Amidships there is a bridge from which the vessel can be navigated when on the surface and a conning tower from which she can be navigated when running awash. Most of the submerged travel is maintained at a comparatively shallow depth so that the submarine can keep an “eye” on the surface. The eye is the periscope of which there are two so that in case of damage to one the other may be used. Periscopes date back to the “Fifties” when they were used on some experimental European submarines. In the Civil War, when the river monitor _Osage_ ran aground in the Red River, her captain, now Rear-Admiral Thomas O. Selfridge, rigged up a periscope with which he could look over the high banks of the river and direct the fire of his guns upon a Confederate force that was attacking him. This periscope consisted merely of a three-inch pipe with a hole at each end cut in opposite sides of the pipe. Small mirrors were set in the pipe so that the light coming in through the upper hole was reflected down through the pipe and out of the lower hole. This crude periscope is the same in principle as the modern submarine periscopes except that the latter are provided with lenses to gather and focus all the light possible on the eyepiece so that the operator will have a perfectly clear view. Periscopes extend fifteen to twenty feet above the roof of the conning tower. While periscopes are insignificant objects on the broad seas when a submarine is moving very slowly, they are made conspicuous by the wake of foam that follows them when the boat is traveling even at a moderate speed. For this reason during the war the Germans developed a telescoping periscope which could be shot up to the surface whenever desired in order to give the commander a glimpse of his surroundings. The submarine is submerged by letting water into the ballast tanks and then turning the hydroplanes to a diving angle. There is an after as well as a forward pair of diving rudders. When running completely submerged the submarine is blind and solely dependent upon the chart and compass. It is impossible to see far under water. Searchlights are of no value at all. They will not make visible an object a hundred feet away. The submarine commander cannot see even the bow of the boat he is piloting. The ordinary magnetic compass will not operate when entirely incased in steel as it is in the hull of the submarine and so a gyroscope compass has to be used instead.