narrow strip of land that separates this part of the bay from the ocean?"

"Quite right, Mr. Benson."

It was to be observed that the major, like naval officers, addressed Jack by the title of "mister," not "captain." This was because, in the military service, Army and Navy titles are not recognized unless conferred by government appointment or commission. Hence, though young Benson was "captain" to his crew and to civilians, officers of the United Service could address, him only as "mister."

"The neck, Mr. Benson," continued Major Woodruff, "is the land best suited for watching our work from to-day. And now, I will state what the object of to-day's tests is. This morning our tug will be engaged in planting certain submarine mines. Mr. Somers will watch our work of planting. Of course the mines will contain no explosives. You young men have, I understand, solved the problem of leaving a submarine boat while it lies on the bottom? You are also able to enter the submarine again from the surface?"

"Quite right, Major," Jack nodded.

"Then, if Mr. Somers watches the planting of the dummy mines, he will have the same advantage as would the commander of an enemy's submarine in knowing where our mines are planted. We shall plant four of them, this morning, and Mr. Somers, after seeing each mine planted, will mark down its position on a chart of the bay. He will then take the boat outside, enter under water, and, without touching any of our mines, while handling the boat, will see if he can stop close by and cut the connecting wires."

"If your mines contain no explosive, Major," Eph inquired, "how are you going to be able to tell whether I collide gently with one of your submarine mines?"

"We shall know at once," smiled Major Woodruff. "If you should collide with one, you will cause, a bell to be rung in the camera obscura room over at the fort. The bell that rings will show us which one of the mines you touched against."

The "camera obscura," as used at a modern fort, is in itself a most interesting contrivance. While no elaborate description of it can be attempted here, it will be enough to explain to the reader that, in the camera room, which is darkened, is a large white table covered with white oil-cloth, or other white substance. On this white surface is drawn a plan of the harbor to be defended. The position of each mine sunk under the water's surface is indicated on this map against the white background. Each mine is numbered. Overhead is a revolving shutter, somewhat on the plan of a camera's lens shutter. This shutter, which turns a reflecting lens on the harbor, can be turned in any direction. Any vessel in the harbor can thus be "caught," and its reflection, in miniature, thrown upon the white map surface.

Suppose an enemy's battleship to be entering the harbor. The camera obscura shutter, in being turned about, suddenly throws upon the white screen-map the miniature picture of the hostile battleship. Henceforth the officer in command sees to it that the shutter is so operated as to keep the image of the battleship always upon the white screen map. Thus the course of the battleship is followed—absolutely. At any second the exact position of that battleship in the harbor is known.

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."

The "periscope" is a device also of the nature of a camera obscura. In the case of the periscope a narrow metallic tube is thrust above the water and the shutter turned about, reflecting all the scene about on a white-covered table in the boat's cabin.

"I think I can beat you, Major," smiled Eph.

"I certainly hope you can," replied Major Woodruff. "That is what we want to see today. We shall watch closely, too, and see whether any plan can be devised for beating a submarine torpedo boat at its own game."

Lieutenant Kline was to remain on board the "Spitfire," both in order to watch the work and to give Eph any instructions that might be necessary in order to make the tests more conclusive.

"If you will come along with me, then, Mr. Benson," suggested Major Woodruff, "I will put you ashore on the neck. On the way over I will give you your instructions."

As the tug came alongside again Jack followed the major over the gang plank to the deck of the other craft.

"Good-bye, Captain Somers," called Jack, laughingly. "Give a fine account of yourself as an enemy of the United States!"

"Oh, you—" began Eph, flaring red, but wisely cutting his speech short.

On the way over to the strip of land known as the "neck" Major Woodruff managed to make his instructions wholly clear to young Benson.