine mine number nineteen. In a breathless instant the current traverses the whole length of the wire. The spark has reached the gun-cotton! There is a dull, booming sound; a great column of water shoots up from the surface. In the midst of the commotion the enemy's battleship is rent, and all on board, perhaps killed. The cool, dry-eyed Army officer bending over the white screen-map sees all this scene of horror depicted under the white surface beneath his eyes. He knows that submarine mine number nineteen, planted out there in the harbor, has done its duty in protecting this portion of the coast of the United States.
Here, at Fort Craven, it was desired to find whether an enemy's submarine boat could creep in, below the surface, find the mine, whose location was already known through spies, and effectively cut the firing wire. If this could be done, then, in war-time, it might be that the sergeant at the wall-board would press the button in vain. No explosion would follow. With the current thus cut off, the officer bending over the white screen would not see the miniature reproduction of the destruction of the enemy's battleship.
A submarine torpedo boat, coming into a harbor underneath the surface, is not pictured on the white table under the camera obscura. So it was desired to see whether Eph could come in, knowing the exact locations of each of the four dummy mines, and quickly cut the firing electric wires. If this could be done, the Army would have to revise its method of firing such submarine mines by means of the camera obscura detection.
As Eph listened to the explanation his mind began to revolve plans rapidly whereby he hoped to succeed in cutting the mine wires.
"You will keep sufficiently below the surface, too, Mr. Somers," continued Major Woodruff. "We do not want you so close to the surface of the water that a ripple would show on the camera obscura table. You cannot, of course, rise and use your periscope to see where you are. Even the periscope would betray you."