“A Sperry gyroscopic compass,” replied the lieutenant. “An ordinary magnetic compass could not be relied on to point in any particular direction if it was shut up in a steel box full of charged electric wires, like the turret of a submarine. We tried to remedy this by building conning-towers of copper, till Mr. Sperry perfected a compass that has no magnetic needle, but operates on the principle of the gyroscope. You know that a heavy, rapidly rotating wheel resists any tendency to being shifted relative to space?” “Yes.” [Illustration: Cross-section of a Periscope.] “The earth, revolving on its axis, is nothing but a big gyroscope--that is why it stays put. The little gyroscope on this compass spins at right angles to the revolution of the earth and so keeps in a due north and south line. But the frame it is mounted on turns with the ship, so the relative positions of the frame and the gyro-axis show in what direction the submarine is heading.” “And you can see what’s ahead of you through the periscope. Who invented that?” “The idea is a very old one. Certain French and Dutch inventors designed submarines with periscopes as long ago as the eighteen-fifties. In the Civil War, the light-draft river-monitor _Osage_ had attached to her turret a crude periscope made by her chief engineer, Thomas Doughty, out of a piece of three-inch steam-pipe with holes cut at each of its ends at opposite sides, and pieces of looking-glass inserted as reflectors. By means of this instrument her captain, now Rear-Admiral, Thomas O. Selfridge, was able to look over the high banks of the Red River when the _Osage_ had run aground in a bend and was being attacked by three thousand dismounted Confederate cavalry, who were repulsed with the loss of four hundred killed or wounded by the fire of the monitor’s 11-inch guns, directed through the periscope.[15] “But as late as 1900 the periscope was so crude and unsatisfactory an instrument that John P. Holland would have nothing to do with it. The credit for bringing it to its present efficiency belongs chiefly to the Germans, who kept many of their scientists working together on the solution of the difficult problems of optics that were involved. “By turning this little crank,” the lieutenant continued, “I can revolve the reflector at the top of the tube. This reflector contains a prism which reflects the image of the object in view down through a system of lenses in the tube to another prism here at the bottom, where the observer sees it through an eyepiece and telescope lenses.”