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2: How long can the diver in this picture stay underwater in the diving bell? How would you redesign the bell to allow the diver to stay underwater longer?

Answer: The diver can stay underwater only until the fresh air in the bell is used up. The air we are used to breathing is about 79 percent nitrogen and 21 percent oxygen. The air we exhale, however, is made up largely of carbon dioxide, a gas that is poisonous to humans.

To prolong the dive, you might make the bell bigger, so it will hold more air. You also might devise a way to supply the bell with fresh air -- but remember that simply running a tube to the surface won't do the trick, because air won't flow downward against the greater pressure underwater.