"Very good, sir. The neck, I take it, is the 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.