Ships cartesian diver and submarines
Bodies which are less dense than water float; those more dense sink. A piece of solid steel sinks, but a ship made of steel floats. Because a ship is hollow and contains air, its average density is less than that of water.
An interesting example of a body whose average density can be varied is the cartesian diver. This is a small hollow figure made of thin glass and having a hollow, open-ended tail (Fig. 12.5). Normally, being full of air, it floats on water. If, however, the diver is put inside a bottle full of water with a nicely fitting cork it can be made to sink by pressing in the cork. Pressure on the cork increases the pressure inside the bottle, with the result that water is forced into the body of the figure through the hollow tail. The diver then consists of a mixture of glass, water, and air. It will sink, remain stationary or rise in the bottle according as to whether its total weight is greater than, equal to or less than the weight of the water it displaces. Similarly, the buoyancy of a submarine depends on the quantity of water in its ballast tanks. When it is required to dive, water is admitted to special tanks. When the water is ejected from the tanks by means of compressed air the submarine rises to the surface and floats just like any other ship.