I need help in my cartesian diver bottle project?

It's been many, MANY years since I saw this one! If you take a medicine dropper and put exactly so much water in it, (you'll have to determine the exact amount my experiment), and drop it - bulb up) into a bottle FULL of water, then put a cork in the bottle, the dropper will barely float, but if you press hard on the cork, the air in the dropped will be slightly compressed, and the dropper will slowly sink to the bottom, Release the pressure and the air will expand and the dropper will again rise to the top of the bottle. This might work better with a plastic bottle, such as a soft drink bottle with the label torn off.