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.