Experiment 13. Magic.
FIG. 26
ACROBATS
Place the pollywog in a bottle filled to overflowing with water, insert the solid rubber stopper, and press it down hard. Does the pollywog sink?
Now release the stopper quickly. Does the pollywog turn somersaults in a most magical manner (1, Fig. 26), and also rise?
Make one or two more pollywogs, place them all in the bottle together (2, Fig. 26), and entertain your friends with a pollywog circus.
The pollywog sinks when you press down on the stopper because you compress the air in it and force water in until it weighs more than the water it displaces.
FIG. 27
DANCING POLLYWOGS
The pollywog rises when you release the stopper because the compressed air drives the water out until the pollywog weighs less than the water it displaces.
The pollywog turns a somersault because the water rushes out sidewise in one direction and forces the nozzle in the other direction.
Air may escape from the pollywog when it is turning a somersault; if so, water will take its place, and may make the pollywog too heavy to float. You can restore its buoyancy by sucking out the water.