While the compass serves as a guide for travel in the horizontal plane the depth gauge must be watched to see that the boat does not dive too deeply. The pressure of the sea increases at the rate of 64 pounds per foot of depth. Two hundred feet below the surface the pressure amounts to about six tons on every square foot of the surface of the submarine. Few submarines can stand a greater pressure than that without being crushed or at least springing serious leaks. When coming to the surface the hydroplanes are used and if the boat is to remain on the surface water is blown out of the ballast tanks. THE “EARS” OF A SUBMARINE Although submarines may be blind under water they are not deaf. Sound detectors are used which enable them to locate other vessels by the throbbing of their engines or the beat of their propellers and so they can avoid collisions when coming up to the surface. Despite the perfection of the submarine the sea is still a great mystery to us. We know only its surface and its shallows. We have sounded a few deep holes and brought up samples of deep-sea life, but we have not been able to penetrate in person its profound depths and explore with our eyes and hands the world that is buried beneath its waves. Three quarters of the globe is covered by water and if we are to claim complete mastery of the earth we must find some way of descending into the heart of the ocean and exploring its deepest valleys. CHAPTER XIII SLIDING, ROLLING, WALKING, AND CREEPING IN ALL forms of transportation friction plays a most important part. If there were no such thing as friction, it would be impossible for us to set an object in motion by the means that are now commonly in use, and once the object were moving it would be impossible to stop it except by bringing it up against a wall set squarely across its path. In transportation on land friction is a much more serious bar to motion than it is in marine or aerial transportation. There are two kinds of friction that we have to contend with; sliding friction when two contacting surfaces are dragged by each other; and rolling friction when one surface rolls upon another. It is impossible to obtain two surfaces that are absolutely smooth; there are minute elevations in each that sink into minute depressions of the other like intermeshing teeth and a grinding action takes place as these microscopic inequalities are broken away. Oil reduces the friction by filling up these inequalities, but the oil itself offers a certain amount of friction just as water does along the sides of a moving ship. ROLLING FRICTION