Procure a small hollow glass vessel, the shape of a balloon, the
lower part of which is open, and place it in water, with the mouth
downwards, so that the air within prevents the water filling it. Then
fill a deep glass jar nearly to the top with water, and place the
balloon to float on its surface; tie over the jar with a bladder, so
as to confine the air between it and the surface of the water. Press
the hand on the bladder, when more water will enter the balloon, and it
will soon sink to the bottom of the jar; but, on removing your hand,
the balloon will again ascend slowly to the surface.