Ships and submarines:

            A submarine can travel over as well as underwater. It also works on the principle of floatation. It floats over water when the weight of water equal to its volume is greater than its weight. Under this condition, it is similar to a ship and remains partially above water level. It has a system of tanks that can be filled with and emptied from seawater. When these tanks are filled with seawater, the weight of the submarine increases. As soon as its weight becomes greater than the upthrust, it dives into the water and remains underwater. To come upon the surface, the tanks are emptied from seawater.