Floating Hourglass

Two perspex tubes, filled with water. (In fact a little bit of water has evaporated over the years, so the tubes are not quite full any more). Each tube contains an hourglass; one is floating at the top of the tube, and the other one has sunk to the bottom of the tube. This is a stable state - everything will stay like this indefinitely. When you turn the whole mechanism upside down, the hourglass that was floating at the top now stays at the bottom of the tube, and the hourglass that was at the bottom is