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.